From pollen tubes to infection threads: recruitment of Medicago floral pectic genes for symbiosis

While the biology of nitrogen-fixing root nodules has been extensively studied, little is known about the evolutionary events that predisposed legume plants to form symbiosis with rhizobia. We have studied the presence and the expression of two pectic gene families in Medicago, polygalacturonases (PGs) and pectin methyl esterases (PMEs) during the early steps of the Sinorhizobium meliloti-Medicago interaction and compared them with related pollen-specific genes. First, we have compared the expression of MsPG3, a PG gene specifically expressed during the symbiotic interaction, with the expression of MsPG11, a highly homologous pollen-specific gene, using promoter-gus fusions in transgenic M. truncatula and tobacco plants. These results demonstrated that the symbiotic promoter functions as a pollen-specific promoter in the non-legume host. Second, we have identified the presence of a gene family of at least eight differentially expressed PMEs in Medicago. One subfamily is represented by one symbiotic gene (MtPER) and two pollen-expressed genes (MtPEF1 and MtPEF2) that are clustered in the M. truncatula genome. The promoter-gus studies presented in this work and the homology between plant PGs, together with the analysis of the PME locus structure and MtPER expression studies, suggest that the symbiotic MsPG3 and MtPER could have as ancestors pollen-expressed genes involved in polar tip growth processes during pollen tube elongation. Moreover, they could have been recruited after gene duplication in the symbiotic interaction to facilitate polar tip growth during infection thread formation.