Does Rhizobium Avoid the Host Response?

Abstract

Nodule development begins when the first cell division of a mitotically quiescent cortical cell is induced by Rhizobium (sensu lato) in the root of its host plant. This cell division brings about a change in cortical cell fate; the derivatives of these cell divisions form a new organ, the nodule. Prior to the initial anticlinal cell division in the root cortex, numerous interactions between the host and symbiont have already taken place, including the chemotaxis of rhizobia to the legume root, the induction of rhizobial nod genes by plant-derived flavonoids, the production of Nod factor—a substituted lipo-oligosaccharide—root hair deformation, and shepherd’s crook formation (the formation of a 360° curl). The latter two responses are the first visible signs of the interaction. The bacteria enter the curled root hair, presumably by degrading the plant cell wall (CALLAHAM and TORREY 1981; see also DART 1977), and evoke the formation of an infection thread, in which the bacteria are contained. The infection thread, formed from an invagination of the plant membrane and the deposition of cell wall material, extends into the root cells toward the nodule primordium. The nodule primordium originates from cortical cell divisions that take place in advance of the extending infection thread (NEWCOMB et al. 1979). Eventually, bacteria, which have multiplied within the matrix of the infection thread, are released from its end, and become enclosed within a host-derived membrane, designated the peribacteroid membrane. The bacteria, enclosed within this membrane, differentiate into novel forms, bacteroids capable of nitrogen fixation. They remain surrounded by host cell membrane until either the plant or bacteroid cells senesce. In response to the interaction with rhizobia, the plant expresses nodule-specific proteins (nodulins; LEGOCKI and VERMA 1980), which have been classed as early (those involved in nodule morphogenesis) and late (those involved in nodule maintenance and function) (GOVERS et al. 1987). Rhizobium infection and nodule development (Fig. 1) have been recently reviewed by BREWIN (1991), KIJNE (1992), and HIRSCH (1992).