Genes and signals in the rhizobium-legume symbiosis.

Rhizobium-legume symbiosis begins with two free living organisms, and ends with an intimate cellular co-existence. Rhizobium bacteria recognize specific plants, provoke development of a root nodule, and invade the plant tissue. Eventually, the Rhizobium cell transfers itself into a host cell, surrounds itself with plant membrane, and arranges a nutrient exchange in which the bacteria brings fixed nitrogen to the plant, receiving in turn the sanctuary and sugars that the plant cell can provide (8,13,17,20). This historical note concerning 25 years of Rhizobium plant research will focus mostly on a few stories related to the discovery of nod genes and signals. I refer the reader mostly to books and reviews for the details of these and of related research stories that are mentioned more briefly in the latter part of this review.