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 expressi on throughout the various developmental stages of the Rhizobium-legume interaction, promoter-gusA fusions for each of three lectin genes fro m Medicago truncatula (Mtlec1, 2 and 3) have been introduced into tran sgenic Medicago varia plants. Histochemical staining for beta-glucuron idase activity has shown that Mtlec1 and Mtlec3 are expressed independ ently of symbiosis in inner cortical and vascular tissues of the root, respectively. Transcription of Mtlec3 in the root stele is markedly e nhanced following inoculation with Rhizobium meliloti. Furthermore, bo th Mtlec1 and Mtlec3 are transcribed in dividing cells of the root nod ule primordium, whether elicited by R. meliloti or by purified nodulat ion (Nod) factors. These findings suggest that these two lectins are i nvolved in developmental processes early during nodule ontogeny. In co ntrast, 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 g ene expression is restricted to non-infected cells of both central and certain peripheral tissues and appears to be correlated with the nitr ogen-fixing capacity of the nodule.