Polymorphism of the RXLR effector genes Avr3a, Avr4 and ipiO in Phytophthora infestans isolates from China

Late blight caused by Phytophthora infestans is one of the most important diseases of potatoes worldwide. Growing usage of resistant cultivars is a major late blight control measurement. However, late blight resistance is frequently overcome by adaptation of the pathogen. Study of the interaction between Phytophthora spp. and its hosts revealed the importance of RXLR effectors in plant infection and colonization. RXLR effectors can act as avirulent factors when recognized by their cognate resistant proteins. In the past decades, a number of RXLR effector genes, such as Avr3a, Avr4, ipiO, have been identified in Phytophthora infestans. So far, little is known about the distribution of RXLR effector genes in the Chinese P. infestans population. In this study, we investigated the distribution of Avr3a, Avr4 and ipiO among 52 P. infestans isolates collected from major potato producing areas in China. Only 4 of them contain Avr3a (KI) allele which is recognized by R3a, whereas all others contain avr3a (EM), the variant which is no longer detected by R3a. No new alleles of Avr3a were detected. Avr4 was found in all isolates to encode truncated protein. These results indicated that virulent avr3a and avr4 predominately occur in the Chinese P. infestans population. Previous studies showed that P. infestans isolates lack class I ipiO are virulent on Rpi-blb1 potatoes. Class I variants were present in all the analyzed isolates, except one from Sichuan, which contains only the class III variant ipiO4, which cannot trigger Rpi-blb1 mediated cell death. In addition, several new ipiO alleles were identified, functional identification of these new alleles are still in process.