Perception and Processing of Nod Factor Signals

Abstract

Plant responses to Nod factors comprise a variety of processes that affect membrane transporters, the apoplastic milieu, metabolism, phytohormone balance and gene expression. While some of these responses occur in seconds after the first encounter of the root (hair) plasma membrane with the appropriate Nod factor, others need minutes, hours or days before becoming detectable. Following Nod factor perception by one or more putative plasma membrane receptors, activation of Ca²⁺ channels is the earliest response observed so far. The resulting Ca²⁺ influx stimulates anion channels which, giving rise to Cl⁻ release, depolarizes the cells and, by releasing organic anions, alkalizes the external space. Although these effects have not been found with all such investigated plant species, a function with respect to permitting the symbiotic rhizobial bacteria access to the root hair symplast appears likely. The conspicuous root hair deformation seems to be caused by a perturbation in apical free Ca²⁺ which causes both a rapid breakdown of the root hair cytoskeleton followed by its rearrangement. Ca²⁺ spiking in the nucleous area, that occurs with a 10 min delay, may be connected with gene activation leading to the early nodulin gene expression in connection with plant development and nodule formation. Although Nod factors (without rhizobia being present) induce the formation of nodules in certain legumes, they may not be the primary signal. As, due to an imbalance of phytohormones (auxin and cytokinin), nodules may spontaneously form, gene products like ENOD40 are very likely the trigger or regulator of cortical cell divisions.