A chromosome-level, fully phased genome assembly of the oat crown rust fungus Puccinia coronata f. sp. avenae: a resource to enable comparative genomics in the cereal rusts

Genome biology and evolution of mating-type loci in four cereal rust fungi

Permanent heterozygous loci, such as sex- or mating-compatibility regions, often display suppression of recombination and signals of genomic degeneration. In Basidiomycota, two distinct loci confer mating compatibility. These loci encode homeodomain (HD) transcription factors and pheromone receptor (Pra)-ligand allele pairs. To date, an analysis of genome level mating-type (MAT) loci is lacking for obligate biotrophic basidiomycetes in the Pucciniales, an order containing serious agricultural plant pathogens. Here, we focus on four species of Puccinia that infect oat and wheat, including P. coronata f. sp. avenae, P. graminis f. sp. tritici, P. triticina and P. striiformis f. sp. tritici. MAT loci are located on two separate chromosomes supporting previous hypotheses of a tetrapolar mating compatibility system in the Pucciniales. The HD genes are multiallelic in all four species while the PR locus appears biallelic, except for P. graminis f. sp. tritici, which potentially has multiple alleles. HD loci are largely conserved in their macrosynteny, both within and between species, without strong signals of recombination suppression. Regions proximal to the PR locus, however, displayed signs of recombination suppression and genomic degeneration in the three species with a biallelic PR locus. Our observations support a link between recombination suppression, genomic degeneration, and allele diversity of MAT loci that is consistent with recent mathematical modelling and simulations. Finally, we confirm that MAT genes are expressed during the asexual infection cycle, and we propose that this may support regulating nuclear maintenance and pairing during infection and spore formation. Our study provides insights into the evolution of MAT loci of key pathogenic Puccinia species. Understanding mating compatibility can help predict possible combinations of nuclear pairs, generated by sexual reproduction or somatic recombination, and the potential evolution of new virulent isolates of these important plant pathogens.

Genome-Enabled Analysis of Population Dynamics and Virulence-Associated Loci in the Oat Crown Rust Fungus Puccinia coronata f. sp. avenae

Puccinia coronata f. sp. avenae (Pca) is an important fungal pathogen causing crown rust that impacts oat production worldwide. Genetic resistance for crop protection against Pca is often overcome by the rapid virulence evolution of the pathogen. This study investigated the factors shaping adaptive evolution of Pca using pathogen populations from distinct geographic regions within the United States and South Africa. Phenotypic and genome-wide sequencing data of these diverse Pca collections, including 217 isolates, uncovered phylogenetic relationships and established distinct genetic composition between populations from northern and southern regions from the United States and South Africa. The population dynamics of Pca involve a bidirectional movement of inoculum between northern and southern regions of the United States and contributions from clonality and sexuality. The population from South Africa is solely clonal. A genome-wide association study (GWAS) employing a haplotype-resolved Pca reference genome was used to define 11 virulence-associated loci corresponding to 25 oat differential lines. These regions were screened to determine candidate Avr effector genes. Overall, the GWAS results allowed us to identify the underlying genetic factors controlling pathogen recognition in an oat differential set used in the United States to assign pathogen races (pathotypes). Key GWAS findings support complex genetic interactions in several oat lines, suggesting allelism among resistance genes or redundancy of genes included in the differential set, multiple resistance genes recognizing genetically linked Avr effector genes, or potentially epistatic relationships. A careful evaluation of the composition of the oat differential set accompanied by the development or implementation of molecular markers is recommended. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Nuclear exchange generates population diversity in the wheat leaf rust pathogen Puccinia triticina

In clonally reproducing dikaryotic rust fungi, non-sexual processes such as somatic nuclear exchange are postulated to play a role in diversity but have been difficult to detect due to the lack of genome resolution between the two haploid nuclei. We examined three nuclear-phased genome assemblies of Puccinia triticina, which causes wheat leaf rust disease. We found that the most recently emerged Australian lineage was derived by nuclear exchange between two pre-existing lineages, which originated in Europe and North America. Haplotype-specific phylogenetic analysis reveals that repeated somatic exchange events have shuffled haploid nuclei between long-term clonal lineages, leading to a global P. triticina population representing different combinations of a limited number of haploid genomes. Thus, nuclear exchange seems to be the predominant mechanism generating diversity and the emergence of new strains in this otherwise clonal pathogen. Such genomics-accelerated surveillance of pathogen evolution paves the way for more accurate global disease monitoring.

Plant and pathogen genomics: essential approaches for stem rust resistance gene stacks in wheat

The deployment of disease resistance genes is currently the most economical and environmentally sustainable method of crop protection. However, disease resistance genes can rapidly break down because of constant pathogen evolution, particularly when they are deployed singularly. Polygenic resistance is, therefore, considered the most durable, but combining and maintaining these genes by breeding is a laborious process as effective genes are usually unlinked. The deployment of polygenic resistance with single-locus inheritance is a promising innovation that overcomes these difficulties while enhancing resistance durability. Because of major advances in genomic technologies, increasing numbers of plant resistance genes have been cloned, enabling the development of resistance transgene stacks (RTGSs) that encode multiple genes all located at a single genetic locus. Gene stacks encoding five stem rust resistance genes have now been developed in transgenic wheat and offer both breeding simplicity and potential resistance durability. The development of similar genomic resources in phytopathogens has advanced effector gene isolation and, in some instances, enabled functional validation of individual resistance genes in RTGS. Here, the wheat stem rust pathosystem is used as an illustrative example of how host and pathogen genomic advances have been instrumental in the development of RTGS, which is a strategy applicable to many other agricultural crop species.

