CLONING OF A WD-REPEAT-CONTAINING GENE FROM ALFALFA (MEDICAGO-SATIVA)- A ROLE IN HORMONE-MEDIATED CELL-DIVISION

Rhizobium meliloti can interact symbiotically with Medicago plants the reby inducing the formation of root nodules. Screening of a young nodu le cDNA library led to the isolation of a cDNA from Medica,oo sativa, Msgbl, that comprises a new member of the RACK1 (Receptor of Activated C Kinase) subfamily of WD-repeat proteins. This subfamily shows homol ogy to the beta-subunit of heterotrimeric G proteins. Besides RACK1, t his subfamily contains several plant genes including the well characte rized auxin-inducible ArcA of tobacco. The Msgbl gene is strongly expr essed in young embryos and in leaves, and is induced upon cytokinin tr eatment of roots. Whereas northern analysis failed to reveal differenc es in expression between total RNA from roots and nodules, in situ hyb ridization demonstrated that the transcript was most abundant in divid ing cells of nodule primordia and in the nodule meristem. Msgbl may be related to the signal transduction acting in response to hormone-medi ated cell division.