PLANT DEFENSE RESPONSES OF HOST PLANTS WITH DETERMINATE NODULES INDUCED BY EPS-DEFECTIVE EXOB MUTANTS OF BRADYRHIZOBIUM-JAPONICUM

The symbiotic phenotype of exoB mutants Delta P5 and Delta P22 of Bradyrhizobium japonicum 110spc4 was analyzed on the host plants Glycine max and G. soja, The extent of the symbiotic defects was host dependent. In combination with G. max, the B. japonicum exoB mutants induced the formation of effective nodules. Infection threads were found in the central nodule tissue of developing nodules, similar to wild-type infected nodules, However, in early stages of the interaction between the mutants and G. max, plant defense reactions occurred, among which phytoalexin accumulation was the earliest effect observed. Later the rhizodermis was disrupted by longitudinal cracks caused by cortical cell proliferations, and rhizodermal strips were frequently peeled off the growing nodules. Our results indicate that the intact EPS of B. japonicum is necessary for the prevention of plant defense reactions during early interaction with soybean. Combinations between G. max and B. japonicum exoB mutants seemed to be impaired only transiently, since they resulted in effective nodule formation. However, enhanced concentrations of chitinase within the central nodule tissue of B. japonicum exoB mutant induced G. max nodules proved the occurrence of plant defense reactions also in later steps of nodule development. On G. soja, B. japonicum exoB mutants lost their infectivity and induced the formation of white, uninfected and ineffective nodulelike structures at the base of lateral roots.