The Identification Of The Avirulent Gene PsAvr3b In Phytophthora Sojae

Phytophthora sojae is an oomycete that causes root and stem rot on soybean. This disease is a serious and endemic problem in soybean-producing areas, with an annual cost worldwide of $1-2 billion.Soybean and P.sojae interact according to the gene-for-gene model, disease control strategies have relied on developing resistant soybean cultivars through the identification and integration of soybean Rps (resistance to Phytophthora sojae) genes.Cloning and characterizing P. sojae avirulence genes will aid in deciphering the molecular events involved in P.sojae pathogenesis and in understanding plant-pathogen interactions in general. Bioinformatics approaches as well as oomycete genomy data show that oomycete Avr genes share a collective characteristics include transcript level,multi-copy and polymorphisms of the sequences.The identification of oomycete Avr genes is now happening at a rapid pace, facilitated by genome sequence data and the discovery of the special pattern of oomycete Avr genes.The interaction between soybean and the phytopathogenic oomycete Phytophthora sojae is controlled by host resistance (Rps) genes and pathogen avirulence (Avr) genes. Eighty single oospore cultures were isolated from the cross between race P6497(R2)and P7076(R19). A screen of these cultures with CAPS maker identified 3 F1 hybrids, indicating an outcrossing frequency of 3.8%. A single F1 hybrid from the cross was self-fertilised to generate F2 populations.F2 population were tested for virulence on a differential set of soybean cultivars(Rps3b).The observed segregation ratio for Avr3b fits a 3:1 ratio consistent with a dominant allele at a single locus,χ2 indicates the probability.This work provides a basis for the positional cloning of Avr3b.The identification of P. sojae Avr genes and the study of their evolutionary origin are important to understand the mechanism of virulence in this pathogen.The research about oomycete Avr genes suggest that these genes have signal peptide and RXLR motif.Est,Microarray and Solexa data raveal that 118 RXLR genes have a high transtcript during zoospore germination and infection stage.Sequence analysis of each of the candidate genes was performed on P.sojae strains P7064, P7074 and P7076, to detect any polymorphisms compared to the sequences from P.sojae P6497. These P.sojae strains differ in virulence characteristics on soybean plants carrying Rps3b, P.sojae P6497 and P7064 is avirulent while P7074 and P7076 are virulent.15 RXLR genes have polymorphisms in virulent strains than in avirulent strains. Combination the copy of genes 7 RXLR candidate genes as likely to encode Avr3b. The results of CAPS marker show that co-segregation of PsAvh307 with PsAvr3b virulence phenotype in F2 progeny,the result of bombardment also suggest that PsAvh307 is PsAvr3b.