Retinoblastoma protein goes green: the role of RBR in Arabidopsis meiosis

In 1986, retinoblastoma was the first identified tumour suppressor gene in animals (Friend et al, 1986). The protein pRB was then recognized as a key negative regulator of cell cycle progression controlling the G1/S transition. Upon phosphorylation by CDK-cyclin complexes, pRB dissociates from bound E2F transcription factors, which in turn are then free to regulate expression of a variety of genes needed for cell cycle progression and differentiation. Later, pRB has been found to play a critical role in chromosome condensation, centromeric function and chromosome stability (reviewed in Sage and Straight (2010)). These findings stress pRB's importance as a node for mediating CDK-cyclin-derived signals to various downstream processes. The mode of operation may be indirect, via the regulation of transcription of E2F-dependent genes, or direct, via binding and sequestration of, e. g., chromatin modifiers like histone methyl transferases or DNA repair-related proteins like BRCA1 (Aprelikova et al, 1999; Sage and Straight, 2010). pRB is conserved and can also be found in higher plants. Arabidopsis contains only a single retinoblastoma-related gene (RBR), whereas pRB has two more relatives (p107 and p130) in animals. So far it has not been possible to study plants homozygous for a RBR loss-of-function allele, as the protein is required for both female and male gametophyte development (Ebel et al, 2004). This issue of The EMBO Journal features an article by Chen et al (2011) highlighting novel functions of the plant retinoblastoma protein, taking advantage of a genetic rescue strategy to grow plants homozygous for a hypomorphic rbr mutant allele. RBR has been studied in plants before concerning its roles in stem cell maintenance, cell differentiation, organ production and gametophyte development (Ebel et al, 2004; Park et al, 2005; Borghi et al, 2010), but the study by Chen and colleagues is the first to unravel a role of pRB in meiosis.