Male fertility in Pyricularia oryzae: Microconidia are spermatia

Pyricularia oryzae: Lab star and field scourge

Pyricularia oryzae (syn. Magnaporthe oryzae), is a filamentous ascomycete that causes a major disease called blast on cereal crops, as well as on a wide variety of wild and cultivated grasses. Blast diseases have a tremendous impact worldwide particularly on rice and on wheat, where the disease emerged in South America in the 1980s, before spreading to Asia and Africa. Its economic importance, coupled with its amenability to molecular and genetic manipulation, have inspired extensive research efforts aiming at understanding its biology and evolution. In the past 40 years, this plant-pathogenic fungus has emerged as a major model in molecular plant-microbe interactions. In this review, we focus on the clarification of the taxonomy and genetic structure of the species and its host range determinants. We also discuss recent molecular studies deciphering its lifecycle.

TAXONOMY

Kingdom: Fungi, phylum: Ascomycota, sub-phylum: Pezizomycotina, class: Sordariomycetes, order: Magnaporthales, family: Pyriculariaceae, genus: Pyricularia.

HOST RANGE

P. oryzae has the ability to infect a wide range of Poaceae. It is structured into different host-specialized lineages that are each associated with a few host plant genera. The fungus is best known to cause tremendous damage to rice crops, but it can also attack other economically important crops such as wheat, maize, barley, and finger millet.

DISEASE SYMPTOMS

P. oryzae can cause necrotic lesions or bleaching on all aerial parts of its host plants, including leaf blades, sheaths, and inflorescences (panicles, spikes, and seeds). Characteristic symptoms on leaves are diamond-shaped silver lesions that often have a brown margin and whose appearance is influenced by numerous factors such as the plant genotype and environmental conditions. USEFUL WEBSITES Resources URL Genomic data repositories http://genome.jouy.inra.fr/gemo/ Genomic data repositories http://openriceblast.org/ Genomic data repositories http://openwheatblast.net/ Genome browser for fungi (including P. oryzae) http://fungi.ensembl.org/index.html Comparative genomics database https://mycocosm.jgi.doe.gov/mycocosm/home T-DNA mutant database http://atmt.snu.kr/ T-DNA mutant database http://www.phi-base.org/ SNP and expression data https://fungidb.org/fungidb/app/.

The MAT1 locus is required for microconidia-mediated sexual fertility in the rice blast fungus

Rice blast fungus (Pyricularia oryzae) is a heterothallic ascomycete that causes the most destructive disease in cultivated rice worldwide. This fungus reproduces sexually and asexually, and its mating type is determined by the MAT1 locus, MAT1-1 or MAT1-2. Interestingly, most rice-infecting field isolates show a loss of female fertility, but the MAT1 locus is highly conserved in female-sterile isolates. In this study, we performed a functional analysis of MAT1 using the CRISPR/Cas9 system in female- and male-fertile isolates and female-sterile (male-fertile) isolates. Consistent with a previous report, MAT1 was essential for sexual reproduction but not for asexual reproduction. Meanwhile, deletion mutants of MAT1-1-1, MAT1-1-2, and MAT1-1-3 exhibited phenotypes different from those of other previously described isolates, suggesting that the function of MAT1-1 genes and/or their target genes in sexual reproduction differs among strains or isolates. The MAT1 genes, excluding MAT1-2-6, retained their functions even in female-sterile isolates, and deletion mutants lead to loss or reduction of male fertility. Although MAT1 deletion did not affect microconidia (spermatia) production, microconidia derived from the mutants could not induce perithecia formation. These results indicated that MAT1 is required for microconidia-mediated male fertility in addition to female fertility in P. oryzae .

Dysfunctional Pro1 leads to female sterility in rice blast fungi

Although sexual reproduction is widespread in eukaryotes, some fungal species can only reproduce asexually. In the rice blast fungus Pyricularia (Magnaporthe) oryzae, several isolates from the region of origin retain mating ability, but most isolates are female sterile. Therefore, female fertility may have been lost during its spread from the origin. Here, we show that functional mutations of Pro1, a global transcriptional regulator of mating-related genes in filamentous fungi, is one cause of loss of female fertility in this fungus. We identified the mutation of Pro1 by backcrossing analysis between female-fertile and female-sterile isolates. The dysfunctional Pro1 did not affect the infection processes but conidial release was increased. Furthermore, various mutations in Pro1 were detected in geographically distant P. oryzae, including pandemic isolates of wheat blast fungus. These results provide the first evidence that loss of female fertility may be advantageous to the life cycle of some plant pathogenic fungi.

Multiple scenarios for sexual crosses in the fungal pathogen Zymoseptoria tritici on wheat residues: Potential consequences for virulence gene transmission

Little is known about the impact of host immunity on sexual reproduction in fungal pathogens. In particular, it is unclear whether crossing requires both sexual partners to infect living plant tissues. We addressed this issue in a three-year experiment investigating different scenarios of Zymoseptoria tritici crosses according to the virulence ('vir') or avirulence ('avr') of the parents against a qualitative resistance gene. Co-inoculations ('vir × vir', 'avr × vir', 'avr × avr') and single inoculations were performed on a wheat cultivar carrying the Stb16q resistance gene (Cellule) and a susceptible cultivar (Apache), in the greenhouse. We assessed the intensity of asexual reproduction by scoring disease severity, and the intensity of sexual reproduction by counting the ascospores discharged from wheat residues. As expected, disease severity was more intense on Cellule for 'vir × vir' co-inoculations than for 'avr × vir' co-inoculations, with no disease for 'avr × avr'. However, all types of co-inoculation yielded sexual offspring, whether or not the parental strains caused plant symptoms. Parenthood was confirmed by genotyping (SSR markers), and the occurrence of crosses between (co-)inoculated and exogenous strains (other strains from the experiment, or from far away) was determined. We showed that symptomatic asexual infection was not required for a strain to participate in sexual reproduction, and deduced from this result that avirulent strains could be maintained asymptomatically "on" or "in" leaf tissues of plants carrying the corresponding resistant gene for long enough to reproduce sexually. In two of the three years, the intensity of sexual reproduction did not differ between the three types of co-inoculation in Cellule, suggesting that crosses involving avirulent strains are not anecdotal. We discuss the possible mechanisms explaining the maintenance of avirulence in Z. tritici populations and the potential impact of particular resistance deployments such as cultivar mixtures for limiting resistance breakdown.