Molecular genetic analysis of the internodal growth of arabidopsis thaliana

In higher plants, the process by which cells expand by elongation is critical for the highly polarized growth of organs. Numerous physiological studies have revealed a series of events during cell enlargement. Briefly, the first step of cell elongation is the establishment and maintenance of cell polarity, in which cytoskeletal elements may play an important role. Subsequently, cell elongation is driven by water uptake in combination with cell-wall loosening. This step is accompanied by deposition of new cell-wall materials and a large increase in vacuole volume. In most cases, the process of cell elongation is regulated by phytohormones or other external stimuli. However, the molecular components that play a direct and critical role in the process of cell elongation have not been clearly identified. The primary plant body is formed by the root and shoot apical meristems that are embryonic throughout life. During the vegetative stage, cell division and development within the shoot apical meristem form leaf primordia in such rapid succession that the stem (shoot axis) cannot divide into nodes and internodes. Later, internodes begin to develop between the nodes by intercalary growth. The duration of internodal elongation depends on the plant species, environmental conditions, and the type of stem. In Arabidopsis, the internodes are practically undeveloped between leaf-bearing nodes, resulting in a rosette form of leaves. After transition to the reproductive stage, flower bud formation is closely linked with internodal elongation between the nodes that bear flowers or paraclades (axillary flowering shoots). This implies the existence of a specific developmental program of internodal elongation at the reproductive stage in rosette type of plants. One of our major interests is to understand the molecular mechanism by which the length of internodal cells is regulated. In an effort to identify genes that are specifically involved in the inflorescence development of Arabidopsis, we have isolated one mutant with flowering stems of greatly reduced length, named acaulis5 (acl5). Unlike previously described mutants with reduced size of organs, the acl5 mutant has severe defect that is restricted to the process of cell elongation after transition to the reproductive stage and shows no phenotype before flowering. The results of RNA blot hybridizations showed that the acl5 mutation causes a striking reduction in the transcript levels of genes encoding the tonoplast intrinsic protein (γ-TIP) and the endoxyloglucan transferase (EXGT-A1), both of which have recently been suggested to be important for cell elongation. Morphological study indicated that the mutation also causes proliferative arrest of the apical inflorescence meristem. These results strongly suggest that, during the reproductive phase, the wild-type ACL5 gene product may play a critical role in the internodal growth and in the continued maintenance of flowerproducing activity of inflorescence meristems. The ACL5 locus has been mapped on chromosome 5 and a chromosome walk to isolate the gene is currently in progress. We are also interested in identifying molecules involved in the development of inflorescences. The erecta (er) mutation of Arabidopsis leads to a compact inflorescence with flowers clustering at the tip and blunt siliques. Using a transgenic approach, we generated Arabidopsis plants showing er phenotype by the antisense expression of the ER gene. In transgenic lines carrying the ER promoter-GUS reporter fusion gene, GUS activity was histochemically detected in shoot apical meristems, Microscopic observation revealed that mutant cells of pedicels were much less in number but larger in size than wild-type cells. These data suggest that the ER gene product may be involved in the regulatory pathway of the cell proliferation in shoot apices. We further isolated erecta-like mutants, named corymbosa (crm). These were named after plants with a corymbinflorescence and defined by at least two loci, crm1 and crm2. Characterization of these mutants will hell elucidate the mechanisms determining the morphology of inflorescences.