ENOD40 expression precedes cell division and affects phytohormone perception at the onset of nodulation

Legume nodule organogenesis is initiated by local dedifferentiation of the root cortex. Nod factors initiate this mitotic reactivation in a spatially controlled manner (Van Brussel et al., 1992; Vijn et al., 1993). In most temperature legumes like pea and alfalfa, cell division is induced in the inner cortex opposite proto-xylem points (Libbenga and Harkes, 1973; Newcomb et al., 1979). Although the mode of action of Nod factors remains unresolved several studies suggest that these compounds probably establish a local change of the auxin/cytokinin ratio resulting in induction of cell division in the root cortex (Allen et al., 1953; Hirsch et al., 1989; Cooper and Long, 1994). In these dividing cortical cells, ENOD40 is induced by Nod factors (Vijn et al., 1993; Yang et al., 1993). We were interested whether the expression of ENOD40 is involved in the initiation of cortical cell division and investigated whether it is involved in establishing a change in phytohormone concentration of sensitivity as an early step in nodule formation.