Cell wall mediated regulation of plant cell morphogenesis : pectin esterification and cellulose crystallinity

Cellular morphogenesis is a fundamental underpinning of development. All cells in the plant kingdom are surrounded by walls that govern shape formation. This extracellular matrix is composed mainly of polysaccharides. How these polysaccharides are modified during cellular development to regulate cell expansion, and thus cell shape, must be understood to link form with function. In plants, the mechanical aspect of cell expansion is known to be mainly influenced by cellulose, the most abundant biopolymer on Earth. Because cellulose microfibrils exhibit a strong tensile strength along their long axis, they may be used to control the expansion process by conferring mechanical properties to the cell wall material that determine the directionality and the magnitude of expansive growth at subcellular level. Another wall polymer that may influence cell expansion is homogalacturonan pectin, the most abundant type of pectin in the primary wall. The spatial distribution and esterification status of homogalacturonan pectin may affect the mechanical aspects of the wall and, therefore, the expansion pattern. I used a genetic approach combined with novel biochemical, biomechanical and imaging strategies to study the impact of the spatio-temporal dynamics of cellulose and homogalacturonan pectin during cell expansion and shape formation. I investigated cell shape formation in two differently shaped types of epidermal cells: those of the cotyledon and of the hypocotyl of Arabidopsis thaliana. I show that undulation formation in pavement cells of the cotyledon requires spatial and temporal changes of cellulose microfibrils and demethyl-esterified pectin. These changes regulate the mechanical stiffness of the periclinal wall at two different stages: lobe initiation and subsequent expansion. Lobe initiation involves an increase in the stiffness of the periclinal wall at the prospective neck region of the undulation through a local accumulation of demethyl-esterified pectin. The subsequent expansion is controlled by the degree of cellulose crystallinity and the perpendicular alignment of the microfibrils at the tangent of the neck side of the undulation at the periclinal wall. During the elongation process and the anisotropic expansion of the epidermal hypocotyl cells, cellulose and homogalacturonan pectin make distinct contributions in each developmental phase of the elongation. During the first developmental phase, reduction in the proportion of demethyl-esterified pectin decreases the wall stiffness and accelerates the elongation. A reduction in the cellulose crystallinity decreases the elongation of the hypocotyl at the second developmental phase. It may be concluded from the two cell systems that cellulose, contrary to a long-established hypothesis, may not be essential for the initiation of morphogenetic events and their function may be reassigned to the feedback-mediated augmentation of cell shaping processes. Moreover, stress-induced shape formation plays a key role during the initiating steps and it is likely to be dominated by the degree of pectin esterification. My data link experimental cell mechanics to functional cell biology and genetics.

Keyphrases: Morphogenesis, Development, Cell Expansion, Cell Wall, Cellulose, Crystallinity, Pectin Esterification, Cortical Microtubule, Brillouin Microscopy, Polarized Fluorescence Microscopy