DEVELOPMENT OF PHASEOLUS-VULGARIS ROOT-NODULES

We report developmental studies on the determinate type of root nodule s in Phaseolus vulgaris induced by Rhizobium leguminosarum biovar phas eoli. We show the first steps of nodule induction, leading to the form ation 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 m ature nodules older invaded cells appear to be located in coincidence with a window generated in the endodermis by the penetration of the in fection threads in the central tissue. This region seems to be the ecc entric origin of a radial gradient of development. A model for the dev elopment of the common bean nodule is proposed.