Chemically-induced resistance on soybean inhibits nodulation and mycorrhization

Induced plant resistance (IR) against pathogen infection can be triggered by various chemical and biological elicitors. The effectiveness of elicitors to induce resistance as a practical means to control plant disease makes use of the plant's own defence mechanisms triggered by resistance inducing agents. The aim of the present study was to examine the possible side effects of IR on the establishment and the efficiency of a rhizobial symbioses (Bradyrhizobium japonicum-soybean) and an arbuscular mycorrhizal symbioses (Glomus mosseae -soybean). IR was triggered by applying, acibenzolar S-methyl (ASM) at 80 mg a.i. L-1 by two ways: seed soaking or foliar spray. Chitinase activity, used as a biochemical marker of IR, increased when ASM was applied both as seed soaking or as foliar spray. In vitro ASM showed no direct effect on the growth of B. japonicum and induced, only at a high concentration, a slight inhibition effect on spore germination of G. mosseae. ASM caused after both treatments a significant decrease in the number of nodules. Nitrogen content in aerial parts and roots decreased. On the other hand, ASM showed no significant effect on the frequency of colonization by G.mosseae but reduced the intensity of colonization and the proportion of arbuscules. The possible interaction between IR and the induction and suppression of defence-like mechanisms during symbiotic processes is discussed.