Three members of the Puccinia genus, Puccinia triticina (Pt), P. striiformis f.sp. tritici (Pst), and P. graminis f.sp. tritici (Pgt), cause the most common and often most significant foliar diseases of wheat. While similar in biology and life cycle, each species is uniquely adapted and specialized. The genomes of Pt and Pst were sequenced and compared to that of Pgt to identify common and distinguishing gene content, to determine gene variation among wheat rust pathogens, other rust fungi, and basidiomycetes, and to identify genes of significance for infection. Pt had the largest genome of the three, estimated at 135 Mb with expansion due to mobile elements and repeats encompassing 50.9% of contig bases; in comparison, repeats occupy 31.5% for Pst and 36.5% for Pgt. We find all three genomes are highly heterozygous, with Pst [5.97 single nucleotide polymorphisms (SNPs)/kb] nearly twice the level detected in Pt (2.57 SNPs/kb) and that previously reported for Pgt. Of 1358 predicted effectors in Pt, 784 were found expressed across diverse life cycle stages including the sexual stage. Comparison to related fungi highlighted the expansion of gene families involved in transcriptional regulation and nucleotide binding, protein modification, and carbohydrate degradation enzymes. Two allelic homeodomain pairs, HD1 and HD2, were identified in each dikaryotic Puccinia species along with three pheromone receptor (STE3) mating-type genes, two of which are likely representing allelic specificities. The HD proteins were active in a heterologous Ustilago maydis mating assay and host-induced gene silencing (HIGS) of the HD and STE3 alleles reduced wheat host infection.
Rust fungi have the most complex life cycles among described fungi, with many stages having discrete morphologies and very distinguishable sexual and asexual propagation. For many rust fungi, these stages occur on two different, unrelated host plants requiring two different infection strategies (heteroecious). For many, such as the cereal rust fungi, the asexual stage can successfully propagate and lead to epidemics as long as the host is present. When this host becomes senescent, the fungus produces the more resilient teliospores allowing for the sexual cycle to occur (Mendgen 1984). In homoecious rust fungi, like flax rust [Melampsora lini (Ehrenb.) Lév.], both stages evolved on the same host (autoecious). In heteroecious rust fungi, such as the cereal rust pathogens, major host jumps occured through evolution of the asexual (uredinial) stage to infect a different host (Savile 1976). These complex interactions result in the production of up to five different rust spore types, requiring very discrete developmental programs, resulting in altered gene expression profiles (Huang et al. 2011; Xu et al. 2011; Upadhyaya et al. 2014).
The obligate biotrophic lifestyle of wheat rust pathogens hampers the ability to culture the fungus in vitro and thus limits biological studies. Genetic studies by crossing individual strains is not trivial, but not impossible, due to the difficulty of breaking teliospore dormancy in order to infect the alternate hosts (Samborski and Dyck 1968; Rodriguez-Algaba et al. 2014). Most of what is known about wheat rust pathogen biology is based on extensive cytology (Bushnell and Roelfs 1984) and isolate interactions with host resistance genes. Many interactions between rust fungi and their cereal hosts have been shown to genetically conform to the gene-for-gene theory (Flor 1942; Loegering, and Powers 1962). The majority of wheat rust resistance genes (McIntosh et al. 1995) have been shown to be dominant or semidominant (Statler 1979, 1982, 2000), and current models imply an interaction between the resistance gene products and fungal effectors (Sperschneider et al. 2013; Petre et al. 2014).
Wheat leaf rust, caused by Puccinia triticina Eriks (Pt), is the most commonly occurring and economically important cereal rust disease worldwide (Singh et al. 2002; Huerta-Espino et al. 2011). Leaf rust on wheat was first recognized as different from stem rust in 1718 (De Candolle 1815), included into a species complex (P. rubigo-vera), and taxonomic refinements resulted in the current classification based on differences in spore morphology and alternate host range (Eriksson 1899; Savile 1984; Anikster et al. 1997). Pt is an obligate biotroph that can complete its sexual cycle on either of two known alternate host species, Thalictrum speciosissimum Loefl. (meadow rue; Jackson and Mains 1921; Saari et al. 1968) or Isopyrum (Brizgalova 1935, 1937). The complete Pt cycle consists of five spore stages (Bolton et al. 2008). The urediniospore is the most common and is asexual and polycyclic. At maturity when leaves begin to senesce, the fungus will form black teliospores on the abaxial side of the leaf. Within the teliospores, karyogamy takes place and promycelia are formed when the teliospores germinate. Four haploid basidiospores, in which the mating types have segregated, are formed and infect the alternate host, forming the pycnium. Dikaryotization occurs through fusion between a receptive flexous hyphae and a pycniospore of a different mating type. After fusion, the dikaryotic state is reestablished and an aecium will form on the underside of the leaf from which aeciospores will be released and travel to the wheat host. After landing on wheat leaves, the aeciospore will germinate forming an appressorium over a stoma. The haustorial mother cell forms in the substomatal cavity and attaches to the host cell wall. The plant cell wall is breached between 24 and 48 hr, forming haustoria. The fungus will spread intercellularly and a uredinium is formed at 7 d, from which urediniospores are produced to complete the life cycle. Wheat suffers from two other major rust diseases: stem rust, caused by P. graminis Pers.:Pers. f. sp. tritici Erikss. & E. Henn. (Pgt; Leonard and Szabo 2005), and stripe rust, caused by P. striiformis Westend. f. sp. tritici Erikss. (Pst; Chen et al. 2014) with very similar biology and spore stages, except Pgt and Pst have Berberis spp. as an alternate host (Statler 1979, 1982, 2000; Jin et al. 2010).
Rust fungi belong to the subphylum Pucciniomycotina that together with the Ustilaginomycotina, the true smut fungi, and the Agaricomycotina, which include mushroom-forming species, make up the phylum Basidiomycota. In this phylum, the sexual cycle typically requires cell–cell fusion governed by both pheromone (mfa) and pheromone receptor (STE3) genes, which then allows the formation of heterodimeric transcription factors coded for by two classes of homeodomain-containing protein genes, HD1 and HD2 (Raudaskoski and Kothe 2010; Kües et al. 2011). In the corn smut fungus Ustilago maydis, the mating-type locus contains both the pheromone receptor gene Pra (the STE3 equivalent) and a pheromone precursor gene mfa (Brefort et al. 2009). In all basidiomycetes studied to date, heterodimeric HD-containing transcription factors have been implicated in the mating process. They are found in pairs of genes each encoding subunits of an HD1 and HD2-containing protein that are divergently transcribed. Originally found with their start sites within 1 kb in Um, many variations exist and, in mushrooms, multiple pairs are often found in arrays of linked diverged copies (Casselton and Kües 2007; Raudaskoski and Kothe 2010), though single gene pairs are predicted in Pleurotus djamor (James et al. 2004) and Pholiota nameko (Yi et al. 2009). Supplemental Material, Figure S1 illustrates various mating-type genes and their organization in a few species of basidiomycete fungi. To date, the mating loci of the wheat rusts have not been carefully analyzed.
