Primary Study On Genetic Analysis And Gene Mapping Of Resistance To P. Sojae In Soybean

Phytophthora root rot(PRR),caused by Phytophthora sojae M.J.Kaufmann & J.W. Gerdemann,is one of the most devastating diseases in soybean[Glycine max(L.) Merr.]in the main soybean production countries such as USA,Brazil,Argentina and China. Utilization of resistant varieties is the most economical and environmentally safe method for controlling this disease.Two types of resistance to Phytophthora root rot have been described in soybean. Specific resistance confers immunity or near immunity to the plant through a hypersensitive response and is mediated by Rps genes.Fourteen genes at 8 loci(Rps1 to Rps8) for resistance to PRR have been designated and mapped to four molecular linkage groups in soybean:F,G,J and N,respectively.Single allele resistance has provided adequate disease management;however,following deployment of each single Rps gene,races of P.sojae were subsequently identified that had a susceptible interaction with the Rps gene.Single allele deployed in a soybean cultivar is only effective for 8 to 15 years,depending on inoculum density and environmental conditions.Continuous and enhanced uses of stable Rps genes in soybean cultivars against P.sojae races has created selection pressures for the evolution of new pathogenic P.sojae races that can overcome resistance conferred by these genes.P.sojae evolves constantly,and the number of physiological races of this oomycete pathogen is rapidly increasing.Partial resistance limits the lesion growth rate of the pathogen in host tissue,and it is understood to be quantitatively inherited.Huanghuai valley is one of main soybean production areas in China.The main goals of this study were:(1) to investigate the distribution and diversity of Phytophthora-resistant soybean and identify sources that confer resistance to multiple races for implementation into breeding programs; (2) to discover genetic mechanism of resistance to P.sojae;(3) to map resistant gene.The results of this study will be helpful for the development of resistant cultivars in this area.1.Resistance identification.96 soybean cultivars or lines from Huanhuai valley were evaluated for their responses to 7 strains of P.sojae using the hypocotyls inoculation technique.96 cultivars or lines elicited 38 different reaction types with the 7 strains of the pathogen.Among them 4 reaction types accorded to that of single gene,ten reaction types were consistent with two gene combinations,five reaction types were consistent with three gene combinations and the others were new reaction types.The 7 of 96 cultivars probably carried gene Rps3a,The 4 cultivars probably carried gene Rps3b,The 1 cultivar probably carried gene Rps3c,while 5 cultivars probably carried gene Rps7.Some cultivars or lines possibly carry new Rps genes that are effective to control Phytophthora root rot of soybean in China.These accessions may provide sources of resistance for control of Phytophthora rot of soybean in the future.2.Method of partial resistance to Phytophthora root rot and screening of soybean cultivar resistance.Three soybean cultivars with known resistance were evaluated for partial resistance to P.sojae with root inoculation.The result was that the root inoculation was simple,rapid and reliable and could be used as basal identification method of partial resistance of soybean to Phytophthora root rot.38 accessions from Huanghuai basin were evaluated for partial resistance to P.sojae using root inoculation technique.The results showed that 10 cultivars(26.3%) were high resistant to PNJ1 of P.sojae.3.Inheritance analysis.In the present study,four crosses between materials with complete or partial resistance were used to reveal the inheritance of resistance.The results showed that the complete resistance and partial resistance were controlled by different genetic systems:the former by a single dominant gene(Rps),while the latter by one additive major gene plus additive-dominant polygene.The heritability values of resistance in development were about 90%,those of major and polygene were 41.31%～74.84%and 15.60%～50.34%in F₂,and 54.21%～77.05%and 13.52%～38.24%in F_2:3,respectively. Both kinds of resistance can be used in soybean improvement,and it would be effective to conduct selection in early generations for high resistance in breeding,combining partial resistance with specific Rps genes to provide long term management of Phytophthora root rot as well as to avoid the boom-and-bust cycle of single gene deployment.4.Gene Mapping.Chinese soybean cultivar(line) Yudou25 and Zheng92116 had the same reaction to the 7 P.sojae isolates and might contain novel resistance loci or alleles.In order to further identify the allele(s) for resistance to P.sojae in Yudou25 and Zheng92116 and to map these allele(s) to a linkage group,two soybean populations,Yudou25×NG6255 and Zheng92116×NH5,were evaluated following inoculation with P.sojae and with SSR markers.Results indicated that the resistance in Yudou25 and Zheng92116 was conferred by a same single dominant gene,and it maybe a new resistance gene,temporarily designated RpsYu25,located on molecular linkage group N in two populations.5.RGA analysis.Most known plant disease-resistance(R) genes include nucleotide binding site(NBS) or leucine-rich repeats(LRRs) and serine/threonine protein kinase(STK) in their encoded products domains.Two primers,XLRRfor/XLRRrev and Pto-kin1/Pto-kin2,were designed based on these conserved domains.By polymerase chain reaction(PCR) and denatured polyacrylamide-gel electrophoresis techniques,disease resistance gene analogs(RGA) have been amplified on 48 soybean materials.The RGA analysis of 48 cultivars produced 53 amplification bands,39 of the 53(73.6%) bands showed polymorphic,and they could be divided into 7 groups at 0.746 similarity coefficient. Although there is no parallelism relationship between group and group in two different types of the clustering,the varieties,such as Fengshouhuang,Kefeng36,and Jimoyoudou with broad spectrum can be fallen into the same group.The result indicated RGAP (resistance gene analog polymorphism) technique,combining the resistant spectrum, provides a useful and efficient way to improve the efficiency of parent materials selection in soybean breeding and to accelerate the process of developing soybean cultivars with resistance.