The Molecular Basis of Infection and Nodulation by Rhizobia: The Ins and Outs of Sympathogenesis

Abstract

Bacteria belonging to the genera Rhizobium, Bradyrhizobium, and Azorhizobium, collectively known as rhizobia, penetrate the roots (or adventitious roots) of their leguminous host plants via tubular structures, the infection threads. During infection of the host plant they trigger the formation of a new organ, the root nodule, in which a differentiated form of rhizobia, the bacteroid, fixes nitrogen into ammonia, which can then be used by the plant. This review presents an update of the recent literature on the molecular biology of the infection and nodulation of plants by rhizobia, with special emphasis on results pertinent to other plant-microbe interactions. Particular attention is given to determinants of host specificity such as flavonoid and lipo-chitin oligosaccharide signal molecules.