Alteration of enod40 expression modifies Medicago truncatula root nodule development induced by Sinorhizobium meliloti

Molecular mechanisms involved in the control of root nodule organogenesis i n the plant host are poorly understood. One of the nodulin genes associated with the earliest phases of this developmental program is enod40. We show here that transgenic Medicago truncatula plants overexpressing enod40 exhib it accelerated nodulation induced by Si-norhizobium meliloti. This resulted from increased initiation of primordia, which was accompanied by a prolife ration response of the region close to the root tip and enhanced root lengt h. The root cortex of the enod40-transformed plants showed increased sensit ivity to nodulation signals. T-1 and T-2 descendants of two transgenic line s with reduced amounts of enod40 transcripts (probably from cosuppression) formed only a few and modified nodulelike structures. Our results suggest t hat induction of enod40 is a limiting step in primordium formation, and its function is required for appropriate nodule development.