A FAMILY OF CYCLIN D-HOMOLOGS FROM PLANTS DIFFERENTIALLY CONTROLLED BY GROWTH-REGULATORS AND CONTAINING THE CONSERVED RETINOBLASTOMA PROTEIN-INTERACTION MOTIF

A new family of three related cyclins has been identified in Arabidops is by complementation of a yeast strain deficient in G(1) cyclins. Ind ividual members show tissue-specific expression and are conserved in o ther plant species. They form a distinctive group of plant cyclins, wh ich we named delta-type cyclins to indicate their similarities with ma mmalian D-type cyclins. The sequence relationships between delta and D cyclins include the N-terminal sequence LXCXE. This motif was origina lly identified in certain viral oncoproteins and is strongly implicate d in binding to the retinoblastoma protein pRb. By analogy to mammalia n cyclin D, these plant homologs may mediate growth and phytohormonal signals into the plant cell cycle. In support of this hypothesis, we s how that, on restimulation of suspension-cultured cells, cyclin delta 3 is rapidly induced by the plant growth regulator cytokinin and cycli n delta 2 is induced by carbon source.