Retinoblastoma proteins in plants

The retinoblastoma protein Rb is part of a conserved pathway that controls the activation of cell division in animals. Rb represses cell cycle transcription factors of the E2F family, and thereby prevents uncontrolled cell proliferation. Rb itself is inactivated when phosphorylated by cyclin-dependent kinases, and the D-type cyclin kinases are particularly important in this process during the reactivation of cell division in quiescent cells. In addition, Rb has important developmental roles in controlling the onset of cellular differentiation in a number of cell types. The recent discovery in plants of both Rb proteins and other components of the Rb pathway suggests that, far from being restricted to the animal kingdom, Rb may have a conserved role in allowing multicellular organisms to develop complex body plans consisting of many different cell types. This review assesses the potential roles of Rb proteins in plant cell cycle control and development.