Control of cell proliferation, endoreduplication, cell size, and cell death by the retinoblastoma-related pathway in maize endosperm

Abstract

The endosperm of cereal grains is one of the most valuable products of modern agriculture. Cereal endosperm development comprises different phases characterized by mitotic cell proliferation, endoreduplication, the accumulation of storage compounds, and programmed cell death. Although manipulation of these processes could maximize grain yield, how they are regulated and integrated is poorly understood. We show that the 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 MAINTE-NANCE 2-7 gene family and PROLIFERATING CELL NUCLEAR ANTI-GEN, 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. The DNA content of mature endosperm increased 43% upon RBR1 downregulation, whereas storage protein content and kernel weight were essentially not affected. 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/organlevel regulation of endosperm/seed homeostasis. seed development | endocycle T he seed endosperm is a triploid tissue resulting from the fusion of one haploid sperm nucleus with the diploid central cell nucleus within the female gametophyte. Development of the endosperm in flowering plants is characterized by acytokinetic mitoses of the primary endosperm nucleus, resulting in a syncytium, cellularization of syncytial nuclear domains, and cell proliferation through mitotic activity that is coupled to cell division (1, 2). Additionally, in the Poaceae (grass) family, the endosperm undergoes a rapid growth phase that coincides with accumulation of storage compounds, such as starch and storage proteins, during a specialized type of cell cycle known as endoreduplication. Endoreduplication is characterized by one or more rounds of DNA synthesis in the absence of mitosis, resulting in polyploid cells (3-5). Endoreduplication is highly correlated with cell size in many plant and animal tissues, but its role in endosperm development has not been established. Upon completion of endoreduplication and storage metabolite synthesis, cereal endosperm cells undergo programmed cell death (PCD), resulting in extensive DNA degradation (5, 6). In maize (Zea mays L.), endosperm cells transition from a mitotic to an endore...

Drought Induces Distinct Growth Response, Protection, and Recovery Mechanisms in the Maize Leaf Growth Zone

“…S2). (Sabelli et al, 2013). Related to that, we found seven minichromosome maintenance transcripts that control DNA duplication (Chong et al, 1996;Aparicio et al, 1997), which are known targets of E2F transcription factors (Vandepoele et al, 2005), and four transcripts of genes encoding subunits of the anaphase-promoting complex (APC) that controls the cell cycle at M-phase exit (Eloy et al, 2006).…”

Zygotic Genome Activation Occurs Shortly after Fertilization in Maize

“…Furthermore, retinoblastoma-related protein genes RBR1 and RBR2 were highly expressed in sperm cells ( Figure 6A, SP versus all: log 2 FC > 2.9*). RBR1 and RBR2 mediate G1-phase arrest by inhibiting E2F TFs, which in turn promote DNA replication (Sabelli et al, 2013). Compared with the other cell types, egg cells showed by far the lowest expression levels of cell cycle genes and lacked a typical cell cycle phase-specific gene expression pattern.…”

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“…At cell and tissue levels, several investigations have characterized specific physiological processes during kernel development (Borrás and Gambín 2010;Ettenhuber et al 2005;Sabelli et al 2013;Young and Gallie 2000) and identified the influence of water deficit (Ober et al 1991;Setter and Flannigan 2001) and genotype (Jones et al 1996) on this process. At the plant canopy level, a number of studies have described the special physiological process (Blewley and Black 1985), source-sink interactions (Borrás et al 2004;Gambin et al 2006;Jones and Simmons 1983), kernel growth-plant growth interactions (Gambin et al 2008), and the effect of dynamic water content during kernel development (Borrás and Westgate 2006;Westgate 1994;Westgate and Boyer 1986).…”