Major proliferation of transposable elements shaped the genome of the soybean rust pathogen Phakopsora pachyrhizi

With >7000 species the order of rust fungi has a disproportionately large impact on agriculture, horticulture, forestry and foreign ecosystems. The infectious spores are typically dikaryotic, a feature unique to fungi in which two haploid nuclei reside in the same cell. A key example is Phakopsora pachyrhizi, the causal agent of Asian soybean rust disease, one of the world's most economically damaging agricultural diseases. Despite P. pachyrhizi's impact, the exceptional size and complexity of its genome prevented generation of an accurate genome assembly. Here, we sequence three independent P. pachyrhizi genomes and uncover a genome up to 1.25 Gb comprising two haplotypes with a transposable element (TE) content of ~93%. We study the incursion and dominant impact of these TEs on the genome and show how they have a key impact on various processes such as host range adaptation, stress responses and genetic plasticity.

Gapless Genome Assembly of Puccinia triticina Provides Insights into Chromosome Evolution in Pucciniales

Chromosome evolution drives species evolution, speciation, and adaptive radiation. Accurate genome assembly is crucial to understanding chromosome evolution of species, such as dikaryotic fungi. Rust fungi (Pucciniales) in dikaryons represent the largest group of plant pathogens, but the evolutionary process of adaptive radiation in Pucciniales remains poorly understood. Here, we report a gapless genome for the wheat leaf rust fungus Puccinia triticina determined using PacBio high-fidelity (HiFi) sequencing. This gapless assembly contains two sets of chromosomes, showing that one contig represents one chromosome. Comparisons of homologous chromosomes between the phased haplotypes revealed that highly frequent small-scale sequence divergence shapes haplotypic variation. Genome analyses of Puccinia triticina along with other rusts revealed that recent transposable element bursts and extensive segmental gene duplications synergistically highlight the evolution of chromosome structures. Comparative analysis of chromosomes indicated that frequent chromosomal rearrangements may act as a major contributor to rapid radiation of Pucciniales. This study presents the first gapless, phased assembly for a dikaryotic rust fungus and provides insights into adaptive evolution and species radiation in Pucciniales. IMPORTANCE Rust fungi (Pucciniales) are the largest group of plant pathogens. Adaptive radiation is a predominant feature in Pucciniales evolution. Chromosome evolution plays an important role in adaptive evolution. Accurate chromosome-scale assembly is required to understand the role of chromosome evolution in Pucciniales. We took advantage of HiFi sequencing to construct a gapless, phased genome for Puccinia triticina. Further analyses revealed that the evolution of chromosome structures in rust lineage is shaped by the combination of transposable element bursts and segmental gene duplications. Chromosome comparisons of Puccinia triticina and other rusts suggested that frequent chromosomal arrangements may make remarkable contributions to high species diversity of rust fungi. Our results present the first gapless genome for Pucciniales and shed light on the feature of chromosome evolution in Pucciniales.

A decade after the first Pucciniales genomes: A bibliometric snapshot of (post) genomics studies in three model rust fungi

Pucciniales (rust fungi) are one of the largest fungal order of plant pathogens. They collectively infect key crops such as wheat and soybean, and threaten global food security. In the early 2010s, the genome sequences of three rust fungi were released: Melampsora larici-populina (the poplar leaf rust fungus), Puccinia graminis f. sp. tritici (the wheat stem rust fungus), and Puccinia striiformis f. sp. triciti (the wheat stripe rust or wheat yellow rust fungus). The availability of those genomes has forwarded rust biology into the post-genomic era, sparking a series of genomics, transcriptomics, in silico, and functional studies. Here, we snapshot the last 10 years of post-genomics studies addressing M. larici-populina, P. graminis f. sp. tritici, and/or P. striiformis f. sp. tritici. This mini-review notably reveals the model species-centered structure of the research community, and highlights the drastic increase of the number of functional studies focused on effectors since 2014, which notably revealed chloroplasts as a central host compartment targeted by rust fungi. This mini-review also discusses genomics-facilitated studies in other rust species, and emerging post-genomic research trends related to fully-phased rust genomes.

Current Status and Future Perspectives of Genomics Research in the Rust Fungi

Rust fungi in Pucciniales have caused destructive plant epidemics, have become more aggressive with new virulence, rapidly adapt to new environments, and continually threaten global agriculture. With the rapid advancement of genome sequencing technologies and data analysis tools, genomics research on many of the devastating rust fungi has generated unprecedented insights into various aspects of rust biology. In this review, we first present a summary of the main findings in the genomics of rust fungi related to variations in genome size and gene composition between and within species. Then we show how the genomics of rust fungi has promoted our understanding of the pathogen virulence and population dynamics. Even with great progress, many questions still need to be answered. Therefore, we introduce important perspectives with emphasis on the genome evolution and host adaptation of rust fungi. We believe that the comparative genomics and population genomics of rust fungi will provide a further understanding of the rapid evolution of virulence and will contribute to monitoring the population dynamics for disease management.