Role of DISTORTED2, GNARLED and SPIRRIG in cell morphogenesis of Arabidopsis thaliana.

The leaf epidermal trichome of Arabidopsis thaliana is an ideal model system to study plant cell architecture due to its many mutants featuring distorted phenotypes. In this thesis, the DISTORTED2 (DIS2), GNARLED (GRL) and SPIRRIG (SPI) genes were cloned and characterized. Both, the morphological as well as subcellular mutant phenotypes were assessed and discussed. The dis2 mutant displayed a pronounced distorted trichome phenotype, by having stubbed and swollen trichomes. The DIS2 gene was cloned by a gene candidate approach and encoded the ARPC2 subunit of the actin nucleating protein 2 and 3 (ARP2/3) complex. Loss of function of the protein disclosed an aberrant actin cytoskeleton, when compared to wild type, and thus also featured an altered microtubule cytoskeleton. Based on the actin and microtubule density in the mutant and wild type, presumptions could be made of how local outgrowth of cells might be restricted to a defined area. A similar trichome phenotype was observed in grl mutants. The GRL gene was identified through a gene candidate approach, revealing several mutations in the GRL gene. Protein homology analysis uncovered GRL as a homolog of the animal NAP125 protein. NAP125 is known to regulate the ARP2/3 complex. Phenotypical morphological analysis showed that grl exhibits the same plant defects, like pavement cell aberrations, hypocotyl cell and root hair defects, as observed in other distorted mutants. The cloning and identification of GRL suggested that the regulatory machinery to control the actin cytoskeleton is conserved in animals and plants. The spi mutant disclosed similar plant defects as described above, but the trichome alterations were more subtle and the root hairs were shorter than those of wild type. Map based cloning attempts, gene candidate approaches and sequencing of several spi mutant alleles led to the identification of SPI. The SPI gene encoded a 3600 amino acids long protein, with seven transmembrane-, one BEACH- and four WD40- domains and is thus a novel protein in determining cell shape. Based on the spi phenotype as well as on the protein domains, mentioned above, a function in membrane trafficking could be suggested