Reduced nodulation in alfalfa induced by arsenic correlates with altered expression of early nodulins

Arsenic (As) reduces legume nodulation by affecting the first stages of the symbiotic interaction, which causes a 90% decrease in rhizobial infections. In this paper, we examine molecular mechanisms underlying this toxic effect, using the model system Medicago sativa–Sinorhizobium. In the presence and absence of As, the expression patterns of seven nodulin genes, markers for the different events leading to nodule formation, were analyzed by RT-PCR and by real-time RT-PCR. A significant decrease was observed, especially from days 1–5 after the inoculation, in the expression of four early nodulins: the genes coding the Nod factor receptor (nork), the transcription factor NIN and the markers for infection progression (N6) and nodule organogenesis (Enod2). On the contrary, the expression of markers for primordium initiation (Enod40) and differentiation (ccs52) was not significantly altered. Finally, the expression of a marker for nitrogen fixation (Legbrc) was also reduced, probably due to the reduction in nodule number induced by As. These results suggest that As affects the expression of nodulation genes that have been associated with processes that take place in the epidermis and the outer cortical cells, and that the expression of genes associated with events that take place in the inner cortical cells is less affected. This is the first report showing changes in the expression of nodulin genes induced by the presence of any toxic metal(loid).