Analysis of regulatory networks underlying morphogenesis of leaf epidermis pavement cells

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The development of multicellular organisms from a fertilized egg cell to complex organisms with specialized tissues and organs requires precise coordination of growth. The basic processes that drive growth are cell division and cell expansion, which generate new cells and increase the size of individual cells, respectively. During cell expansion, cells acquire various distinct shapes to fulfill their specialized functions (cell differentiation). In plants, cell expansion is controlled and restricted by a rigid cell wall that surrounds individual cells outside of the plasma membrane and physically glues neighboring cells. Deposition of cell wall material is mediated by the plant cytoskeleton, comprised of microtubules and actin filaments, which form a highly dynamic intracellular network. The concerted activity of the cell wall and the cytoskeleton is coordinated by complex signaling networks that integrate endogenous and external signals for precise regulation of growth. Leaf epidermis pavement cells form one of the most complex cell shapes in plants with multiple lobes and indentations in their anticlinal walls. In addition to their importance for leaf growth and their protective role for photosynthetically active tissues, pavement cells thus are a popular and suitable model system to study multipolar growth in the context of a tissue of mechanically coupled cells. Genetic and pharmaceutical studies, mostly in the model plant Arabidopsis thaliana, have identified several factors that control pavement cell morphogenesis. The precise mechanisms and principles underlying shape formation, however, remain largely enigmatic. In our previous work we identified a member of a plant-specific class of microtubule-associated proteins, IQ67 DOMAIN5 (IQD5), as novel regulator of pavement cell shape and cell wall composition, which likely integrates calcium signaling at the microtubule cytoskeleton. In addition, we have developed a software-tool, which for the first time enables fully automatic detection and comparative quantitative analysis of pavement cell shape from microscopy images. Our first analyses of pavement cell shape in naturally occurring Arabidopsis ecotypes revealed large intraspecific variation that provide a suitable basis to identify associated genetic loci. Within the proposed project we aim to functionally characterize the role of IQD5 in regulation of microtubule organization and in cellular signaling pathways, using cell biology and reverse-genetics approaches. In addition, we will use forward-genetics approaches to identify novel regulators by linking genotypic and phenotypic variation within Arabidopsis accessions and by screening of mutant collections. Collectively, the combined analysis of pavement cell shape regulation by reverse- and forward-genetics approaches will provide a suitable framework to investigate mechanisms of cell morphogenesis and will contribute to a more holistic understanding of the underlying principles.