Nodule development induced by mutants of Bradyrhizobium japonicum defective in cyclic B-glucan synthesis

The soybean response to inoculation with B. japonicum mutants defective in cyclic beta-(1-->3), beta-(1-->6) glucan synthesis was examined by electron microscopy and by monitoring the expression of early and late nodulin genes. Two mutants were examined. Strain AB-14 is an ndvB mutant and is unable to synthesize beta-glucans. Strain AB-1 is an ndvC mutant and produces cyclic glucans containing 95 to 100% beta-(1-->3) glycosidic linkages. Nodules formed by either mutant were defective in nitrogen fixation activity. Soybean plants inoculated with strain AB-14 formed nodules roughly at the same rate as the wild-type strain USDA110, but nodulation by strain AB-1 was significantly delayed. Microscopy of nodules formed by strain AB-14 showed an overall ultrastructure similar to nodules formed by the wild type. However, in some nodules bacteroids were limited to only one part of the infected cells and in others the few bacteroids present showed signs of degradation. Nodulelike structures were formed by strain AB-1 that showed some signs of cellular differentiation. For example, clear parenchyma and sclerenchyma tissue could be seen. However, no infection threads or bacteria were evident in these structures. The expression of early (e.g., ENOD2 and ENOD55) and late (e.g., NOD26 and leghemoglobin) nodulins was examined. Nodules formed by strain AB-14 expressed all of the nodulins tested but at a reduced level. Expression of late nodulins was delayed in strain AB-14-induced nodules. Nodules formed by strain AB-1 were more strongly affected in nodulin expression. Although leghemoglobin synthesis was not detected, infection-specific nodulin transcripts of GmN93 and ENOD55 were detected. However, expression of the early nodulins ENOD2 and ENOD55 was considerably delayed and only apparent when assayed 21 days postinoculation. A low level of expression of intermediate nodulin GmN70 and late nodulin NOD26 could also be detected by 21 days postinoculation. The microscopy data show that tissue differentiation occurs in these nodules even in the absence of active bacterial invasion. These results demonstrate the importance of cyclic beta-(1-->3), beta-(1-->6) glucan synthesis to symbiotic development in soybean.