Submitted to: Korean Journal of Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2004
Publication Date: N/A
Interpretive Summary: Sinorhizobium sp. strain MUS10 forms nodules on the roots and stems of Sesbania rostrata, a green manure crop. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by legumes for growth and development. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. In this study, anatomical and morphological changes leading to the formation of stem nodules are reported. Information provided in this basic study will lead to a better understanding of biological nitrogen fixation in green manure crops. Further research in this area will help farmers to increase crop yield through less chemical fertilizer input and more recycling of plant nutrients.
Technical Abstract: Sinorhizobium sp. strain MUS10 forms nitrogen-fixing stem nodules on Sesbania rostrata, a tropical green-manure crop. In this study, the ultrastructural events associated with the formation of stem nodules were investigated. Sinorhizobium sp. strain MUS10 entered the host tissue through cracks created by the emerging adventitious root primordia and multiplied within the intercellular spaces. During early phases of infection, host cells adjacent to invading bacteria revealed cellular damage that is typical of hypersensitive reactions, while the cells at the inner cortex exhibited meristematic activity. Infection threads were numerous in 5-day-old nodules and often were associated with the host cell wall. In several cases, more than one infection thread was found in individual cells. The junction at which the host cell walls converged was often enlarged due to fusion of intracellular branches of infection threads resulting in large infection pockets. The infection threads were made up of a homogeneous, amorphous matrix that enclosed the bacteria. Several finger-like projections were seen radiating from these enlarged infection threads and were delineated from the host cytoplasm by the plasma membrane. As in Azorhizobium caulinodans-induced root nodules, the release of Sinorhizobia from the infection threads into the plant cells appears to be mediated by "infection droplets." A 15-day-old Sesbania stem nodule revealed typical ultrastructure features of a determinate nodule, containing several bacteroids within symbiosomes.