A family of cyclin D homologs from plants differentially controlled by growth regulators and containing the conserved retinoblastoma protein interaction motif

Abstract

A new family of three related cyclins has been identified in Arabidopsis by complementation of a yeast strain deficient in G1 cyclins. Individual members show tissue-specific expression and are conserved in other plant species. They form a distinctive group of plant cyclins, which we named δ-type cyclins to indicate their similarities with mammalian D-type cyclins. The sequence relationships between δ and D cyclins include the N-terminal sequence LXCXE. This motif was originally identified in certain viral oncoproteins and is strongly implicated in binding to the retinolastoma preotein pRb. By analogy to mammalian cyclin D, these plant homologs may mediate growth and phytohormonal signals into the plant cell cycle. In support of this hypothesis, we show that, on restimulation of suspension-cultured cells, cyclin δ3 is rapidly induced by the plant growth regulator cytokinin and cyclin δ2 is induced by carbon source.