Time Course of Cell Biological Events Evoked in Legume Root Hairs by Rhizobium Nod Factors: State of the Art

Abstract

In many common legumes, when host-specific nodule bacteria meet their legume root they attach to it and enter through root hairs. The bacteria can intrude these cells because they instigate in the hairs the formation of an inward growing tube, the infection thread, which consists of wall material. Prior to infection thread formation, the bacteria exploit the cell machinery for wall deposition by inducing the hairs to form a curl, in which the dividing bacteria become entrapped. In most species, Nod factor alone (a lipochito-oligosaccharide excreted by bacteria) induces root hair deformation, though without curling, thus most aspects of the initial effects of Nod factor can be elucidated by studying root hair deformation. In this review we discuss the cellular events that host-specific Nod factors induce in their host legume root hairs. The first event, detectable only a few seconds after Nod factor application, is a Ca2+influx at the root hair tip, followed by a transient depolarization of the plasma membrane potential, causing an increase in cytosolic [Ca2+] at the root hair tip. Also within minutes, Nod factors change the cell organization by acting on the actin cytoskeleton, enhancing tip cell wall deposition so that root hairs become longer than normal for their species. Since the remodelling of the actin cytoskeleton precedes the second calcium event, Ca2+spiking, which is observed in the perinuclear area, we propose that the initial cytoskeleton events taking place at the hair tip are related to Ca2+influx in the hair tip and that Ca2+spiking serves later events involving gene expression.