We show here that MtMMPL1, a Medicago truncatula
nodulin gene previously identified by transcriptomics, represents a novel and specific marker for root and nodule
infection by Sinorhizobium meliloti. This was established by determining the spatial pattern of MtMMPL1 expression and evaluating gene activation in the context of various plant and bacterial symbiotic mutant interactions. The MtMMPL1
protein is the first
nodulin shown to belong to the large matrix metalloendoproteinase (
MMP) family. While plant
MMPs are poorly documented, they are well characterized in animals as playing a key role in a number of normal and
pathological processes involving the remodeling of the extracellular matrix. MtMMPL1 represents a novel
MMP variant, with a substitution of a key
amino acid residue within the predicted active site, found exclusively in expressed sequence tags corresponding to legume
MMP homologs. An RNA interference approach revealed that decreasing MtMMPL1 expression leads to an accumulation of rhizobia within
infection threads, whose diameter is often significantly enlarged. Conversely, MtMMPL1 ectopic overexpression under the control of a constitutive (35S) promoter led to numerous abortive
infections and an overall decrease in the number of nodules. We discuss possible roles of MtMMPL1 during Rhizobium
infection.