As for other obligate pathogens, genome sequencing of rust fungi has advanced the basic understanding of these organisms, which are otherwise recalcitrant to laboratory study. Initial molecular analyses and phylogenetic data indicated that, within each lineage of these three rust pathogens, adaptation to the wheat host had occurred independently (Zambino and Szabo 1993). Genome differences were identified in EST sequencing studies, where it was shown that 40% of Pt ESTs did not have a match to Pgt and Pst (Xu et al. 2011). Comparison of contigs from BAC and genome sequencing have shown synteny between the three genomes; however, there are regions of gene insertions, expansion by mobile elements, and inversions (Cantu et al. 2011; Fellers et al. 2013).
Prior to this work, the genomes of Pgt and the poplar leaf rust M. larici-populina (Mlp) were sequenced and compared. Out of 17,773 and 16,399 predicted genes, respectively, a core set of genes was identified representing the biotrophic nature of rusts (Duplessis et al. 2011). More recently, a second de novo genome assembly of Pgt was completed (Upadhyaya et al. 2014). Three genome sequencing projects have been described for Pst (Cantu et al. 2011, 2013; Zheng et al. 2013). The first two focused on identifying the effector complement, and the third study on identifying heterozygosity between two isolates of Pst. In each study, the total number of predicted genes varied across the projects (22,815 vs. 25,288, respectively). The genome of the flax rust M. lini (Mli) has also been sequenced (Nemri et al. 2014). Comparative analyses with other basidiomycete genomic resources have provided initial insights into the relatedness of subsets of genes (Xu et al. 2011; Nemri et al. 2014), but a comprehensive analysis among wheat rusts is missing.
Here, we have generated draft genome sequences of the wheat rust fungi Pt race 1 (BBBD) and Pst race PST-78, updated the gene set of Pgt race SCCL, and utilized these sets to define the shared and unique properties of these three related pathogens. To examine gene content evolution, we compared predicted proteins to those of other high-quality basidiomycete genomes. We examined the three wheat rust pathogens for conservation of gene families, including effector genes, and compared them to other sequenced rust fungi. We identified predicted secreted proteins including gene families found only in the wheat rust pathogens and found differences in expression levels between Pt life cycle stages, including sexual stages on the alternate host and wheat infection. In addition, we analyzed the mating-type gene complexes, revealed their evolutionary placement among basidiomycetes, tested the functionality of several Pt homeodomain proteins through heterologous expression in U. maydis (Um), and demonstrated a role for mating-type genes during wheat infection by HIGS.
Material and Methods
For detailed descriptions of isolates, sequencing strategies, genome assemblies and annotation, polymorphism analyses, DNA and RNA isolation procedures, RNA sequencing (RNA-Seq), and cloning methods, see File S1.
Puccinia isolates and growth conditions
Pt race 1, BBBD was selected as the race to be sequenced. This isolate was first collected in 1954 (Ordoñez and Kolmer 2009) and represents the earliest race characterized in North America. For Pst, isolate 2K41-Yr9 was selected (race PST-78). PST-78 was collected from the Great Plains in 2000 and is a representative of races that were first identified in the US in 2000 and then subsequently identified in other countries (Wellings et al. 2003; Hovmøller et al. 2008).
DNA and RNA isolation
Genomic DNA was isolated from Pt and Pst urediniospores. RNA was isolated from three stages of Pt race 1: fresh mature, “dormant” urediniospores that had been collected at 10 d postinoculation (DPI); urediniospores that were germinated on water but harvested at 8 hr postgermination initiation; and from heavily infected wheat tissue at 6 DPI to represent the formation of urediniospores, initiation of secondary infection, and most of the infection structures. RNA was also isolated and sequenced for two stages from the alternate host, for Pt, T. speciosissimum (meadow rue) and Pgt, Berberis spp. These stages represented pycnia with their pycniospores and a mixture of pycnia and aecia with aeciospores. RNA was isolated from two stages of Pst, infected wheat tissue at 8 DPI, isolated haustoria, and purified as described (Yin et al. 2009).
Genome sequencing and assembly
For Pt genome sequencing, various sizes of genomic DNA libraries and platforms were used. In short, libraries of 3 and 8 kb fragment inserts were sequenced using Roche 454 FLX chemistry and two large insert libraries were end-sequenced using Sanger technology: a 40 kb insert Fosmid library (30,731 clones) and ∼100 kb insert BAC library [15,000 clones (Fellers et al. 2013); Table S1]. An initial assembly of FLX and Sanger data was generated with Arachne (HybridAssemble) (Jaffe et al. 2003). The assembly was updated to incorporate the FLX+ data by first generating a new de novo assembly of all data using Newbler runAssembly, with parameters –het and –large, and merging the output with contigs uniquely present in the first assembly.
Three similar Pst genomic DNA insert libraries were sequenced using FLX chemistry with a Roche 454. In addition, paired-end Illumina reads were generated for three additional library sizes: fragment, 3–5 kb insert, and 40 kb Illumina-adapted Fosmids (Fosill library, Williams et al. 2012; Table S2). Three initial assemblies were generated using different algorithms: Life Technologies’ Newbler program, the CLC (QIAGEN, Hilden, Germany) de novo assembler, and ALLPATHS-LG (Gnerre et al. 2011). To provide the most complete representation of the genome, contigs from the ALLPATHS-LG assembly were first selected, and then unique contigs from the CLC assembly were incorporated; see File S1 for details.
All assemblies were evaluated for regions that could correspond to both haplotypes that were independently assembled due to higher than typical divergence. One approach compared the ortholog representation (see below) across the three Puccinia genomes for two-copy paralogs in a single gene set, which could suggest either independent assembly of allelic copies in a single assembly or, alternatively, a gene duplication in that genome. As an independent approach, we aligned the repeat-masked sequence of each assembly to itself using nucmer (version 3.22, with parameter –maxmatch) (Kurtz et al. 2004); alignments were filtered to select alignments with 95% identity and 1 kb or greater length. Scaffolds with alignments covering 50% or greater of the repeat-masked contig length were considered as potentially representing the second haplotype and their total size was reported; alternatively, these could include segmental duplications present at different locations in the genome.
Heterozygous positions within the sequenced isolates of Pt and Pst were identified from Illumina data. Reads were aligned to each assembly using BWA (v0.5.9) (Li and Durbin 2010), and SNP positions were identified with GATK v2.1.9 UnifiedGenotyper, and then filtered by GATK VariantFiltration; see File S1 for details.
Strand-specific libraries were constructed with poly(A)-selected RNA samples using the dUTP second strand marking method (Parkhomchuk et al. 2009; Levin et al. 2010) for most samples. See File S1 for details of library construction and expression analysis.
