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MsEnod12A Expression Is Linked to Meristematic Activity During Development of Indeterminate and Determinate Nodules and Roots

During development of nodules and roots, the alfalfa early nodulin gene MsEnod12A is expressed adjacent to the meristem. Using transgenic alfalfa carrying a MsEnod12A promoter-gusA fusion, we investigated the regulation of MsEnod12A expression in mature nodules and roots by GUS assays and reverse transcription-PCR. We found that in alfalfa indeterminate nodules induced by various Fix^- Rhizobium meliloti mutants and in spontaneous nodules devoid of rhizobia, MsEnod12A was expressed at the distal end when a persistent meristem was present. However, this gene was not expressed when the meristem was lacking in nodules arrested in development. The MsEnod12A-gusA fusion was introduced into Lotus corniculatus plants that form determinate nodules devoid of a persistent meristem. Using these plants we found MsEnod12A-gusA expression only in young nodules and a disappearance in mature nodules. Moreover, when alfalfa roots were treated with auxins a lateral band of MsEnod12A expression was observed surrounding the club-shaped root apex and coinciding with induced lateral meristematic activities. Thus, in all cases MsEnod12A expression was associated with meristematic activities, suggesting that MsEnod12A plays a role in the differentiation processes of nodule and root cells and that it may serve as molecular tool for analyzing meristem establishment during nodule development.