ArabidopsisRETINOBLASTOMA-RELATED Is Required for Stem Cell Maintenance, Cell Differentiation, and Lateral Organ Production  

Abstract

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...

AUXIN BINDING PROTEIN1: The Outsider

“…Similar approaches in Arabidopsis and tobacco confirmed that several promotive cell cycle components, such as cyclins CYCD3 or CYCD6, are downregulated in response to ABP1 knock-down and with corresponding phenotypes (Braun et al, 2008;Tromas et al, 2009). ABP1 downregulation also led to an increase in RETINOBLASTOMA RELATED (RBR) expression, which negatively affects cell division and has recently been related to cell differentiation (Borghi et al, 2010). Overexpression of CYCD3;1 or downregulation of RBR in ABP1 knock-down plants partially restored the cell proliferation defects (Tromas et al, 2009), indicating that downregulation of ABP1 affects the cell cycle via the CYCD/RBR pathway.…”

Defects in a New Class of Sulfate/Anion Transporter Link Sulfur Acclimation Responses to Intracellular Glutathione Levels and Cell Cycle Control

“…Subsequently, E2F/DP-dependent transcription of cell cycle genes promotes S phase entry and cell cycle progression. Because of its central role in controlling the transcription of cell cycle genes, RB serves as a convergence point for regulating the cell cycle in response to internal and external mitogenic signals (Nakagami et al, 2002;Stevaux et al, 2002;Cobrinik, 2005;Dimova and Dyson, 2005;Wikenheiser-Brokamp, 2006;Jullien et al, 2008;van den Heuvel and Dyson, 2008;Borghi et al, 2010;Henriques et al, 2010;Johnston et al, 2010;Chen et al, 2011;Gutzat et al, 2011Gutzat et al, , 2012Weimer et al, 2012). Recent studies also provide evidence that RB depletion causes metabolic reprogramming and suggest that the RB pathway in animals exerts part of its effect on cell proliferation through the control of Gln metabolism (Nicolay et al, 2013;Reynolds et al, 2014).…”

PhyscomitrellaCyclin-Dependent Kinase A Links Cell Cycle Reactivation to Other Cellular Changes during Reprogramming of Leaf Cells

“…This coordination appears to depend at least in part on certain cell cycle regulators themselves. For example, in Arabidopsis thaliana, reduction of RBR expression levels in the shoot meristem caused diminished expression of CLAVATA3 and WUSCHEL regulating stem cell identity (Borghi et al, 2010). Similarly, the induction of a kinase-negative form of CDKA in the shoot meristem caused a portion of the meristem cells to expand and exhibit endoreduplication and thus resemble differentiated cells (Gaamouche et al, 2010).…”