Parasitic traits and plant defenses in the rhizobia-legume symbiosis

LODEIRO, A.R.; S. L. LÓPEZ GARCÍA; M.J. ALTHABEGOITI; E.J. MONGIARDINI; J. PÉREZ GIMÉNEZ; J.I. QUELAS

Recent Research Developments in Plant Pathology

Research Signpost, Trivandrum, INDE

Lugar: Kerala; Año: 2004; p. 125 - 166

Rhizobia are soil bacteria that establish symbioses with dicot plants. This diverse group of bacteria includes parasites such as Agrobacterium, while the other species, comprising six genera of α-proteobacteria and a few β-proteobacteria are mutualistic legumes symbionts. The mutualistic relationship is based on the common ability of most rhizobia to fix atmospheric N2 inside special plant organs called nodules. The nodule developmental program is encoded in the plant genome but is specifically induced by rhizobia mainly in the legume roots, and at least in one case, in the shoot. Rhizobia invade the nodules by two different ways: infection through root hairs (infection threads) or intercellular spreading (crack entry). Once into the nodule, rhizobia differentiate into non-replicating forms called bacteroids, which are able to fix N2. The processes of invasion and differentiation, the number of nodules, and the N2 fixation rates, are tightly controlled by the plant and the bacteria in what appears to be a balance between the rhizobial need to persist in the nodules for spreading after nodule senescence, and the plant advantage from being able to survive in extremely N-deficient soils. The evolution of this relationship started around 70 million years ago, when the main rhizobial and legume genera already existed. The rhizobial ability to infect the roots seems to have been acquired and distributed by horizontal gene transfer and uses some bacterial strategies common to parasites such as polysaccharide-mediated plant defense evasion and suppression, and transfer of effector molecules to the plant cell cytosol. The plant partner seems to have recruited signal transduction and developmental pathways from other symbioses and tissue differentiation processes to establish the bacterial recognition and the nodule developmental programs. A number of mutations affecting these pathways either in the rhizobia or in the legumes shift this symbiosis from mutualism to parasitism, with clear plant defense reactions against the invading rhizobial cells. These observations support the view that considers this mutualistic symbiosis as a refined parasitism.