Genome annotation and protein set comparisons
Gene sets were annotated by incorporating RNA-Seq data and predicted gene structures from multiple de novo predictions as previously described (Haas et al. 2011); see File S1 for a detailed description. The gene sets of the three Puccinia genomes were compared to each other and those of eight other fungi: M. lini, M. larici-populina, Microbotryum lychnidis-dioicae, Mixia osmundae, Sporobolomyces roseus, Coprinopsis cinerea, Cryptococcus neoformans var. neoformans, and U. maydis. Orthologs were identified using OrthoMCL (Li et al. 2003) with expectation value 1e−5. The resulting orthologs were input to DAGchainer (Haas et al. 2004) to identify syntenic blocks, requiring a minimum of four genes in the same order and orientation in the compared genomes. Synteny plots were generated using a custom perl script, using the GDgraph library; code is available at https://github.com/gustavo11/syntenia.
Cloning, expression, and functional analysis of Pt mating-type genes
The various Pt HD mating-type alleles were amplified by PCR from cDNA generated from total RNA isolated at 5 DPI from infected wheat cv. “Thatcher” leaves infected with Pt race 1 or from urediniospores germinated over water. These alleles were subsequently cloned in a Ustilago-specific, integrative vector for heterologous expression from the strong constitutive Hsp70 promoter and terminator elements. Constructs were stably transformed into Um518 (Kronstad and Leong 1989) or strain FB1 (Banuett 1991), and in U. hordei. To test the function of various mating-type genes during infection of wheat by Pt, HIGS experiments were performed. To create the RNAi silencing vectors, fragments of size 393, 430, 351, and 345 bp of the genes PtbW1, PtbE1, PtSTE3.3, and PtSTE3.1, respectively, were amplified by PCR, cloned into the vector pENTR/D-TOPO (ThermoFisher, Waltham, MA), and subsequently recombined with the binary destination vector pIPK007 (Himmelbach et al. 2007) using the LR GateWay recombination reaction to create the silencing vectors pRNAi-PtbW1, pRNAi-PtbE1, pRNAi-PtSTE3.3, and pRNAi-PtSTE3.1, respectively. Agroinfiltration assays, using Agrobacterium tumefaciens strain COR308, subsequent challenge by Pt, and fungal biomass measurements using quantitative PCR measurements were performed as described previously (Panwar et al. 2013). For details on these procedures, see File S1.
Data access in NCBI: all genome assemblies and annotations are available with the following accessions ADAS00000000 (P. triticina), AJIL00000000 (P. striiformis f. sp tritici), and AAWC00000000 (P. graminis f. sp tritici). All sequence is linked to the following BioProjects: PRJNA36323 (P. triticina), PRJNA41279 (P. striiformis f. sp tritici), and PRJNA18535 (P. graminis f. sp. tritici).
Genome expansion in Pt associated with repetitive element proliferation
High-quality genome assemblies of Pt and Pst were generated by combining data from multiple sequencing technologies. A range of insert size libraries for both genomes were sequenced using Roche 454, Illumina, and Sanger Technologies (Table S1 and Table S2). The assembled genome of Pt was the largest of the three wheat rust pathogens, totaling 135.3 Mb (Table 1); this assembly included 14,818 scaffolds of an N50 length of 544 kb. The assembly of Pst totaled 117.31 Mb and consisted of 9715 scaffolds with N50 length of 519 kb. The total contig size of 79.3 is slightly larger than the total of contig assemblies generated for other strains at lower coverage levels (Cantu et al. 2011, 2013); both the scaffold and contig total are lower than those reported for the CY32 strain (Zheng et al. 2013). RNA-Seq was used to guide gene prediction for both Pt and Pst, and to improve the gene set of Pgt (Materials and Methods). Of the three rust pathogens, Pst had the highest number of genes predicted with 19,542, though fewer than the number reported for other Pst genomes (Cantu et al. 2011; Zheng et al. 2013), while Pt had the smallest total of the three with 14,880 genes. All three genomes have high coverage of a core eukaryotic gene (CEG) set (Parra et al. 2007; Table 1). The CEG coverage of this Pst gene set (97%) is notably higher than that of the PST-130 assembly (66%), the only other publicly available Pst gene set, due to a higher fraction of partial gene matches in PST-130 (Figure S2). In addition, comparison of a larger set of basidiomycete conserved orthologs supports the notion that few genes appear missing in the three wheat rust fungal genomes (Figure S3). Together, these gene conservation metrics suggest that these assemblies contain highly complete gene sets.
The assemblies of the three wheat rust fungi varied significantly in size, ranging from 89 Mb for Pgt to 135 Mb for Pt. While the Pst assembly totals 117 Mb, the genome may in fact be smaller than the assembly size, as the high percentage of gaps (32%) in scaffolds suggests that small contigs fall into some of the gap regions. In comparison, the Pt assembly consists of 21% and the Pgt assembly only 8% gap regions. While all assemblies are impacted by the high heterozygosity (see below), differences in repeat content and organization, as well as the use of different sequencing technologies, likely contribute to these differences. Each of the three wheat rust pathogen genomes was evaluated for content of repeated elements using both de novo predicted repeats and fungal elements from RepBase, which included 413 Puccinia sequences (File S1). The larger genome of Pt includes a higher fraction of repetitive elements, encompassing 50.9% of contig bases, whereas repeats covered only 31.5% of Pst and 36.5% of Pgt (Table 2). The expanded repeat content of Pt includes roughly twofold more of both class I retroelements and class II DNA elements. After excluding the identified repeats, the nonrepetitive portions of the genomes are very similar, totaling 53.4 Mb for Pt, 54.4 Mb for Pst, and 51.8 Mb for Pgt.
Comparison of syntenic regions between Pt and Pgt highlights that the genome expansion in Pt is due to disperse integration of repetitive elements. In total, 4319 orthologs are found between the two species in syntenic blocks of between 4 and 52 genes (Material and Methods). However, the size of the syntenic blocks in Pt are 30.1% larger overall than in Pgt; syntenic regions cover 46.7 Mb of Pt and 35.9 Mb of Pgt. In contrast, regions of Pst are 71.2% the size of syntenic regions of Pgt, suggesting a compaction of Pst; however, this analysis is impacted by the high percentage of gaps in the Pst assembly, reducing the resolution of blocks that can be detected. Within the expanded regions of Pt and Pst, there are larger blocks of repetitive sequence interleaved between the orthologs (Figure 1), highlighting that the genome expansion appears due to disperse integration of repetitive elements.
High heterozygosity across all wheat rusts and evaluation of haploid assemblies
The cereal rust pathogens exist as dikaryotic (n + n) organisms for most of their life cycle, with a high level of heterozygosity between haplotypes. For Pt, 269,370 heterozygous SNPs were identified across the genome based on Illumina read alignment (File S1). Across the genome, the average rate of heterozygosity was 2.53 SNPs/kb, though a higher rate was observed in intergenic regions (2.80 SNPs/kb) than in genic regions (1.69 SNPs/kb). In contrast, for Pst, 473,282 heterozygous SNPs were identified, for an average rate of 5.97 SNPs/kb; genic regions show a higher rate (7.49/kb) than intergenic regions (4.96/kb), a much higher rate than the 0.68 SNPs/kb previously reported for Pst with an assembly of the CY32 strain (Zheng et al. 2013) but in the same range of 5.29 SNPs/kb averaged over another five Pst isolates (varying from 2.23 to 7.11 SNPs/kb; Cantu et al. 2013). The rate of heterozygosity for Pt is similar to that reported for Pgt (Duplessis et al. 2011), where higher rates in genic regions (2.28 SNPs/kb) were found compared to intergenic regions (1.72 SNPs/kb), although the sequencing technology and methods differ between these studies. Heterozygosity levels in both Pgt and Pt are more than double those reported for genic and intergenic regions of Mlp (0.84 and 0.87 SNPs/kb, respectively), supporting a high level of allelic diversity in these two wheat rust pathogens.
