Identification of an Avirulence Gene in the Fungus Magnaporthe grisea Corresponding to a Resistance Gene at the Pik Locus

Effects of Pyramiding Quantitative Resistance Genes pi21, Pi34, and Pi35 on Rice Leaf Blast Disease

Development of resistant cultivars has been an effective method for controlling rice blast disease caused by Magnaporthe oryzae. Quantitative blast resistance genes may offer durable resistance because the selection pressure on M. oryzae to overcome resistance is low as a result of the genes' moderate susceptibility. Because the effects of individual resistance genes are relatively small, pyramiding these genes in rice cultivars is a promising strategy. Here, we used near-isogenic and backcross lines of rice cultivar Koshihikari with single- or two-gene combinations of blast resistance genes (pi21, Pi34, and Pi35) to evaluate the suppression of leaf blast. The severity of the disease was assessed throughout the infection process. Resistance varied among the lines: Pi35 conferred the strongest resistance, while Pi34 showed the weakest effects. Two types of combined-gene interactions were observed, and they varied on the basis of gene combination and characteristic of the infection: (i) the combination of two resistance genes was more effective than either of the genes individually or (ii) the combination of two resistance genes was similar to the level of the most effective resistance gene in the pair. The most effective gene combination for the suppression of leaf blast was pi21 + Pi35.

Distribution of Mating Type and Sexual Status in Chinese Rice Blast Populations

A collection of 520 field isolates of the rice blast fungus (Magnaporthe oryzae) originating from five provinces in China was assessed for mating type and sexual fertility. One of the two tester sets was composed of isolates collected from barley and the other from rice. Two mating types (MAT1-1 and MAT1-2) were identified among the 443 fertile isolates. The two mating types were roughly in balance with one another in the southwestern region but one or the other predominated in the southeastern and southern regions. Male-only fertile isolates were the most common, and only a few hermaphroditic and no female only fertile isolates were detected. The fertility level of the isolates was variable. Isolates from Jiangsu were more fertile than those from Fujian. The mating capacity of the testers collected from barley was higher than that of those collected from rice, but this was because the MAT1-2 testers differed very significantly from one another. In contrast, the mating capacities of the two MAT1-1 testers were similar to one another.

Induced Rice Resistance to Blast Varies as a Function of Magnaporthe grisea Avirulence Genes

Incompatibility reactions between rice and the blast fungus Magnaporthe grisea produce various degrees of lesions, from large brown flecks to small, nearly invisible lesions. We previously identified four avirulence genes (AvrPia, AvrPii, AvrPit, and Avr-Hattan3) in M. grisea isolates by genetic analysis of progeny from crosses between isolates with differing pathogenicity. Using progeny known to contain a specific avirulence gene, we demonstrated that the type of resistance lesion produced in rice by an avirulent isolate and the degree of leaf blast suppression by preinoculation with that isolate were determined by the combination of avirulence and resistance genes in the isolate and the cultivar. The degree of leaf blast suppression by preinoculation with an avirulent isolate increased with larger resistance lesions. When two genes were involved in an isolate's avirulence, lesions appeared that resembled those expected for the gene that produced the smaller lesion. The degree of leaf blast suppression by the isolate with two avirulence genes was comparable with that induced by the isolate with the avirulence gene that produced the smaller effect. The ability of specific resistance gene combinations that effectively suppress blast disease is discussed for each avirulence gene.

Genome organization and evolution of the AVR-Pita avirulence gene family in the Magnaporthe grisea species complex

The avirulence (AVR) gene AVR-Pita in Magnaporthe oryzae prevents the fungus from infecting rice cultivars containing the resistance gene Pi-ta. A survey of isolates of the M. grisea species complex from diverse hosts showed that AVR-Pita is a member of a gene family, which led us to rename it to AVR-Pita1. Avirulence function, distribution, and genomic context of two other members, named AVR-Pita2 and AVR-Pita3, were characterized. AVR-Pita2, but not AVR-Pita3, was functional as an AVR gene corresponding to Pi-ta. The AVR-Pita1 and AVR-Pita2 genes were present in isolates of both M. oryzae and M. grisea, whereas the AVR-Pita3 gene was present only in isolates of M. oryzae. Orthologues of members of the AVR-Pita family could not be found in any fungal species sequenced to date, suggesting that the gene family may be unique to the M. grisea species complex. The genomic context of its members was analyzed in eight strains. The AVR-Pita1 and AVR-Pita2 genes in some isolates appeared to be located near telomeres and flanked by diverse repetitive DNA elements, suggesting that frequent deletion or amplification of these genes within the M. grisea species complex might have resulted from recombination mediated by repetitive DNA elements.

Genetic mapping of Pyrenophora teres f. teres genes conferring avirulence on barley

A Pyrenophora teres f. teres cross between isolates 0-1 and 15A was used to evaluate the genetics of avirulence associated with barley lines Canadian Lake Shore (CLS), Tifang, and Prato. 15A is avirulent on Tifang and CLS, but virulent on Prato. Conversely, 0-1 is avirulent on Prato, but virulent on Tifang and CLS. Avirulence:virulence on Tifang and CLS segregated 1:1, whereas avirulence:virulence on Prato segregated 3:1. An AFLP-based linkage map was constructed and used to identify a single locus derived from 15A (AvrHar) conferring avirulence to Tifang and CLS. Virulence on Prato was conferred by two epistatic genes (AvrPra1 and AvrPra2). AvrPra2 co-segregated with AvrHar, but the two genes from opposite parents conferred opposite reactions. This work provides the foundation for the isolation of these avirulence genes.

Evidence of Genetic Exchange by Parasexual Recombination and Genetic Analysis of Pathogenicity and Mating Type of Parasexual Recombinants in Rice Blast Fungus, Magnaporthe oryzae

ABSTRACT A selectable marker gene conferring resistance to bialaphos (BI) was introduced into rice blast isolate Y90-71BI and another conferring resistance to blasticidin S (BS) into isolate 3514-R-2BS of Magnaporthe oryzae to demonstrate exchange of DNA. Colonies obtained from co-cultures of these two isolates were resistant to both BI and BS and had both resistance genes as shown by Southern blot analysis of their genomic DNA. Conidia from these BI-BS-resistant isolates had only one nucleus per cell after staining with 4',6-diamidino-2-phenylindole (DAPI). Using flow cytometry, however, these BI-BS-resistant isolates were found to be haploid. Segregation of BI-BS-resistant isolates for pathogenicity (avirulence to virulence) on rice line K59-1 was consistent with a 1:1 ratio, as was segregation for mating type. These BI-BS-resistant isolates were thus apparently derived from parasexual exchange of DNA and the segregation of pathogenicity and of mating type of the parasexual recombinants might correspond to that of the progeny of the offspring of the sexual cross.