| Abstract: | Nod factors, which are signaling molecules produced by Rhizobia, are the principal determinants of host specificity in Rhizobium-legume symbiosis. Nod factors can elicit a number of characteristic developmental responses in the roots of legumes, such as depolarization of the membrane potential in epidermal cells, specific expression of early nodulin genes and changes in the flux of calcium in root hairs, deformation of root hairs, cell division in the root cortex and formation of the nodule primordium. Whether the rice plant can respond to signaling molecules (i.e. Nod factors) is an important question, as it could establish the potential for symbiotic nitrogen fixation in rice. The promoter of the soy-bean (Glycine max) early nodulin gene Gmenod2B fused to the b-glucuronidase (GUS) reporter gene was used as a mo-lecular marker to explore whether Nod factors can be recog-nized by rice cells as signaling molecules. Transgenic rice plants harboring the chimeric gene Gmenod2BP-GUS were obtained via an Agrobacterium tumefaciens-mediated system. NodNGR factors produced by a broad-host-range Rhizobium strain NGR234(pA28) were used as probes to investigate the activity of the Gmenod2B promoter in rice. Our results showed that the early nodulin gene Gmenod2B promoter was induced by NodNGR factors in transgenic rice, and that it was specifically expressed in rice plant roots. Moreover, GUS gene expression driven by the Gmenod2B promoter in trans-genic rice was regulated by nitrogen status. These findings indicated that rice possessed the ability to respond to Nod factor signals, and that this signal transduction system re-sulted in activation of the Gmenod2B promoter. Thus, we predict that the Nod-factor inducible nodulin expression system, which is similar to Rhizobium-legume symbiosis, may also exist in rice. |