Regions of high heterozygosity could carry enough differences to prevent haploid assembly and could inflate the gene count, as alleles would appear as duplicated genes. Therefore, we examined the orthology assignment of genes found in all three wheat rust pathogens for conserved genes with two copies in only one species (Materials and Methods). Among the wheat rust fungi, Pt has only 230 species-specific two-copy paralogs (2:1:1; Pt:Pgt:Pst). Pst has an intermediate value of 361 species-specific paralogs while Pgt has 465 species-specific paralogs. This suggests that the new assemblies of Pt and Pst do not contain more duplicate conserved genes than the well-assembled Pgt genome. The presence of independently assembled haplotypes was also evaluated by identifying high identity and high coverage regions of self-alignment for each assembly (Materials and Methods); such regions cover 690 kb in Pt, 383 kb in Pgt, and 737 kb in Pst. The Pgt total is similar to the 326 kb estimated previously when sequence depth was also considered (Duplessis et al. 2011). While more stringently supported for Pgt, comparison of gene count conservation metrics between the assemblies suggests that independent assembly of both haplotypes is minimal in all three wheat rust pathogens, consistent with the use of assembly strategies that take heterozygosity into account.
Core protein comparisons and orthology
To examine gene content variation between the three wheat rust pathogens and with other basidiomycetes, we compared the predicted proteins of Pt, Pgt, and Pst to those of related genomes. These included Mlp and Mli, the smuts Um and Mi. lychnidis-dioicae, the fern parasite M. osmundae, the unicellular plant phylloplane “red” yeast S. roseus, the human facultative pathogen C. neoformans, and the woodrotter Co. cinerea. By identifying orthologs across these genomes, we inferred the phylogenetic relationship of the species using single-copy orthologs; Pgt and Pt are most closely related, with Pst being an earlier diverged outgroup (Figure 2), consistent with previous findings from phylogenetic analysis of the ITS ribosomal DNA region (Zambino and Szabo 1993). While Pgt and Pt are most closely related based on phylogeny, some features may be more conserved or have evolved in parallel in Pgt and Pst, which share the same alternative host.
The wheat rust pathogens have very different gene content from other basidiomycetes, including a large fraction of species-specific genes. Less than half of the genes in each wheat rust pathogen, an average of 6867, were conserved among other basidiomycetes. All of the rust fungi (Pt, Pst, Pgt, Mlp, and Mli) contained an average of 6482 species-specific predicted genes. Among the other compared basidiomycetes, only Co. cinerea contained a similar number of species-specific genes. Among the wheat rust pathogens, Pt and Pgt had similar numbers of species-specific genes (5443 and 4901, respectively), while the Pst number was higher at 8955. In addition, a large fraction of the genes were conserved across the wheat rust pathogens but not other fungi; an average of 3440 were conserved in at least two genomes and 2164 were found in all three.
To assess functional differences based on variation in genes between the three wheat rust pathogens and other fungi, we identified significant differences in the number of predicted protein domains. The three wheat rust pathogens were compared to Mli, Mlp, and to six other basidiomycetes, to determine what protein families exhibit significant gain or loss in rust fungi. The majority of these are protein families involved with nucleotide binding and modification, transcription factor regulation, and protein modification (Table 3). These include the NAM-associated transacting factor family, the most significantly enriched protein domain overall; this domain is specific to the rusts among basidiomycetes, with seven or 10 copies in Mlp and Mli and between 49 and 134 copies in the three wheat rusts. Three other Zn-finger transacting factors, and a fungal-specific transcription factor, families are also enriched. Four enriched protein families are associated with carbohydrate active enzymes; trehalose phosphatase, pectinesterases, glycoside hydrolase (GH) family 26 (GH26), which processes mannan and galactomannan, and the GH76 family of α-1,6 mannanases. Other families are involved in carbohydrate processing and transportation, cell metabolism, and metabolite transportation (Table 3). A notable depleted family, NmrA, belongs to a family of transcriptional repressors involved in controlling nitrogen metabolite repression in fungi (Table S3; Stammers et al. 2001). Other genes involved in nitrate metabolism are lost in Pt and Pst, as previously noted for Pgt (Duplessis et al. 2011). Overall, these domains highlight recent adaptation of gene regulation and host-interaction via gene duplication and diversification.
Effector repertoire mining and conservation
Wheat rust pathogen candidate secreted effector proteins (CSEPs) are predicted to be expressed and secreted during host infection and are likely involved in host interactions. In this analysis, CSEPs predicted for Pt were compared to those previously identified in Pst and Pgt. From a starting set of 15,685 predicted proteins, including variant proteins encoded by alternate transcripts and novel genes detected by RNA-Seq data, a total of 1358 CSEP-encoding genes were predicted for Pt (Figure S4). Of these, a total of 914 Pt CSEPs grouped in 385 families or “tribes” previously assigned to Pst or Pgt CSEP tribes (Table S4; Cantu et al. 2013). From the remaining CSEPs, an additional 111 contained BLASTP sequence similarity (at ≤ e–20) to Pst and Pgt predicted proteins, of which 72 did not contain a predicted signal peptide at the expected initiation codon. The remaining 333 CSEPs were specific to Pt, of which 246 were unique without any paralogs in the Pt protein set. The remaining 87 CSEPs belonged to Pt-specific gene families having from two to eight members per tribe. This highlights that, while the vast majority of Pt CSEPs share sequence similarity with those in the other wheat rust pathogens, a subset is unique to each species. In addition, a disproportionate fraction of the wheat rust pathogen-specific genes mentioned above are predicted to code for secreted proteins. In Pt, a total of 17.0% of the wheat rust specific genes are predicted to encode secreted proteins compared to 9.6% of all predicted genes. This suggests that the genes specific to the wheat rust fungi include a high fraction of CSEP proteins.
Based on gene ontology (GO) term assignment and similarities to Pfam domains, the molecular functions of the Pt CSEPs appear highly diverse, though some frequent categories were observed. The largest subcategories include a total of 123 CSEPs that have hydrolase activity, 76 contain ion-binding activity, and 44 have oxidoreductase activity (Figure 3). Pfam domain comparisons also revealed some potential common and unique functions among rust fungi (Figure S5). Since protein targeting is dependent on intrinsic protein motifs such as a nuclear localization signal or a chloroplast transit peptide, we analyzed all predicted CSEPs minus their signal peptides for their potential localization in the host to deduce possible functions. The distribution of their subcellular localization prediction in the plant indicates that 388 CSEPs are potentially targeted to the cytoplasm, 361 to the nucleus, and 292 to plastids. A total of 190 of these proteins are targeted to membranes, 16 to the apoplast, seven to the Golgi system, four to the vacuole, and one to the ER (Table S4).
