Lectin genes are expressed throughout root nodule development and during nitrogen-fixation in the Rhizobium-Medicago symbiosis

In order to study the spatio-temporal patterns of lectin gene expression throughout the various developmental stages of the Rhizobium-legume interaction, promoter-gusA fusions for each of three lectin genes from Medicago truncatula (Mtlec1, 2 and 3) have been introduced into transgenic Medicago varia plants. Histochemical staining for beta-glucuronidase activity has shown that Mtlec1 and Mtlec3 are expressed independently of symbiosis in inner cortical and vascular tissues of the root, respectively. Transcription of Mtlec3 in the root stele is markedly enhanced following inoculation with Rhizobium meliloti. Furthermore, both Mtlec1 and Mtlec3 are transcribed in dividing cells of the root nodule primordium, whether elicited by R. meliloti or by purified nodulation (Nod) factors. These findings suggest that these two lectins are involved in developmental processes early during nodule ontogeny. In contrast, Mtlec2 is not expressed in uninoculated roots or during early stages of the symbiotic interaction. At later stages, all three lectin promoters are functional in mature nodules. In this context, lectin gene expression is restricted to non-infected cells of both central and certain peripheral tissues and appears to be correlated with the nitrogen-fixing capacity of the nodule.