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

The euAP1 protein MPF3 represses MPF2 to specify floral calyx identity and displays crucial roles in Chinese Lantern development in Physalis

The Chinese lantern phenotype or inflated calyx syndrome (ICS) is a postfloral morphological novelty in Physalis. Its origin is associated with the heterotopic expression of the MADS box gene 2 from Physalis floridana (MPF2) in floral organs, yet the process underlying its identity remains elusive. Here, we show that MPF3, which is expressed specifically in floral tissues, encodes a core eudicot APETALA1-like (euAP1) MADS-domain protein. MPF3 was primarily localized to the nucleus, and it interacted with MPF2 and some floral MADS-domain proteins to selectively bind the CC-A-rich-GG (CArG) boxes in the MPF2 promoter. Downregulating MPF3 resulted in a dramatic elevation in MPF2 in the calyces and androecium, leading to enlarged and leaf-like floral calyces; however, the postfloral lantern was smaller and deformed. Starch accumulation in pollen was blocked. MPF3 MPF2 double knockdowns showed normal floral calyces and more mature pollen than those found in plants in which either MPF3 or MPF2 was downregulated. Therefore, MPF3 specifies calyx identity and regulates ICS formation and male fertility through interactions with MPF2/MPF2. Furthermore, both genes were found to activate Physalis floridana invertase gene 4 homolog, which encodes an invertase cleaving Suc, a putative key gene in sugar partitioning. The novel role of the MPF3-MPF2 regulatory circuit in male fertility is integral to the origin of ICS. Our results shed light on the evolution and development of ICS in Physalis and on the functional evolution of euAP1s in angiosperms