Expression profiles across life cycle stages and two hosts
Functionally important gene expression was evaluated using RNA-Seq across diverse life cycle stages. RNA was sequenced from samples of dormant and germinating urediniospores, infected wheat leaf tissue at 6 DPI representing most of the infection structures of the uredinial spore genesis of the life cycle, and two stages collected from the alternate host, T. speciosissimum (meadow rue), at the pycnia sexual stage and a later stage mixture of both pycnia and aecia. Comparing normalized counts across conditions revealed that two urediniospore samples were most highly correlated, and that both were similar to the mixed sample of pycnia and aecia (Table S5). In contrast, the pycnia and infected wheat leaf samples appeared the most different from the others.
To closely evaluate how secreted proteins change in expression across these conditions, all Pt CSEP genes were assessed and 199 were identified with a minimum of fivefold change. From these, 138 Pt CSEP genes were highly induced in wheat-infected leaves (Figure 3) with 30 assigned to known proteins with hydrolase (10), peptidase (four), oxidoreductase (three), ion-binding (two), phosphatase (two), carbohydrate-binding (two), lipase (two), chitinase (one), protease (one), phospholipase (one), phosphoglucono-lactonase (one), and esterase (one) activities. Twenty-six CSEP genes were highly induced exclusively in germinating urediniospores. Among other stage-specific sets were 16 CSEP transcripts highly accumulated in dormant urediniospores compared to the germinated spore stage. When focusing on infection of the alternate host Thalictrum, 16 were highly expressed in pycnia and three during the later stage of the mixed pycnia and aecia sample (p + ae). One to 10 hydrolase-encoding CSEPs are highly induced in various datasets, except for the “mixed” p + ae stages. Among those, members of the GH superfamily were specifically highly expressed: GH18 in wheat-infected tissue, GH16 and GH17 in germinating urediniospores, GH26 in dormant urediniospores, and GH16 in pycnia.
The predominant gene classes expressed in each stage were also examined by testing for functional enrichment in differentially expressed genes. Roughly half of the genes are induced during wheat infection, compared to dormant spores, and encode predicted secreted proteins (Table S6). Other gene families that are enriched during wheat infection include the GH18 family, DNA binding proteins, peroxidases, and amino acid permeases (Table S7). In contrast, genes expressed in pycnia relative to dormant urediniospores are enriched for GMC oxidoreductase, LON proteases, potassium uptake, osmotic stress response, and chitin synthases (Table S8).
Pheromone receptors and precursors:
Three homologs of the Um pheromone receptor gene Pra were found in each of the Pt, Pgt, and Pst dikaryotic genomes (Table S9). All corresponding genes had a similar structure including five introns (Figure S6). The predicted proteins ranged in size from 379 to 395 amino acids, and had the characteristic seven transmembrane domains typical of these G protein-coupled membrane-inserted receptors (Bölker et al. 1992). A molecular phylogeny was calculated for these and the receptors from the poplar and pine rust pathogens, including known STE3/PRA proteins from several other basidiomycetes (Figure 4). This analysis revealed that the STE3 receptors from the Pucciniales formed a clade (blue and gray boxes) well-separated from the Agaricomycotina (no color), the Ustilaginomycotina (orange box), and the Microbotryomycetes (yellow box). The rust clade encompassed two major groups, STE3.1 homologs (light blue box) and another branch with two subgroups, representing STE3.2 and STE3.3 (dark blue boxes). Since we did not have separate haplotype genome assemblies, we performed an in-depth analysis for Pt. Two allelic SNPs were found for PtSTE3.1 in Pt race 1, resulting in two nonsynonymous amino acid changes (Figure S7A). The same two SNPs were confirmed in 25 out of 29 other recently sequenced Pt genomes (data not shown). Race 1 RNA-Seq transcriptome analysis revealed two allelic transcript populations confirming the presence of two alleles, with both expressed roughly equally in various life cycle stages (Figure S7B). Complete digestion of a PCR product representing the 3′-end of the race 1 PtPRA3.1 gene with restriction enzyme Cac8I was able to distinguish one of the SNPs (Figure S7C). This analysis indicated the presence of two PtSTE3.1 alleles in the Pt race 1 genome. In contrast, no SNPs were found for the PtSTE3.2 and PtSTE3.3 genes, which may represent the two allelic specificities, one in each haplotype. Additional homologs, such as for Mlp, likely represent more recent, lineage-specific duplication and divergence events (gray box in Figure 4). Comparison among 16 resequenced Pst genomes revealed 34 SNPs in STE3.1, no SNPs in STE3.2, and two SNPs in the STE3.3 gene, whereas these numbers are very similar among 15 resequenced Pgt isolate genomes: 38 SNPs in the STE3.1 gene, no SNPs in STE3.2, and one SNP in STE3.3.
Using annotated Pt EST or known mfa sequences (File S2), three contigs in Pt and Pgt and two in Pst were found to contain putative mfa genes (Table S10). A putative Ptmfa2 gene coding for a 33 amino acid protein with a characteristic C-terminal CAAX motif was identified on supercontig 2.517; while no gene model was initially predicted here, evidence of expression was detected in several life cycle stages (Figure S6). Extensive searches of genomic reads and RNA-Seq data could not identify other mfa genes. In all three Puccinia species, the predicted mfa2 and STE3.2 genes are divergently transcribed and are approximately 500-700 bp apart (605 bp in Pt; Figure S6), an organization reminiscent of Ustilagomycete a2 loci. The Pgt STE3.3 allele is 24 kb away from a potential Pgt mfa gene on supercontig 2.2 (Table S9 and Table S10). In Um and Sporisorium reilianum, the a2 loci each harbor two additional genes, lga2 and rga2, that are located in between the Pra2 and mfa2 genes. The LGA2 and RGA2 proteins localize to mitochondria and are implicated in mitochondrial fusion processes in that fungus (Bortfeld et al. 2004), but no obvious homologs could be identified in the Puccinia species.
