Genetic dissection of plant cell-wall biosynthesis.

The plant cell wall is a complex structure consisting of a variety of polymers including cellulose, xyloglucan, xylan and polygalacturonan. Biochemical and genetic analysis has made it possible to clone genes encoding cellulose synthases (CesA). A comparison of the predicted protein sequences in the Arabidopsis genome indicates that 30 divergent genes with similarity to CesAs exist. It is possible that these cellulose synthase-like (Csl) proteins do not contribute to cellulose synthesis, but rather to the synthesis of other wall polymers. A major challenge is, therefore, to assign biological function to these genes. In an effort to address this issue we have systematically identified T-DNA or transposon insertions in 17 Arabidopsis Csls. Phenotypic characterization of "knock-out" mutants includes the determination of spectroscopic profile differences in mutant cell walls from wild-type plants by Fourier-transform IR microscopy. A more precise characterization includes cell wall fractionation followed by neutral sugar composition analysis by anionic exchange chromatography.

Farnesylation is involved in meristem organization in Arabidopsis.

Although studies in plant and animal cell culture systems indicate farnesylation is required for normal cell cycle progression, how this lipid modification of select proteins translates into whole-organism developmental decisions involving cell proliferation or differentiation is largely unknown. The era1 mutant of the higher plant Arabidopsis thaliana (L.) Heynh. offers a unique opportunity to understand the role farnesylation may play in regulating various processes during the development of a multicellular organism. Loss of farnesylation affects many aspects of Arabidopsis growth and development. In particular, apical and axillary meristem development is altered and these phenotypes are contingent on the growth conditions.

Cell cycling and cell enlargement in developing leaves of Arabidopsis.

Cell cycling plays an important role in plant development, including: (1) organ morphogenesis, (2) cell proliferation within tissues, and (3) cell differentiation. In this study we use a cyclin::beta-glucuronidase reporter construct to characterize spatial and temporal patterns of cell cycling at each of these levels during wild-type development in the model genetic organism Arabidopsis thaliana (Columbia). We show that a key morphogenetic event in leaf development, blade formation, is highly correlated with localized cell cycling at the primordium margin. However, tissue layers are established by a more diffuse distribution of cycling cells that does not directly involve the marginal zone. During leaf expansion, tissue proliferation shows a strong longitudinal gradient, with basiplastic polarity. Tissue layers differ in pattern of proliferative cell divisions: cell cycling of palisade mesophyll precursors is prolonged in comparison to that of pavement cells of the adjacent epidermal layers, and cells exit the cycle at different characteristic sizes. Cell divisions directly related to formation of stomates and of vascular tissue from their respective precursors occur throughout the period of leaf extension, so that differing tissue patterns reflect superposition of cycling related to cell differentiation on more general tissue proliferation. Our results indicate that cell cycling related to leaf morphogenesis, tissue-specific patterns of cell proliferation, and cell differentiation occurs concurrently during leaf development and suggest that unique regulatory pathways may operate at each level.

A protein farnesyl transferase involved in abscisic acid signal transduction in Arabidopsis.

The hormone abscisic acid (ABA) modulates a variety of developmental processes and responses to environmental stress in higher plants. A collection of mutations, designated era, in Arabidopsis thaliana that confer an enhanced response to exogenous ABA includes mutations in the Era1 gene, which encodes the beta subunit of a protein farnesyl transferase. In yeast and mammalian systems, farnesyl transferases modify several signal transduction proteins for membrane localization. The era1 mutants suggest that a negative regulator of ABA sensitivity must be acted on by a farnesyl transferase to function.