Light microscopy of early stages in the symbiosis of soybean with a delayed-nodulation mutant of Bradyrhizobium japonicum

Abstract

An α-ketoglutarate dehydrogenase mutant (LSG184) of Bradyrhizobium japonicum USDA110 has a delayed nodulation phenotype when inoculated onto soybean (Glycine max L.). To pinpoint the defective stage of symbiotic development, light microscopic techniques were used to monitor early responses of soybean to inoculation with the mutant as compared to the wildtype strain. Methylene blue was used to visualize curled root hairs and a convenient haematoxylin staining method was developed that could detect nodule primordia as early as 2 d after inoculation. The results demonstrate that early symbiotic events occur with normal timing after inoculation with LSG184 and that its developmental delay is first evident during the progression of nodule primordia into emergent nodules. The timing of this delay suggests that LSG184 is not deficient in Nod factor production, at least during the early stages of symbiosis, but rather may have a defect in infection thread initiation or elongation. The results further imply that the rate of development of advanced soybean nodule primordia is, in part, dependent on the metabolic capabilities of the invading bacterium.