Homeodomain-containing transcription factors:
Two allelic homologs of both HD1- and HD2-containing protein genes were found in each of the three Puccinia species and were termed bE-HD2 and bW-HD1 (Table S11). Gene models in the genome assemblies were found to be partial, and complete transcript sequences were constructed using de novo RNA-Seq assemblies (Figure S8). The predicted Puccinia HD2 proteins are ∼374 amino acids in length whereas the HD1 proteins are ∼620 amino acids in length. PtbE2-HD2 and PtbW2-HD1 are located close together and are divergently transcribed (Figure S9 and Table S11). However, in the fragmented genome assembly, PtbE1-HD2 and PtbW1-HD1 are each located on a small contig, so no direct inferences of linkage could be made for this pair. Comparative analysis of aligned DNA and protein sequences for the two alleles of each PtbE and PtbW gene revealed the conserved HD-specific domains within an overall conserved C-terminal region, whereas the proteins were more diverged at the N-terminus, similar to the paradigm established in Ustilago species (Figure S8). A similar pattern of conservation was noted for the corrected Pgt and Pst alleles. A molecular phylogeny was generated to establish the relatedness among the HD-containing mating-type proteins in the three cereal rust fungi, compared to single homologs from the poplar and pine rust fungi (Figure 5). The allelic variants were closer to each other in each Puccinia species as they were among the species, since they are alleles and their sequences are evolving in a concerted fashion. Thus, among the rust fungi compared, the HD1- and HD2-containing transcription factors are each separated in defined clades, as is seen when many basidiomycetes are compared, indicating an ancient system in which allelic specificities are maintained because of their functionality (Bakkeren et al. 2008; Kües et al. 2011).
In all three wheat rust pathogens, a large contig with a complete divergently-transcribed pair of bE and bW genes is found while the other sometimes partial alleles are found on small contigs. This analysis highlights the challenges faced when assembling very similar sequences such as the conserved C-terminal domains, likely belonging to two different haplotype genomes. Therefore, to investigate the physical arrangement of both bE-bW pairs in the Pt race 1 genome, primers to the conserved 3′-ends of each gene (Table S12) were used in a PCR reaction, which yielded a single product of 3.9 kb from total gDNA isolated from germinating urediniospores. In dikaryotic urediniospores, both pairs are assumed to be present. Analysis of the sequences had revealed that nucleotide polymorphisms in restriction enzyme sites for Xma1 and Spe1 could be used to distinguish the allelic pairs. To verify whether or not the 3.9 kb PCR product contained both divergently-transcribed bW and bE gene pairs, it was digested with these enzymes for a prolonged period of time to yield fragments consistent with the presence of both allelic pairs, confirming the suspected organization (Figure 6).
Pt HD genes can functionally interact in U. maydis
We previously demonstrated the feasibility of using Um as a heterologous expression system for Pt genes (Hu et al. 2007). To examine the role of the candidate Pt mating-type genes, cDNA-derived Pt HD-containing transcripts were expressed in Um. Upon stable transformation of each of the PtbE1- or PtbW1-expressing constructs into either Um haploid strains a1b1 or a2b2, the resulting transformants yielded cells that had changed morphology from growth by budding to a filamentous growth (Figure 7). When introduced into Ustilago cells, constructs expressing b mating-type genes of a different specificity or from different Ustilago species, transformants display these changed morphologies similar to regular mating interactions between cells of opposite mating types (Gillissen et al. 1992; Bakkeren and Kronstad 1993). This indicates that each of the PtbE-HD2 or PtbW-HD1 proteins can productively interact with the respective resident Ustilago b-gene subunit to initiate the switch to filamentous growth.
Next, we wanted to see whether or not a pair of Pt-specific HD proteins could substitute for the resident pair in Ustilago. Two Uh strains of opposite mating type but each deleted for both bE and bW alleles [Uh553 (a1 b0) and Uh530 (a2 b0); Bakkeren and Kronstad 1996] were each transformed with the above-tested single Pt HD-containing gene constructs (PtbE1, PtbE2, PtbW1, or PtbW2). Several independent stable transformants for each strain and construct were paired on a mating-type plate assay. Transformants of opposite mating type (a1 vs. a2) should initiate proper cell fusion (brought about by the pra and mfa genes) allowing the respective Pt HD proteins to interact. “Fuzz+” colony phenotypes would then be indicative of productive heterodimer formation and initiation of filamentous growth. Whereas control pairings of Uh Uh100 × Uh112 wild-type cells produced very “fuzzy” colonies after 48 hr, all pairings of various combinations of transformants (five per construct) did not produce colonies with significant aerial hyphae production. Upon microscopic analysis of the cells from such colonies, no convincing production of dikaryotic straight-growing hyphae, as seen in wild-type mating interactions, could be seen, although mating hyphae were present and fusion initiated (data not shown).
Pt mating-type genes are functional during wheat infection
Analysis of the transcriptomes revealed significant expression levels of several of the PtSTE3, PtbE-HD2, and PtbW-HD1 alleles during various life cycle stages, though expression in urediniospores was relatively low (Figure S10 and Figure S11). Although expected to play a role during the sexual stage on the alternate host (certainly the pycniospore stage), a diversified role for them during infection could be envisaged. To examine such a role for the HD-containing alleles, PtSTE3.1, and PtSTE3.3, the Agrobacterium-mediated HIGS technique was used (Panwar et al. 2013). The silencing constructs containing the 3′ sequences representing PtbW1, PtbE1, and PtSTE3.1 would each target both alleles because of their conserved nature (Figure S7, Figure S8, and File S1). Extensive searches by BLAST of the targeted Pt gene sequences to all available wheat and Pt genomic resources could not identify potential off-target sequences. Prior expression of silencing constructs in the wheat host targeted at these pathogen genes significantly reduced fungal development, as measured by biomass reduction and disease symptoms such as sporulation, upon infection with Pt urediniospores (Figure 8). The exception was the HIGS construct, which targeted the PtSTE3.1 alleles, resulting in no measurable reduction in Pt biomass, similar to the control wheat TaPDS silencing construct. This correlated with the very limited number of PtSTE3.1 transcripts in the Pt wheat-infected transcriptome (Figure S7 and Figure S10) and hence demonstrated the specificity of the system, as was previously extensively shown for several other pathogenicity genes (Panwar et al. 2013).
The genomes of Pt and Pst, sequenced here and compared to those from Pgt and other Basidiomycetes, are notable for their expanded size and high level of heterozygosity. While each genome was assembled using different sequencing technology, each of the gene sets appears to be of similar quality, with high representation of core genes. The genome of Pt in particular has been expanded due to multiple classes of repetitive elements; while this higher repeat content was found to be dispersed across the genome assembly, repeat elements could impact the expression of nearby genes and could also contribute in this way to differences between related strains of the same species. Notably, we find that Pst has the highest level of heterozygosity and that this measure is larger than previously reported (Zheng et al. 2013). While some of this difference could be attributed to the isolate sequenced, the much larger size of the CY32 genome used in this previous study may result in an underestimation of heterozygosity, such as in cases where both alleles of a gene were assembled independently.
