Molecular analysis of cell expansion in Arapidopsis thaliana

Research project P 14477 Molecular Analysis of Cell Expansion in Arabidopsis Thaliana Marie-Theres HAUSER 26.6.2000 During the past years Arabidopsis thaliana has been widely used as model organism in advancing our understanding of the biology of plants. The most exciting progress in the regulation of plant development has been made by the identification of informative mutations in order to assign functions to genes that play a specific role in plant growth and development. A major challenge in the study of plant development is to unravel the molecular basis of morphogenesis. Since cell division and expansion are the major parameters of morphogenesis, their temporal and spatial regulation leads to the characteristic form of plant organs. Although cell expansion is fundamental to establish cell size, shape and polarity, little is known about the molecular mechanisms which control the orientation and the extent of this morphogenic parameter. Using Arabidopsis thaliana, mutants with a root and hypocotyl defect in cell expansion were identified by the applicant. 22 of them are alleles of pom1 and all fail to control the extent and orientation of cell expansion of the two root tissues, epidermis and cortex. Moreover, in pom1 mutants the cell volume of the affected root tissues is significantly greater than that of wild type cells which demonstrates its role in regulating the extent of cell expansion. The defect in root and hypocotyl cell expansion can be modulated by different growth conditions. Hence, pom1 mutants belong to the group of conditional expansion mutants. This project aims to characterise the POM1 gene expression on a transcriptional and translational level. Furthermore, the subcellular localisation of the POM1 gene product will be studied in detail. It is expected that different growth conditions may alter the POM1 activity. The proposed experiments aim to differentiate between a transcriptional, translational or post-translational regulation. To isolate further components of the regulatory pathway of cell expansion we will clone an enhancer of pom1 - the EPO gene - which we isolated in a previous project. Genetic and phenotypic analyses indicate that EPO substitutes for a part of the POM1 function and that the POM1 gene counterbalances the function of the mutated EPO gene. Characterisation of the two cell expansion regulators will be a major contribution toward the understanding of the organ and cell type specific cell volume and polarity control.

Morphogenesis involves complex developmental processes leading to the characteristic form of cells, tissues, organs and organisms and is a prerequisite for their functionality. Plant cells are surrounded and firmly attached to each other by a rigid cell wall preventing cell migration during development. Hence, the position of a given cell is the result of controlled cell divisions and the polarity and extent of cell expansion determine the final form of a plant and its organs. Hence, a major challenge in the study of plant development and in particular morphogenesis is to unravel the molecular mechanisms of the temporal and spatial control of cell division and expansion. Since several years we search for the molecular mechanisms responsible for root morphogenesis of the model plant Arabidopsis thaliana (thale cress). In preparatory investigations we have identified several genetic loci which were involved in the regulation of cell expansion in roots. In this project and in collaboration with other international research groups we have isolated and characterized several of these genes. Most of them are directly or indirectly involved in the synthesis of cellulose, the major component of plant cell walls and the most frequent renewable and degradable macromolecule. Another class of genes regulating cell expansion is involved in the regulation of the cytoskeleton and is important for the last step of cell division - cytokinesis. During cytokinesis of plants a new cell wall is established between the separated daughter nuclei and this process in accomplished by a particular cytoskeletal structure - the phragmoplast. Thus the result of the project did not only identified and functionally characterize new regulators of cellulose synthesis but also an evolutionary conserved component of the cytoskeleton. Parts of the results are already published in highly ranked international journals.