Cell cycling and cell enlargement in developing leaves of Arabidopsis

Cell cycling plays an important role in plant development, including: (1) o rgan morphogenesis, (2) cell proliferation within tissues, and (3) cell dif ferentiation. In this study we use a cyclin::beta-glucuronidase reporter co nstruct to characterize spatial and temporal patterns of cell cycling at ea ch of these levels during wild-type development in the model genetic organi sm Arabidopsis thaliana (Columbia). We show that a key morphogenetic event in leaf development, blade formation, is highly correlated with localized c ell cycling at the primordium margin. However, tissue layers are establishe d by a more diffuse distribution of cycling cells that does not directly in volve the marginal zone. During leaf expansion, tissue proliferation shows a strong longitudinal gradient, with basiplastic polarity. Tissue layers di ffer in pattern of proliferative cell divisions: cell cycling of palisade m esophyll precursors is prolonged in comparison to that of pavement cells of the adjacent epidermal layers, and cells exit the cycle at different chara cteristic sizes. Cell divisions directly related to formation of stomates a nd of vascular tissue from their respective precursors occur throughout the period of leaf extension, so that differing tissue patterns reflect superp osition of cycling related to cell differentiation on more general tissue p roliferation. Our results indicate that cell cycling related to leaf morpho genesis, tissue-specific patterns of cell proliferation, and cell different iation occurs concurrently during leaf development and suggest that unique regulatory pathways may operate at each level. (C) 1999 Academic Press.