Prior to this work, gene content surveys focused on genes expressed during infection and other life cycle stages. An extensive EST data set of 13,328 unique ESTs was created by sampling several stages in Pt; however, functional annotation was generally low (Hu et al. 2007; Xu et al. 2011). During this genome project, ESTs and newly generated RNA sequences were used to refine gene models and predict alternatively spliced forms in each of the genomes. Notably, Pst contained the largest set of predicted genes at 19,542, despite not having the largest genome. This total is similar to what has been found in other Pst genome projects. In the sequence of four other Pst races, the gene count varied from 18,149 to 21,030, which may have been impacted by differing levels of heterozygosity (Cantu et al. 2013). It is intriguing that in Pst there are many more CSEPs than in Pt or Pgt; in one study, 2999 CSEPs were predicted in five consolidated Pst genomes, compared to 1333 and 1173 in Pgt and Mlp, respectively (Cantu et al. 2013). Virulence variability among Pst isolates is high and larger than for Pgt and Pt, likely due to a CSEP gene expansion and diversification to elude host recognition. In this regard, it may be significant that Pst can be found on 126 species of grasses among 20 genera (Line 2002; Cheng et al. 2016). Overall, the number of genes within the three rust fungal genomes is higher than that in other plant pathogenic fungi. Smut fungi have fairly low gene counts (6500–7000), but plant pathogenic fungi have as many as 17,735 in Fusarium oxysporum (Ma et al. 2010) and 16,448 in the necrotroph Botrytis cinerea (Amselem et al. 2011). Mli and Mlp have gene numbers of 16,271 and 16,399, respectively (Duplessis et al. 2011; Nemri et al. 2014), indicating a similar number of genes to wheat rust pathogens. Higher gene numbers may support the multiple spore stages and more complex life cycle in the rust fungi.
The large genome expansion in Pt due to repetitive elements was suggested by an earlier study of selected genome regions (Fellers et al. 2013). The genomes of other rust fungi are also enriched for repetitive elements, though smaller in number and total DNA content. Pst and Pgt have similar repeat element numbers, while Pt is more like Mli, for which repeats occupy 87 Mbp or 46% of the genome (Nemri et al. 2014). While in some fungi the process of repeat-induced point mutation helps control the expansion of transposable elements, the activity of repeat-induced point mutation in the rust fungi (Pgt and Mlp) appears much lower than in other fungi (Amselem et al. 2015). Mobile elements are now considered to be essential “genome modifiers” that replicate and randomly reinsert to drive recombination, addition, and/or deletion events, sometimes leading to protein neo-functionalizations. Regions of the genome enriched in repetitive elements have also been shown to be a source of genetic diversity, particularly within effector repertoires of pathogens for possible adaptation to their hosts (Haas et al. 2009; Raffaele and Kamoun 2012; Ali et al. 2014).
Similar to two previously sequenced wheat rust pathogen genomes (Duplessis et al. 2011; Zheng et al. 2013), 8% of the identified Pt transcript repertoire encodes potential secreted effectors. The three Puccinia species share a complement of secreted proteins, yet each has a group that is specific to its own species (Figure S5 and Table S4). Although all three are pathogens of wheat, their indigenous worldwide distribution and therefore evolutionary path, environmental (host) adaptation, and life histories are different, as are their symptom formation and alternate host selection; this will have likely translated into a varied complement of CSEPs. Comparison among available rust fungus inventories allowed us to identify a preliminary set of CSEPs specific to the wheat rusts. However, poor annotation of candidate effectors, currently a common challenge in plant pathology, makes deducing biological meaning from specific subsets difficult. Nevertheless, based on Pfam domain searches, specific wheat rust CSEPs were members of GH families (GH15, GH17, and GH88), trehalose-phosphatases, members of the DyP-type peroxidase family, glyoxal oxidase, and proteins with prokumamolisin, thaumatin, and alcohol dehydrogenase-like domains (Figure S5). Intriguingly, 140 of the unidentified proteins were predicted to target the cytoplasm of the host and could be candidates with a role in the interplay with the host immune system.
Gene expression during the key stages of the fungal life cycle is quite different. Many CSEPs were strongly expressed in plant host tissues in comparison to the (germinating) urediniospore stages, suggesting their particular role during infection. Although a large number of highly induced CSEPs could not be functionally annotated, a significant number fall into groups with hydrolase, peptidase, and oxidoreductase activities. In the uredinial, pycnial, and aecial spore stages, many of the genes are associated with sugar, amino acid, and membrane modification, or are amino acid transporters, nucleotide binding proteins, or transcription factors. However, prior to uredinia formation, the fungus induces the protein manufacturing machinery and the most highly expressed genes are associated with ribosomes.
A recent study of mating-type genes in a basal basidiomycete lineage, Leucosporidium scotii, strongly suggested a biallelic pheromone receptor recognition system to be ancestral in the basidiomycetes (Maia et al. 2015), separated into two ancient clades, tentatively called STE3.1 and STE3.2 in Figure 4 (in red). This is generally seen in genomes among the Ustilaginomycotina, Agaricomycotina, and the more recently identified Microbotryomycetes, though variations have become apparent. In the Ustilaginaceae, Sphacelotheca reilianum has three pra (STE3) alleles, one possibly evolved through recombination (Schirawski et al. 2005), whereas in closely related Uh and Um only two are found. In the latter, a pseudo pheromone gene (mfa’ in Figure S1) suggests one specificity might have been lost. A recent study among members of the Ustilaginaceae found three highly syntenic pheromone receptor alleles to be prevalent, which led Kellner et al. (2011) to propose a triallelic recognition system to be ancestral in this family. In the mushrooms, two clades of pheromone receptors are found but, in each, expansion by duplication and mutation is very common leading to several allelic series (Raudaskoski and Kothe 2010). Our analysis to date of members of the genus Puccinia suggests that the biallelic recognition system is indeed ancestral in the basidiomycetes, represented by STE3.2 and STE3.3 (for consistency in this speculative scenario, we called them STE3.2-2 and STE3.2-3; dark blue boxes and red lettering in Figure 4). They each were found to be expressed during the sexual and the wheat infection stages, at approximately equal levels (Figure S10). The close proximity organization of the PtSTE3.2 and Ptmfa2 genes is reminiscent of the P/R organization found in several basidiomycetes (Figure S6), whereas in Pgt the STE3.3 allele is 24 kb away from a potential Pgt mfa gene on supercontig 2.2 (Table S9 and Table S10). In addition, almost no SNPs are identified for each of these two genes per species among a number of resequenced isolates, suggesting a biallelic recognition system. Further duplication and divergence of some of the allelic STE3 genes in certain species may have occurred, such as for MlpSTE3.4 (gray box), similar to mushrooms. The limited synteny, presence of homologous genes at variable spacing, and multiple TEs and repeats, are in agreement with accelerated evolutionary potential in STE3-containing regions (File S2). The well-separated clade containing the Pucciniales STE3.1 homologs (speculatively called STE3.2-1, light blue box and red lettering in Figure 4) could represent an ancient duplication and divergence event, with a possible neo-functionalization. This is supported by the finding of two alleles in each haplotype of Pt race 1, the weak synteny that is apparent among the investigated three cereal rusts (Figure S12, Figure S13 and File S2), and the many SNPs found in this gene among resequenced isolate genomes for all three species.
