Nodule Morphogenesis in the Absence of Rhizobium

Abstract

It is generally assumed that Rhizobium and Bradyrhizobium bacteria are the causative agent for nodulation in legumes. The analysis of bacterial genes and their products is focused on the elucidation of the signal-transduction pathway leading to cell division, differentiation and infection (Caetano-Anollés, Gresshoff, 1991a). Bacteria are chemoattracted to the root surface (Bauer, Caetano-Anollés, 1990) where plant flavonoids induce bacterial gene expression (Peters, Verma, 1990) and secretion of factors that trigger the first morphological responses in the host, root hair curling and cortical cell division (Dénarié et al. 1992). Concurrent anticlinal and periclinal cell divisions, usually beneath the infection site, initiate a nodule primordium that forms a persistent nodule meristem in its distal end and gives rise to a central tissue with infected and uninfected cells.