RESUMO
Transgenic tobacco lines expressing Arath-CYCD2 or Arath-CYCD3 genes under a cauliflower mosaic virus 35S promoter are modified in the timing of their development, but not in the phenotype of their vegetative organs. They display an increased rate of leaf initiation, which is shown to be associated with distinct changes in the structural organization of their shoot apical meristem (SAM). Constitutive expression of Arath-CYCD2 leads to a progressive modification of the SAM structural organization with predominant periclinal divisions in the L3 layer and to the loss of the classical cytophysiological zonation, the central zone being reduced to the central cells of the L1 and L2 layers. These changes reveal a particular sensitivity of the corpus cells (L3) to Arath-CYCD2 over-expression and suggest a role for CYCD2 in controlling the planes of cell division in these cells. The SAM structural modifications in the Arath-CYCD3 over-expressing lines are less drastic; only an increased cell number together with a reduced cell size, particularly in the L1 layer, characterizes the peripheral zones. This could be related to the shortening of the G1-phase duration that renders cell growth incomplete between successive mitoses. Cell proliferation continues beyond the SAM in the developing internodes and confers a delayed senescence to Arath-CYCD3 over-expressing juvenile tissues. In addition, the ploidy levels of mature stem tissues in both types of transgenic lines are unaffected, suggesting that the studied G1 to S cell-cycle genes have no effect on the extent of endoreduplication in tobacco stem tissues.
Assuntos
Proteínas de Arabidopsis/fisiologia , Ciclinas/fisiologia , Meristema/crescimento & desenvolvimento , Nicotiana/crescimento & desenvolvimento , Nicotiana/genética , Proteínas de Arabidopsis/genética , Ciclo Celular/fisiologia , Ciclinas/genética , Expressão Gênica , Meristema/genética , Mitose/fisiologia , Folhas de Planta/genética , Folhas de Planta/crescimento & desenvolvimento , Caules de Planta/genética , Plantas Geneticamente Modificadas , PloidiasRESUMO
In most plants, sucrose is the major transported carbon source. Carbon source availability in the form of sucrose is likely to be a major determinant of cell division, and mechanisms must exist for sensing sugar levels and mediating appropriate control of the cell cycle. We show that sugar availability plays a major role during the G(1) phase by controlling the expression of CycD cyclins in Arabidopsis. CycD2 mRNA levels increase within 30 min of the addition of sucrose; CycD3 is induced after 4 h. This corresponds to induction of CycD2 expression early in G(1) and CycD3 expression in late G(1) near the S-phase boundary. CycD2 and CycD3 induction is independent both of progression to a specific point in the cell cycle and of protein synthesis. Protein kinase activity of CycD2- and CycD3-containing cyclin-dependent kinases is consistent with the observed regulation of their mRNA levels. CycD2 and CycD3 therefore act as direct mediators of the presence of sugar in cell cycle commitment. CycD3, but not CycD2, expression responds to hormones, for which we show that the presence of sugars is required. Finally, protein phosphatases are shown to be involved in regulating CycD2 and CycD3 induction. We propose that control of CycD2 and CycD3 by sucrose forms part of cell cycle control in response to cellular carbohydrate status.
Assuntos
Proteínas de Arabidopsis , Arabidopsis/citologia , Arabidopsis/genética , Metabolismo dos Carboidratos , Ciclo Celular/genética , Ciclinas/genética , Arabidopsis/metabolismo , Carbono/metabolismo , Divisão Celular/genética , Ciclina D3 , Ciclinas/metabolismo , Fase G1/genética , Regulação da Expressão Gênica de Plantas , Fosfoproteínas Fosfatases/metabolismo , Reguladores de Crescimento de Plantas/metabolismo , Sementes/metabolismo , Sacarose/metabolismoRESUMO
Three D-cyclin genes are expressed in the apical meristems of snapdragon (Antirrhinum majus). The cyclin D1 and D3b genes are expressed throughout meristems, whereas cyclin D3a is restricted to the peripheral region of the meristem, especially the organ primordia. During floral development, cyclin D3b expression is: (a) locally modulated in the cells immediately surrounding the base of organ primordia, defining a zone between lateral organs that may act as a developmental boundary; (b) locally modulated in the ventral petals during petal folding; and (c) is specifically repressed in the dorsal stamen by the cycloidea gene. Expression of both cyclin D3 genes is reduced prior to the cessation of cell cycle activity, as judged by histone H4 expression. Expression of all three D-cyclin genes is modulated by factors that regulate plant growth, particularly sucrose and cytokinin. These observations may provide a molecular basis for understanding the local regulation of cell proliferation during plant growth and development.
Assuntos
Ciclinas/genética , Regulação da Expressão Gênica no Desenvolvimento/genética , Regulação da Expressão Gênica de Plantas/genética , Meristema/metabolismo , Proteínas de Plantas/genética , Plantas/genética , Sequência de Aminoácidos , Sequência de Bases , Ciclo Celular , Divisão Celular/genética , Ciclina D , Ciclinas/química , Primers do DNA , Proteínas de Ligação a DNA , Dados de Sequência Molecular , Células Vegetais , Homologia de Sequência de Aminoácidos , Fatores de TranscriçãoRESUMO
Cytokinins are plant hormones that regulate plant cell division. The D-type cyclin CycD3 was found to be elevated in a mutant of Arabidopsis with a high level of cytokinin and to be rapidly induced by cytokinin application in both cell cultures and whole plants. Constitutive expression of CycD3 in transgenic plants allowed induction and maintenance of cell division in the absence of exogenous cytokinin. Results suggest that cytokinin activates Arabidopsis cell division through induction of CycD3 at the G1-S cell cycle phase transition.