Mating and compatibility have been very difficult to study in the (cereal) rusts because many are macrocyclic, completing their sexual stage on a different (sometimes obscure or unknown) alternate host plant. Several studies have attempted to shed light on the mating-type system in rust fungi. Conclusions and speculations vary from rust fungi having a simple ± bipolar system in several Puccinia and Uromyces species (Anikster and Eliam 1999) to a more complicated tetrapolar system with multiple allelic specificities in Mli (Lawrence 1980) and the related oat crown rust pathogen, P. coronata (Narisawa et al. 1994). Our genome analysis demonstrates that the proposed simple ± bipolar system in the cereal rust fungi is more complex. The limited number of a locus Pra and mfa alleles in smuts indicates a small repertoire of haploid fusion capabilities in nature (though promiscuity has been observed; Bakkeren and Kronstad 1996); this contrasts with multiple (allelic) arrays often found in mushrooms. Similarly, single bE/bW pairs are found in smuts with very few allelic variants identified in nature for the bipolars but many more (up to 33) for tetrapolar Um. The organization is often more complex in mushrooms where one to multiple HD1–HD2 pairs representing various alleles are found in arrays in many of their analyzed genomes, accounting for the myriad of sexually productive specificities recognized in nature (Fraser et al. 2007; Bakkeren et al. 2008; Kües et al. 2011; Nieuwenhuis et al. 2013; Kües 2015). Closer to the Pucciniomycetes, a bipolar system with limited number of alleles for the HD-pair and Pra-mfa genes has been found in Mi. violaceum (Petit et al. 2012). A “pseudobipolar” system with loose linkage of the HD-pair and pheromone receptor genes, estimated to be 1.2 Mb apart, was described in Sporidiobolus salmonicolor, resulting in the discovery of multiple allelic HD-pairs in nature (Coelho et al. 2010). The Puccinia species genome analyses described here did not indicate close linkage of the STE3/mfa and HD genes. The current assembly and preliminary mapping data in Pt indicate these loci to be at least 216 kb apart (File S2), though a loose linkage has not been ruled out. An inventory of HD alleles among a wide collection of isolates may answer some of these questions.
When introducing one particular Pt bE or bW allele into a wild-type haploid Um strain, filamentous growth is triggered through the production of the respective HD protein, functional interaction with the Ustilago counterpart, and subsequent transcriptional activation of a subset of genes by the formed bispecies HD dimer, as shown for Um (Wahl et al. 2010). While we have shown such active interactions to occur between b alleles from different species within the smuts (Bakkeren and Kronstad 1993), such activity across quite diverged members of the basidiomycetes is astounding and suggests an ancient origin of these proteins. However, the experiment introducing PtbW1 and PtbE2 alleles, each in a compatible Uh strain lacking b genes, did not trigger a switch to hyphal growth upon mating. Although fusion of mating hyphae was confirmed, this suggests that no productive interaction within the dikaryotic heterologous cell occurred. Given that we found only two allelic pairs of bE and bW in these Puccinia species and the overwhelming evidence of the productive interaction between such heterodimers in many very diverse basidiomycetes studied to date, it is unlikely that the PtbE and PtbW HD proteins would not interact. Failure to initiate filamentous growth in Uh then may indicate that the Pt HD proteins lack domains or the specificity necessary for Ustilago-specific downstream interactions, nuclear import, and/or for binding to Ustilago promoter elements that normally initiate the transcription of genes involved in the switch to filamentous growth (Scherer et al. 2006; Kahmann and Schirawski 2007); when Pt-Uh HD-heterodimers are formed, such functionality may be provided by the Uh component (Figure 7). Indeed, the predicted PtbE proteins are, at 374 amino acids, ∼100 residues shorter than the Ustilago homologs. The compositions of the helices that constitute the HD are relatively well-conserved between the Pt and Ustilago b-proteins; however, their location within the protein is significantly different and may have evolved Puccinia lineage-specific adaptations.
The HIGS experiments demonstrated that some of the Pt mating-type genes were additionally functional in dikaryotic hyphae during wheat infection (Figure 8), as well as the assumed activity during the sexual stage on Thalictrum spp. The involvement of the a mating-type genes in pathogenicity of the dikaryotic cell type has been demonstrated in Um (Hartmann et al. 1996; Urban et al. 1996). Silencing of PtSTE3.1 had less of an effect than of PtSTE3.3 and was correlated with the observed expression levels during wheat infection (Figure S10). Differing expression levels for specific alleles at different life cycle stages could indicate functional diversification and possibly a loss in function in determining MAT-specificity, as seen in many mushrooms. The sequences in the HD silencing constructs were designed such that they would silence both alleles. This was clearly detrimental to the infection process, and therefore shows that they are important for pathogenicity. They could play a role in the maintenance of the dikaryotic stage and/or induction or persistence of pathogenicity gene expression, such as demonstrated for Um where the bE/bW heterodimer was shown to be essential for initiating the induction of a set of genes involved in the pathogenic life style (Brachmann et al. 2001; Wahl et al. 2010).
Wheat rust diseases are a major impediment to economic production of wheat in many areas in the world, and because of their rapid adaptation to newly introduced resistant cultivars and fungicides, they are a threat to envisaged increased yield for a growing population. Genome research on these elusive biotrophic pathogens has tremendously accelerated our understanding of their interaction with their host, and the presentation of a Pt and another Pst genome and the comparative analysis to other rust fungi in this study has highlighted similarities and differences that can now be exploited for targeted crop protection strategies. Conserved and essential effectors, expressed during infection, and their intended host targets, would be interesting components for further study; a search for natural or engineered resistance genes recognizing such effectors could be effective.
We thank the Broad Genomics Platform for generating DNA and RNA sequences and the Michael Smith Genome Sciences Centre in Vancouver for sequencing the BAC ends. C.A.C thanks G. Cerqueira for sharing the syntenia code. G.B. thanks M. Coelho for helpful discussions and acknowledges funding from the Canadian Genomics Research and Development Initiative. This project was supported by the United States Department of Agriculture (USDA) Cooperative State Research, Education, and Extension Service (awards 2008-35600-04693 and 2009-65109-05916). Mention of a trademark of a proprietary product does not constitute a guarantee of warranty of the product by the USDA, and does not imply its approval to the exclusion of other products that may also be sui. USDA is an equal opportunity provider and employer.
Author contributions: C.A.C., G.B., L.J.S., S.H., X.C., and J.P.F. designed the study and experiments. G.B., X.S., X.A., L.F., J.Z.L., Y.A., M.B., M.W., C.Y., B.M., L.J.S., and J.P.F. undertook sample collection and preparation, and the generation of constructs. G.B. and V.P. performed experiments. J.M.G., S.Y., and Q.Z. performed assembly and annotation. C.A.C., G.B., H.B.K., D.J., R.L., S.S., B.M., and J.P.F. analyzed the data. C.A.C., G.B., H.B.K., and J.P.F. wrote the paper.
Supplemental material is available online at www.g3journal.org/lookup/suppl/doi:10.1534/g3.116.032797/-/DC1.
Communicating editor: M. S. Sachs
- Received June 24, 2016.
- Accepted October 24, 2016.
- Copyright © 2017 Cuomo et al.
This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.