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.

CYCD3;1 expression in Arabidopsis is associated with proliferating tissues such as meristems and developing leaves but not with differentiated tissues. Constitutive overexpression of CYCD3;1 increases CYCD3;1-associated kinase activity and reduces the proportion of cells in the G1-phase of the cell cycle. Moreover, CYCD3;1 overexpression leads to striking alterations in development. Leaf architecture in overexpressing plants is altered radically, with a failure to develop distinct spongy and palisade mesophyll layers. Associated with this, we observe hyperproliferation of leaf cells; in particular, the epidermis consists of large numbers of small, incompletely differentiated polygonal cells. Endoreduplication, a marker for differentiated cells that have exited from the mitotic cell cycle, is inhibited strongly in CYCD3;1 -overexpressing plants. Transcript analysis reveals an activation of putative compensatory mechanisms upon CYCD3;1 overexpression or subsequent cell cycle activation. These results demonstrate that cell cycle exit in the G1-phase is required for normal cellular differentiation processes during plant development and suggest a critical role for CYCD3 in the switch from cell proliferation to the final stages of differentiation.

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.

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.The shoot apical meristem, which gives rise to the aerial part of the plant, has classically been divided into two or more zones (Clowes, 1961). Cells in the central zone are considered to be undifferentiated and can divide indefinitely. Cell proliferation in the central zone causes the displacement of cells into the peripheral zone, where they divide more quickly (Lyndon and Robertson, 1976) and contribute to the formation of the stem and of lateral organs such as leaves, bracts, and floral organs. While most cells in the stem and mature organs eventually cease division, some tissues (e.g. vascular cambium) retain the capacity for continued proliferation throughout the lifetime of the plant. Other tissues, such as secondary meristems, may remain dormant for some time before re-activation.The molecular basis for localized differences in cell proliferation within the meristem is unclear, but could, as in other organisms, involve changes in the length of the cell division cycle. For example, the duration of the cell cycle in Drosophila is increased by the successive addition of G2 and then G1 phases (Edgar and O'Farrell, 1989). An alternative, but not exclusive, control mechanism operates later in larval development, when entry into and exit from the cycle is modulated, leading to localized differences in cell proliferation rates (Edgar and Lehner, 1996).Entry into the cell cycle in both animals and yeast is under the control of external signals which act through regulating the level of G1 cyclins (Matsushime et al., 1991; for review, see Pines, 1995). These specific G1 cyclins in turn activate cyclin-dependent protein kinases (CDKs) and thus drive cell cycle progression. Examples include the Cln genes in budding yeast and the D-cyclin genes in animals (for review, see Sherr, 1995). Plant D-cyclins were recently identified by functional complementation in a yeast strain deficient for G1 cyclins (Soni et al., 1995). On restimulation of growth-arrested plant suspension-cultured cells, transcription fro...

Studies of the interaction between Arabidopsis thaliana and the necrotrophic fungal pathogen Sclerotinia sclerotiorum have been hampered by the extreme susceptibility of this model plant to the fungus. In addition, analyses of the plant defense response suggested the implication of a complex interplay of hormonal and signaling pathways. To get a deeper insight into this host-pathogen interaction, we first analyzed the natural variation in Arabidopsis for resistance to S. sclerotiorum. The results revealed a large variation of resistance and susceptibility in Arabidopsis, with some ecotypes, such as Ws-4, Col-0, and Rbz-1, being strongly resistant, and others, such as Shahdara, Ita-0, and Cvi-0, exhibiting an extreme susceptibility. The role of different signaling pathways in resistance was then determined by assessing the symptoms of mutants affected in the perception, production, or transduction of hormonal signals after inoculation with S. sclerotiorum. This analysis led to the conclusions that i) signaling of inducible defenses is predominantly mediated by jasmonic acid and abscisic acid, influenced by ethylene, and independent of salicylic acid; and ii) nitric oxide (NO) and reactive oxygen species are important signals required for plant resistance to S. sclerotiorum. Defense gene expression analysis supported the specific role of NO in defense activation.