DEVELOPMENT OF PHASEOLUS-VULGARIS ROOT-NODULES

We report developmental studies on the determinate type of root nodules in Phaseolus vulgaris induced by Rhizobium leguminosarum biovar phaseoli. We show the first steps of nodule induction, leading to the formation of the nodule primordium from cells of the root outer cortex and describe the development of the central tissue, including invaded and uninvaded cells. Cytological observations, such as cell size and the pattern of starch accumulation, show that cells of the central tissue are a heterogeneous population. In situ hybridization shows a specific pattern of expression of the leghemoglobin and uricase-II genes. In mature nodules older invaded cells appear to be located in coincidence with a window generated in the endodermis by the penetration of the infection threads in the central tissue. This region seems to be the eccentric origin of a radial gradient of development. A model for the development of the common bean nodule is proposed.