Silencing of defense-related genes reveals different mechanisms leading to ace-specific resistance to Phytophthora in soybean

Agrobacterium rhizogenes mediated RNAi gene silencing has been used to determine the roles of several defense-related genes in a series of Williams isolines carrying various Rps genes for race specific resistance to Phytophthora sojae. The Rps Lines examined included those carrying Rps 1, 1b, 1c, 1k, 2, 3, 3b, 3c, 4, 5 and 7. The genes silenced included isoflavone synthase (IFS) and chalcone reductase (CHR) for isoflavone biosynthesis, PR-1a, the glucan elicitor releasing endoglucanase (PR-2), a unique multidomain metallothionein gene (MMT), two soybean Npr homologs and an isoflavone specific beta-glucosidase (ISBG). Phenotypes were examined in transformed hairy roots, and included resistance or susceptibility to infection by P. sojae and various cell death and redox responses to infection or treatment with the cell wall glucan elicitor (WGE) from P. sojae. An interesting new finding was that while the isoflavones were essential to HR cell death in lines carrying Rps 2 or alleles at the Rps 1 locus, cell death in lines carrying Rps 7 or alleles at Rps 3 was isoflavone-independent. This suggests that different classes of Rps genes may modulate different cell death pathways. An additional finding is that silencing the Npr homologs led to enhanced general (partial, basal) resistance. This unexpected finding is under further investigation.