INVESTIGADORES
QUELAS Juan Ignacio
artículos
Título:
Parasitic traits and plant defenses in the rhizobia-legume symbiosis
Autor/es:
LODEIRO, AR; LÓPEZ-GARCÍA, SL; ALTHABEGOITI, MJ; MONGIARDINI, EJ; PÉREZ GIMÉNEZ, J; QUELAS, JI
Revista:
Recent Res. Devel. Plant Pathology
Editorial:
Research Signpost
Referencias:
Lugar: Kerala, India; Año: 2004 vol. 3 p. 125 - 166
Resumen:
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 a-proteobacteria and a few b-
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.