Screening For Resistance Source And Mapping Of Resistance Gene To Phytophthora Sojae In Soybean

Phytophthora sojae Kaufmann & Gerdemann （P. sojae） is a soil borne pathogen that causes Phytophthora root rot of soybean （PRR）, which is one of the most serious soybean diseases. PRR has been reported in most soybean-growing regions throughout the world and caused consistent yield losses of 0.68×106-1.55×106 tonnes from 1996 to 2010 in the United States. In China, PRR was first observed in the Northeast region in 1989, and has subsequently been observed in main soybean production regions. In recent years, the yields of soybean and the income of soybean producers have decreased dramatically in the China.The most effective and economical way to reduce damage from PRR would be to plant resistant cultivars. Two distinct types of host resistance have been described:（i） race-specific resistance conditioned by a single dominant Rps （Resistance to P. sojae） gene and （ii） partial resistance conferred by multiple genes acting together. The main goals of this study were:（1） screening for resistance sources to phytophthora root rot in soybean; （2） genetic analysis and fine mapping of RpsJS; （3） genetic analysis and gene mapping of RpsHN; （4） association mapping for partial resistance to Phytophthora sojae in soybean. These results will help to understand the genetic basis of resistance, and facilitate future marker assistant selection （MAS） aimed to improve the resistance to P. sojae and reduce disease-related mortality in soybean.1. Screening for resistance sources. Two hundred and one soybean cultivars from 25 provinces in china were evaluated for their responses to 5 isolates of P. sojae, namely P6497, PNJ1, AH, P7063, and Pm31, using the hypocotyls inoculation technique in greenhouse, were analyzed using gene postulating procedure. Two hundred and one cultivars elicited 25 different reaction types with the 5 isolates of the pathogen, Twenty five cultivars were identified having resistance to 4-5 isolates of P. sojae, among which probably carried Rps genes Rps1a, Rps3a, Rps1c or a new Rps gene. The study indicated that the most abundant resistance diversity was occurred in cultivars from South China. Five cultivars, which were identified having resistance to 5 isolates of P. sojae, were MCC055, MCC060, MCC079, MCC080, and MCC178.2. Genetic analysis and fine mapping of RpsJS. Chinese soybean line Nannong 10-1 is resistant to many P. sojae isolates, and shows different reaction types to P. sojae isolates as compared with those with known Rps genes, which suggests that the line may carry novel Rps genes or alleles. A mapping population of 231 F2 individuals from the cross of Nannong 10-1 （Resistant, R） and 06-070583 （Susceptible, S） was used to map the Rps gene. We found that the resistance in Nannong 10-1 was conferred by one independent Rps gene, designated RpsJS. The results showed that RpsJS was mapped on soybean molecular linkage group G （chromosome 18） flanked by SSR （simple repeat sequences） markers BARCSOYSSR₁8₁859 and SSRG60752K at a distance of 0.9 and 0.4 cM, respectively. Among the 14 genes annotated in this 138.9 kb region between the two markers, three genes （Gfyma18g51930, Glyma18g51950 and Glyma18g51960） are the nucleotide-binding site and a leucine-rich repeat （NBS-LRR） type gene, which may be involved in recognizing the presence of pathogens and ultimately conferring resistance. Based on marker assisted resistance spectrum analyses of RpsJS and the mapping results, we inferred that RpsJS was a novel gene or a new allele at the Rps4, Rps5 or Rps6 loci.3. Inheritance analysis and gene mapping of RpsHN. Chinese soybean cultivar Meng8206 shows the different reaction to the 15 P. sojae isolates and might carry novel Rps gene. Two soybean RIL （Recombined inbred lines） populations derived from Meng8206 （resistant, R） × Linhedafenqing （susceptible, S） and Meng8206 （R） × Zhengyang148 （S）, were identified the genes for resistance to P. sojae and mapped these genes to linkage groups. The results showed that the complete resistance genes in Meng8206 to strain HeN35 was controlled by a single gene, designated RpsHN, which was located on chromosome 3 （molecular linkage group N） and closely linked with Bin250 and Bin282 with the physical distance 672.8Kb. RpsHN was a novel gene or a new allele at the Rpsl locus.4. Association mapping for partial resistance to P. sojae. In this study, we evaluated a total of 201 soybean accessions by the hypocotyl inoculation method, and 163 were susceptible. The 163 susceptible accessions were then evaluated for P. sojae partial resistance using slant board assays. The 163 accessions were screened with 138 simple sequence repeat （SSR） markers that generated 727 SSR alleles.138 SSR markers were used for association mapping by the TASSEL general linear model （GLM） program. This soybean population could be divided into two subpopulations. Four SSR markers, Satt634 （LG Dlb）, Satt135 （LG D2）, Satt221 （LG Dla）, and Sat₂22 （LG D2）, associated with partial resistance to P. sojae were detected by both GLM method.