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Several genes involved in the regulation of postembryonic organ initiation and growth have been identified. However, it remains largely unclear how developmental cues connect to the cell cycle. RETINOBLASTOMA RELATED (RBR) is a plant homolog of the tumor suppressor Retinoblastoma (pRb), which is a key regulator of the cell cycle. Using inducible RNA interference (RNAi) against Arabidopsis thaliana RBR (RBRi), we reduced RBR expression levels at different stages of plant development. Conditional reduction or loss of RBR function disrupted cell division patterns, promoted context-dependent cell proliferation, and negatively influenced establishment of cell differentiation. Several lineages of toti-and pluripotent cells, including shoot apical meristem stem cells, meristemoid mother cells, and procambial cells, failed to produce appropriately differentiated cells. Meristem activity was altered, leading to a disruption of the CLAVATA-WUSCHEL feedback loop and inhibition of lateral organ formation. Release of RBR from RNAi downregulation restored meristem activity. Gene profiling analyses soon after RBRi induction revealed that a change in RBR homeostasis is perceived as a stress, even before genes regulated by RBR-E2F become deregulated. The results establish RBR as a key cell cycle regulator required for coordination of cell division, differentiation, and cell homeostasis. INTRODUCTIONRetinoblastoma (RB) was the first tumor suppressor gene identified in animals (Friend et al., 1986) and later was associated with cell cycle regulation, thus establishing pRb as a key regulator of cell proliferation. In animals, three members of the RB family of proteins (pRb, p107, and p130), also referred to as pocket proteins (Mulligan and Jacks, 1998), negatively regulate the G1-to-S phase transition during the cell cycle. It is now widely accepted that mitogenic signals activate several cyclin-dependent kinase (CDK)-cyclin complexes during progression through G1 to phosphorylate pRb, thereby releasing it from interactions with E2F/DP transcription factor complexes to facilitate S-phase entry (Weinberg, 1995). Subsequently, pRb was also found to regulate tissue-specific transcription factors that regulate cell differentiation, such as MyoD, which activates muscle-specific genes (De Falco et al., 2006). pRb also regulates germ cell proliferation, differentiation, and survival (Toppari et al., 2003) and is required for osteoblast, keratinocyte, and macrophage differentiation (Nead et al., 1998;Liu et al., 1999;Thomas et al., 2003). Together with the tumor suppressor p53, pRb regulates adipocyte differentiation and function (Hallenborg et al., 2009). The requirement for RB in cell differentiation therefore extends beyond regulation of cell cycle entry. For example, the loss of RB function in cardioblasts affects cell differentiation by modulating the activity of cardiogenic factors (Papadimou et al., 2005).RB-related genes, termed RETINOBLASTOMA-RELATED (RBR; Ach et al., 1997), are also found in monocots (Grafi et al., 1996;Xie et al...

The ability of plant cells to translate dicistronic mRNAs that mimic a segment of the polycistronic 35S RNA from cauliflower mosaic virus has been tested. The chloramphenicol acetyltransferase and beta-glucuronidase open reading frames (ORFs) were fused in-frame to the second viral cistron (ORF I). Efficient reporter expression from the corresponding plasmids in plant protoplasts was observed only upon cotransfection with viral DNA. The trans-activating gene maps at ORF VI, which is expressed from a separate, monocistronic messenger (19S RNA). Deletion analysis shows that trans-activation selectively enhances downstream gene expression; the high expression of the upstream ORF is not further increased. The major reporter transcript remained bicistronic upon trans-activation, and its abundance varied only to a limited extent. Results indicate that trans-activation enhances the translation of downstream ORFs on polycistronic mRNAs derived from cauliflower mosaic virus.

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SUMMARYSeedling establishment is a crucial phase during plant development when the germinating heterotrophic embryo switches to autotrophic growth and development. Positive regulators of embryonic development need to be turned off, while the cell cycle machinery is activated to allow cell cycle entry and organ primordia initiation. However, it is not yet understood how the molecular mechanisms responsible for the onset of cell division, metabolism changes and cell differentiation are coordinated during this transition. Here, we demonstrate that the Arabidopsis thaliana RETINOBLASTOMA-RELATED protein (RBR) ortholog of the animal tumor suppressor retinoblastoma (pRB) not only controls the expression of cell cycle-related genes, but is also required for persistent shut-down of late embryonic genes by increasing their histone H3K27 trimethylation. Seedlings with reduced RBR function arrest development after germination, and stimulation with low amounts of sucrose induces transcription of late embryonic genes and causes ectopic cell division. Our results suggest a model in which RBR acts antagonistically to sucrose by negatively regulating the cell cycle and repressing embryonic genes. Thus, RBR is a positive regulator of the developmental switch from embryonic heterotrophic growth to autotrophic growth. This establishes RBR as a new integrator of metabolic and developmental decisions.