Genetic analysis and molecular mapping of the avirulence gene PRE1, a gene for host-species specificity in the blast fungus Magnaporthe grisea

Resolving the polyphyletic nature of Pyricularia (Pyriculariaceae)

Species of Pyricularia (magnaporthe-like sexual morphs) are responsible for major diseases on grasses. Pyricularia oryzae (sexual morph Magnaporthe oryzae) is responsible for the major disease of rice called rice blast disease, and foliar diseases of wheat and millet, while Pyricularia grisea (sexual morph Magnaporthe grisea) is responsible for foliar diseases of Digitaria. Magnaporthe salvinii, M. poae and M. rhizophila produce asexual spores that differ from those of Pyricularia sensu stricto that has pyriform, 2-septate conidia produced on conidiophores with sympodial proliferation. Magnaporthe salvinii was recently allocated to Nakataea, while M. poae and M. rhizophila were placed in Magnaporthiopsis. To clarify the taxonomic relationships among species that are magnaporthe- or pyricularia-like in morphology, we analysed phylogenetic relationships among isolates representing a wide range of host plants by using partial DNA sequences of multiple genes such as LSU, ITS, RPB1, actin and calmodulin. Species of Pyricularia s. str. belong to a monophyletic clade that includes all P. oryzae/P. grisea isolates tested, defining the Pyriculariaceae, which is sister to the Ophioceraceae, representing two novel families. These clades are clearly distinct from species belonging to the Gaeumannomyces pro parte/Magnaporthiopsis/Nakataea generic complex that are monophyletic and define the Magnaporthaceae. A few magnaporthe- and pyricularia-like species are unrelated to Magnaporthaceae and Pyriculariaceae. Pyricularia oryzae/P. grisea isolates cluster into two related clades. Host plants such as Eleusine, Oryza, Setaria or Triticum were exclusively infected by isolates from P. oryzae, while some host plant such as Cenchrus, Echinochloa, Lolium, Pennisetum or Zingiber were infected by different Pyricularia species. This demonstrates that host range cannot be used as taxonomic criterion without extensive pathotyping. Our results also show that the typical pyriform, 2-septate conidium morphology of P. grisea/P. oryzae is restricted to Pyricularia and Neopyricularia, while most other genera have obclavate to more ellipsoid 2-septate conidia. Some related genera (Deightoniella, Macgarvieomyces) have evolved 1-septate conidia. Therefore, conidium morphology cannot be used as taxonomic criterion at generic level without phylogenetic data. We also identified 10 novel genera, and seven novel species. A re-evaluation of generic and species concepts within Pyriculariaceae is presented, and novelties are proposed based on morphological and phylogenetic data.

Comparative analysis of pathogenicity and phylogenetic relationship in Magnaporthe grisea species complex

Outbreaks of rice blast have been a threat to the global production of rice. Members of the Magnaporthe grisea species complex cause blast disease on a wide range of gramineous hosts, including cultivated rice and other grass species. Recently, based on phylogenetic analyses and mating tests, isolates from crabgrass were separated from the species complex and named M. grisea. Then other isolates from grasses including rice were named as M. oryzae. Here, we collected 103 isolates from 11 different species of grasses in Korea and analyzed their phylogenetic relationships and pathogenicity. Phylogenetic analyses of multilocus sequences and DNA fingerprinting revealed that the haplotypes of most isolates were associated with their hosts. However, six isolates had different haplotypes from the expectation, suggesting potential host shift in nature. Results of pathogenicity tests demonstrated that 42 isolates from crabgrass and 19 isolates from rice and other grasses showed cross-infectivity on rice and crabgrass, respectively. Interestingly, we also found that the isolates from rice had a distinct deletion in the calmodulin that can be used as a probe.

Identification of genes for resistance to a Digitaria isolate of Magnaporthe grisea in common wheat cultivars

Common wheat cultivars are resistant to Magnaporthe grisea, a crabgrass (Digitaria sanguinalis)-specific species of the blast fungus. To dissect the genetic basis of this "nonhost" type of resistance, we need an exceptional cultivar that is susceptible to M. grisea. A screening under various conditions revealed that Triticum aestivum 'Chinese Spring' (CS) was susceptible to M. grisea isolate Dig41 when incubated at high temperature (26 degrees C) after inoculation. By contrast, T. aestivum 'P168', 'Shin-chunaga' (Sch), 'Norin 4' (N4), 'Norin 26' (N26), 'Norin 29' (N29), 'Red Egyptian' (RE), and 'Salmon' (Slm) and Triticum compactum 'No. 44' (Cmp) were highly resistant even at the high temperature. When F2 seedlings derived from crosses between the resistant cultivars and CS were inoculated with Dig41, they segregated in a 3:1 ratio of resistant to susceptible, suggesting that the resistance of each cultivar is controlled by one major gene. Crosses of N4 with P168, Sch, N26, N29, and Cmp yielded no susceptible F2 seedlings, suggesting that these six cultivars share the same gene. Similarly, a cross between RE and Slm yielded no susceptible F2 seedlings, suggesting that these two cultivars share the same gene. On the other hand, crosses between the N4 group and the RE group produced resistant and susceptible seedlings in a 15:1 ratio, indicating that these two groups carry different genes inherited independently. The gene in N4 was located on chromosome 4A by a monosomic analysis and designated Rmg4, while the gene in RE was located on chromosome 6D using a series of chromosome substitution lines and designated Rmg5. These results suggest that the resistance of common wheat to M. grisea, an inappropriate species of the blast fungus, is under a simple genetic control.

Identification of OsbZIP72 as a positive regulator of ABA response and drought tolerance in rice

Abscisic Acid (ABA) is an important phytohormone involved in abiotic stress resistance in plants. A group of bZIP transcription factors play important roles in the ABA signaling pathway in Arabidopsis. However, little is known about the function of their orthologs in rice, where they may hold a great potential for developing drought resistant food crops. In this study, our phylogenetic analysis showed that this group of bZIPs was evolutionarily conserved between Arabidopsis and rice, which implies that they may share similar functions. We demonstrated with quantitative RT-PCR that the expressions of most of these OsbZIPs were significantly induced by ABA, ACC, and abiotic stresses. OsbZIP72, a member of this group, was proved to be an ABRE binding factor in rice using the yeast hybrid systems. We showed that it could bind to ABRE and transactivate the downstream reporter genes in yeast, and the transactivity was depending on its N-terminal region. Transgenic rice overexpressing OsbZIP72 showed a hypersensitivity to ABA, elevated levels of expression of ABA response gene such as LEAs, and an enhanced ability of drought tolerance. These results suggest that OsbZIP72 plays a positive role in drought resistance through ABA signaling, and is potential useful for engineering drought tolerant rice.

Telomeres in the rice blast fungus Magnaporthe oryzae: the world of the end as we know it

The subtelomeres of many microbial eukaryotes are highly enriched in genes with roles in niche adaptation. Host and cultivar specificity genes in the rice blast fungus Magnaporthe oryzae also tend to be located near telomeres. In addition, the M. oryzae telomeres are highly variable chromosome regions. These observations suggested that plant pathogenic fungi might also use subtelomere regions for the amplification of genes with adaptive significance. Targeted sequencing of the M. oryzae telomeres provided an opportunity to test this hypothesis, and has yielded valuable insights into the organization and dynamics of these important chromosome regions.