Ex-Planta and In-Planta Signals in Legume-Rhizobium Interaction

Nodule organogenesis is a highly-programmed developmental process triggered by rhizobial signal molecules and controlled by both Rhizobium and plant (Verma, 1992; Fisher and Long, 1992; Hirsch, 1992). Formation of Rhizobium-legume symbiosis provides a unique experimental system for genetic and molecular studies for communication between bacteria and host plants. Plant flavonoids and rhizobial Nod factors (modified oligosaccharides) are two classes of unique signal molecules that represent direct communication between these two organisms. The host roots exude flavonoids that act as chemotactic agents to attract the rhizobia to move towards the root surface and, more importantly, as strong inducers to initiate rhizobial nod gene expression. The nod genes encode enzymes responsible for biosynthesis and secretion of Nod factors (Carlson et al., 1994). Induction of the nod gene expression results in the production of Nod factors that elicit morphological changes in the host roots. These changes include root hair curling, formation of nodule primodia in the cortical cell and nodule organogenesis.