Promoter Activities of Medicago Lectin Genes During the Symbiotic Interaction Between R. Meliloti and Transgenic Alfalfa

Abstract

Under conditions of nitrogen limitation leguminous plants can form associations with soil bacteria of the genus Rhizobium culminating in the formation of symbiotic root nodules. In such nodules rhizobia convert atmospheric nitrogen into ammonia, which then serves as nitrogen source for the plant. Mutual recognition of the symbiotic partners is central to the establishment of the Rhizobium-legume symbiosis. An important characteristic of this recognition process is its specificity. Recent research has shown that rhizobia produce species-specific extracellular lipo-oligosaccharides known as Nod factors which elicit, in the appropriate plant host, a variety of symbiosis-related events such as root hair deformation, cortical cell divisions and nodule formation [1]. However, host-specificity also appears to be determined by the plant partner. In 1989, Díaz et al. [2] showed that white clover roots carrying a pea lectin gene were susceptible to infection and nodulation by a pea-specific Rhizobium strain which normally cannot nodulate white clover. This finding supported an earlier hypothesis [3, for reviews see 4-6] that plant lectins (carbohydrate-binding proteins) might be determinants of host-specificity through their specific binding to bacterial polysaccharides. As a means of determining which symbiotic events might be mediated by lectins, we have adopted the complementary approach of studying lectin gene expression throughout the various successive stages of the Rhizobium-legume interaction. For this purpose, we have characterized the three members of a lectin gene family from the model legume Medicago truncatula. Each of the three lectin promoters was fused to the E. coli gusA reporter gene (encoding ßglucuronidase = GUS) and these constructions were introduced into alfalfa (M. varia) plants using an Agrobacterium tumefaciens transformation approach.KeywordsWhite ClovergusA Reporter GeneLectin GeneTransgenic AlfalfaClover RootThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.