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 Brad yrhizobium japonicum 110spc4 was analyzed on the host plants Glycine m ax and G. soja, The extent of the symbiotic defects was host dependent . In combination with G. max, the B. japonicum exoB mutants induced th e formation of effective nodules. Infection threads were found in the central nodule tissue of developing nodules, similar to wild-type infe cted nodules, However, in early stages of the interaction between the mutants and G. max, plant defense reactions occurred, among which phyt oalexin accumulation was the earliest effect observed. Later the rhizo dermis was disrupted by longitudinal cracks caused by cortical cell pr oliferations, and rhizodermal strips were frequently peeled off the gr owing nodules. Our results indicate that the intact EPS of B. japonicu m is necessary for the prevention of plant defense reactions during ea rly interaction with soybean. Combinations between G. max and B. japon icum exoB mutants seemed to be impaired only transiently, since they r esulted in effective nodule formation. However, enhanced concentration s of chitinase within the central nodule tissue of B. japonicum exoB m utant induced G. max nodules proved the occurrence of plant defense re actions also in later steps of nodule development. On G. soja, B. japo nicum exoB mutants lost their infectivity and induced the formation of white, uninfected and ineffective nodulelike structures at the base o f lateral roots.