Infection Processes and Involvement of Defense-Related Genes in the Expression of Resistance in Cultivars of Subterranean Clover (Trifolium subterraneum) to Phytophthora clandestina

Abstract

Studies on infection processes and gene expression were done to determine differential responses of cultivars of Trifolium subterraneum resistant and susceptible to infection by races of Phytophthora clandestina. In the infection process study, one race was inoculated onto the roots of T. subterraneum cvs. Woogenellup and Junee (compatible or incompatible interactions, respectively). There were no differences in relation to the processes of cyst attachment, germination, and hyphal penetration. There were, however, major differences in infection progression observed post-penetration between compatible and incompatible interactions. In susceptible cv. Woogenellup, hyphae grew into the vascular bundles and produced intercellular antheridia and oogonia in the cortex and stele by 4 days postinoculation (dpi), oospores in the cortex and stele by 8 dpi, when sporangia were evident on the surface of the root. Infected taproots were discolored. Early destruction of taproots prevented emergence of lateral roots. Roots of resistant cv. Junee showed no oospores or sporangia and no disease at 8 dpi. In the gene expression studies, two races of P. clandestina were inoculated onto three cultivars of T. subterraneum. Results showed that three genes known to be associated with plant defense against plant pathogens were differentially expressed in the roots during compatible and incompatible interactions. Phenylalanine ammonia lyase and chalcone synthase genes were activated 4 h postinoculation (hpi) and cytochrome P450 trans-cinnamic acid 4-monooxygenase gene was activated 8 hpi in the incompatible interactions in cvs. Denmark and Junee following inoculation with Race 177. In contrast, in compatible interactions in cv. Woogenellup, there were no significant changes in the activation of these three genes following inoculation, indicating that these three genes were associated with the expression of resistance to Race 177 of the pathogen by the host. To confirm this result, in the second test, cv. Woogenellup was challenged by Race 000 of P. clandestina. In this incompatible interaction, cv. Woogenellup was resistant and expressed highly all three genes in the manner similar to the incompatible interactions observed in the first test.