Rhizobium meliloti can interact symbiotically with Medicago plants thereby inducing the formation of root nodules. Screening of a young nodule cDNA library led to the isolation of a cDNA from Medicago sativa, Msgbl, that comprises a new member of the RACK1 (Receptor of Activated C Kinase) subfamily of WD-repeat proteins. This subfamily shows horology to the β-subunit of heterotrimeric G proteins. Besides RACK1, this subfamily contains several plant genes including the well characterized auxin-inducible ArcA of tobacco. The Msgbl gene is strongly expressed in young embryos and in leaves and is induced upon cytokinin treatment of roots. Whereas northern analysis failed to reveal differences in expression between total RNA from roots and nodules, in situ hybridization demonstrated that the transcript was most abundant in dividing cells of nodule primordia and in the nodule meristem. Msgbl may be related to the signal transduction acting in response to hormone-mediated cell division.
Bibliographical noteFunding Information:
The authors wish to thank Petra Bauer for providing the RNA samples from hormone-treated roots. Jacob Seel-er is gratefully acknowledged for his aid in computer-related matters, including protein structure predictions. H.I.M. was supported by a National Science Foundation (USA) post-doctoral fellowship in Plant Science, F.F. was the recipient of a fellowship of the Min-istère franc¸ais de l’Enseignement Superieur et de la Recherche, G.P. was supported within the framework of the Jumelage Program between the Gif and Szeged laboratories, T.C. was the recipient of a fellowship of the Caixa Galicia Foundation, Spain, and E.J. was the recipient of an EMBO long-term postdoctoral fellowship.
- G-protein β-subunit
- RACK1 subfamily
- WD-repeat proteins