Nodulation of groundnut byBradyrhizobium: a simple infection process by crack entry

Abstract

Infection of legumes by rhizobia may occur by immediate intercellular penetration of root cells (crack entry) as an alternative mode to the more elaborate infection through infection threads. The intercellular spreading mode of infection is exemplified through a comprehensive description of root infection by Bradyrhizobium and nodule organogenesis in Arachis hypogaea (groundnut). The role of axillary root hairs and the processes of plant penetration and intercellular spreading, of internalization and intracellular multiplication, and of bacteroid differentiation are described. Then flavonoids and phytoalexins, Nod factors, lectins, and surface poly(oligo)saccharides pass in review. The roles of these various (macro)molecules in the chemical communication between the two symbionts are discussed. Attention is given to special features of groundnut nodules; the presence and functions of oleosomes and other bodies, the presence and functions of nodule lectins, and the evidence for the export of amides from the nodules are discussed. Finally, a speculative model for the groundnut infection process is presented.

Getting around the legume nodule: I. The structure of the peripheral zone in four nodule types

“…Although it is often mentioned for peanut that the bacteria take advantage of the wounds induced by lateral root emergence and thus use a crack-entry mechanism to penetrate the root cortex (Allen and Allen 1940;Sprent 2000), this has been considered to be inaccurate by Boogerd and van Rossum (1997). The term ''crack-entry'' is probably misleading because rhizobia that enter the root do so in the region of tufted hairs that originate from the epidermis of a lateral root primordium (Boogerd and van Rossum 1997). Chandler (1978) and more recently Uheda et al (2001) have described the infection site as being located between a root hair and an adjoining cell, be it either another epidermal cell or a cortical cell.…”

Involvement of Root Hair during <i>Rhizobial</i> Invasion in Cultivated Peanut (<i>Arachis hypogaea</i> L.)

“…In another word, RoH is only present in nodulating peanut while absent in non-nodulating peanut plants [15]. These observations signal a close relationship between RoH and peanut nodulation, though previous reports indicated that peanut root infection by Bradyrhizobia is not through root hair because Bradyrhizobia enter the plant at epidermal ruptures [6].…”

“…The bacteria are then entrapped in the pocket, which further form ITs [5] in the epidermal cells. The rhizobia engulfed in the ITs penetrate the root hair through ITs into the cells, which were inductively formed by actively dividing cortical cells below the site of bacterial infection in the epidermis [6]. Finally, rhizobia are released in the root cortical nodule primordium cells to differentiate into bacteroids.…”

“…It was noticed that basal enlarged cells with root hair became the first to be infected by the invading rhizobia cells. Different from the ITs mode of infection, successful infection in crack entry mode of infection is restricted to penetration sites [6] American Journal of Plant Sciences [7].…”

“…Peanut forms a symbiotic association with rhizobia mainly known as Bradyrhizobia through crack entry since ITs have never been seen in the root hair nor epidermal cells, nor developing nodules of peanut [6] [7] [8]. No IT observation during peanut rhizobial infection may mean that root hair might not be necessary for peanut root infection by Bradyrhizobia, since the involvement of root hair often results into the formation of ITs [10].…”