Retinoblastoma related protein is a key negative regulator of cell division in metazoans. In plants, it controls the transition in the cell cycle from G1 to S phase, by repressing E2F transcription factors (Ebel et al., 2004). This pRB:E2F cycle is present in all higher plants.
RNAi experiments in Arabidopsis have demonstrated that RBR is a key cell cycle regulator required for coordination of cell division, differentiation, and cell homeostasis. 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. (Borghi et al., 2010, Gutzat et al., 2011, Sablowski et al., 2022).
In maize there are at least 6 RBR genes. It has been shown that the maize Retinoblastoma-related (RBR) pathway controls key aspects of endosperm development in maize. Down-regulation of RBR1 by RNAi resulted in up-regulation of RBR3-type genes, as well as the MINICHROMOSOME MAINTENANCE 2-7 gene family and PROLIFERATING CELL NUCLEAR ANTIGEN, which encode essential DNA replication factors. Both the mitotic and endoreduplication cell cycles were stimulated. Developing transgenic endosperm contained 42-58% more cells and ∼70% more DNA than wild type, whereas there was a reduction in cell and nuclear sizes. In addition, cell death was enhanced. Down-regulation of both RBR1 and CYCLIN DEPENDENT KINASE A (CDKA);1 indicated that CDKA;1 is epistatic to RBR1 and controls endoreduplication through an RBR1-dependent pathway. However, the repressive activity of RBR1 on downstream targets was independent from CDKA;1, suggesting diversification of RBR1 activities. Furthermore, RBR1 negatively regulated CDK activity, suggesting the presence of a feedback loop. These results indicate that the RBR1 pathway plays a major role in regulation of different processes during maize endosperm development and suggest the presence of tissue/organ-level regulation of endosperm/seed homeostasis (Sabelli et al., 2013, Wu et al., 2022).
Last updated June 2023 by John Gray
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Wu H, Becraft PW, Dannenhoffer JM. Maize Endosperm Development: Tissues, Cells, Molecular Regulation and Grain Quality Improvement. Front Plant Sci. 2022 Mar 7;13:852082. doi: 10.3389/fpls.2022.852082. PMID: 35330868; PMCID: PMC8940253.