MtENOD20, a Nod factor-inducible molecular marker for root cortical cell activation

The spatio-temporal expression pattern of the Medicago truncatula ENOD20 ge ne during early stages of nodulation has been analyzed with transgenic alfa lfa (M. varia) expressing a pMtENOD20-GUS chimeric fusion, Our results show that transcriptional activation of this gene occurs initially in dividing inner cortical cells corresponding to sites of nodule primordium formation and subsequently in root hairs containing infection threads. Use of Sinorhi zobium meliloti nod gene mutants that uncouple nodule organogenesis from in fection has confirmed that early MtENOD20 transcription is tightly linked t o cortical cell activation (CCA). Furthermore, these experiments have revea led that an S. meliloti nodH mutant, defective in Nod factor sulfation and epidermal cell activation, is nevertheless able to elicit low-level CCA. Pu rified S. meliloti Nod factors trigger MtENOD20 transcription very rapidly (within 12 to 24 h) in the root cortex, and studies with transgenic alfalfa show that Nod factors are able to elicit gene expression coupled to CCA at concentrations as low as 10(-11) M. Finally, we have made use of a range o f synthetic lipo-chitooligosaccharides to show that fatty acid chain length is an important structural parameter in the capacity of Nod factors to eli cit CCA, Taken together, our results suggest that pMtENOD20-GUS transgenic lines should prove valuable tools in future studies of Rhizobium and Nod fa ctor-elicited CCA.