Developmental biology of legume nodulation

Abstract

SUMMARY Many legumes respond to Rhizobium inoculation by developing unique structures known as nodules on their roots. The development of a legume nodule in which rhizobia convert atmospheric N2 into ammonia is a finely tuned process. Gene expression from both partners of the symbiosis must be temporally and spatially coordinated. Exactly how this coordination takes place is an area of intense study. Nodule morphogenesis appears to be elicited by at least two distinct signals: one from Rhizobium, a product of the nod genes (Nod factor), and a second signal, which is generated within plant tissues after treatment with Nod factor. The identity of the second signal is unknown but changes in the balance of endogenous plant hormones or the sensitivity of plant tissues to these hormones are likely to be involved. These hormonal changes may be triggered by endogenous flavonoids produced by the root in response to inoculation with Rhizobium. There is some controversy as to whether the legume nodule is an organ sui generis or a highly derived lateral root. A resolution of this question may become more critical as attempts to induce nodules on non‐legume hosts, such as rice or maize, increase in number and scope.

Root nodulation of Sesbania rostrata

“…The shape of a nodule depends on the location and the period of activity of its meristem. In indeterminate nodules, the lengthy activity (several weeks) of the apical meristem is responsible for the elongation of the nodules (14,24,41). In S. rostrata root nodules, the activity of the peripheral meristem, restricted to a shorter period of time following inoculation, accounts for the uniform enlargement of the nodules, with the infected tissue surrounding the initial infection site.…”

“…The persistent activity of the meristem, located at the outer extremity of the nodule, ensures nodule elongation for a number of weeks (41). While the meristem is active, rhizobia are released from the infection threads into the plant cell cytoplasm (5,14,17,24). The differentiation of both symbionts leads to the establishment of a central zone of the nodule, in which nitrogen is reduced (22,41).…”

“…In these plants, the infection process occurs via root hairs and results in the formation of rhizobium-containing infection threads, which grow towards the root cortex (8,33,42). Simultaneously with infection, inner root cortical cells dedifferentiate to form a nodule primordium and then a nodule meristem (11,14,19). The persistent activity of the meristem, located at the outer extremity of the nodule, ensures nodule elongation for a number of weeks (41).…”

“…There are two main types of nodule development, the so-called indeterminate and determinate types (5,14,22,24). Indeterminate nodules generally develop on temperate legumes, such as alfalfa, vetch, or pea.…”

“…Nodulation is a highly complex process involving a succession of interactions between the host plant and the bacterium, from initial recognition and infection to nodule development and nodule function (5,8,14,20,22).…”