Partial Resistance to Root-Borne Infection by Phytophthora sojae in Three Allelic Necrotic Root Mutants in Soybean

Abstract

Disease lesion mimic mutations that affect aerial parts of plants have been characterized in several species, but root disease mimics have not been previously described. Our objective was to characterize genetically and physiologically three independently derived necrotic root mutants found in soybean. The three mutants were inherited as a single recessive gene, were allelic, and were not linked to any of seven marker traits tested. When inoculated via the hypocotyl, necrotic root plants were susceptible to hyphal infection by a compatible race of the fungal pathogen Phytophthora sojae. In contrast, the roots showed partial resistance to infection by P. sojae zoospores, which correlated with the accumulation of isoflavonoid phytoalexins and group 2 peroxidases under axenic conditions. However, activation of the defense response in roots of the necrotic root mutants was insufficient to prevent nodulation by the symbiotic microorganism Bradyrhizobium japonicum.