MOLECULAR ANALYSIS OF POLYMORPHISMS IN DIFFERENTIATING Phytophthora infestans RACES

Potato late blight caused by oomycete Phytophthora infestans (Mont.) de Bary is economically significant disease of worldwide importance. The traditional classification of specialized races of P. infestans is based on eleven resistance genes (R genes) introgressed from Solanum demissum to cultivated potato S. tuberosum. The selection of potato varieties, each comprising one of these R genes, is referred to as the Mastenbroek-Black set of differential plants. This set has been employed to establish the virulence genes (r genes) in isolates and strains of P. infestans, and collections of such individual strains have been maintained as tool sets of differential races for discerning R genes in cultivated and wild Solanum plants. While widely used in potato breeding for late blight resistance, these differential races have not been sufficiently explored by present-day molecular methods. We studied 11 differential races (1; 3; 4; 10; 11; 1.2; 1.3; 1.4; 1.2.3; 1.2.4; 1.2.3.4) maintained in the Institute of Phytopathology for over forty years and isolate 161 possessing all 11 genes of virulence. When the differential races of P. infestans were genotyped with the standard set of 12 microsatellite (simple sequence repeat, SSR) loci, these races were distinct from reference A1 strains and highly aggressive lines lately dominant in the Western and Central Europe. SSR clusters of differential races did not match their r gene profiles. To assess the profiles of virulence genes in the differential races of P. infestans, we cloned three avirulence genes (Avr genes): ipiO = Avr-blb1, which recognizes the Rpi-blb1 = Rpi-sto1 gene of S. bulbocastanum and S. stoloniferum characterized by broad resistance to P. infestans races, and also Avr3a and Avr4 corresponding to R3a and R4 of S. demissum. These Avr genes were found in all differential races under study, and each gene was represented by several alleles. The complex patterns of Avr genes are in sharp contrast with the conventional concept of «simple» monogenic races. In the case of ipiO (NCBI GenBank accession numbers KP308170-KP308174, KF154431-KF154433 and KF154434-KF154439) race 1 was represented by the alleles of classes I and II, whereas races 3 and 4 comprised only the class I genes. None of three races contained the most virulent class III ipiO gene. The virulent allele Avr3a_EM was found in all investigated races, while the avirulent allele Avr3a_KI was discerned only in races 1, 3 and 1.2.3 (KF154421- KF154426, KF154430, KP317568, KP317572, KP317580-KP317584, KP317588, KP317589). The Avr4 gene was cloned from differential races 1, 3, 1.4 and 11 (KF188215-KF188223). All these races contained the virulent allele, and race 11 comprised both avirulent and virulent alleles. The molecular and phytopathological evidence for Avr and r genes, respectively, matched only in 30 % of races. Probably, these discrepancies are due to the accumulation of mutations in the Avr genes of differential races in the course of their long-term maintenance in the collection and more complex composition of R genes in plants which were initially to select the differentiating races.

• Kuznetsova M.A.
• Alekseev Ya.I.
• Khavkin E.E.
• Malychenko O.P.
• Morozova E.V.
• Sokolova E.A.
• Ulanova T.I.

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