Xyloglucans and Microtubules Synergistically Maintain Meristem Geometry and Phyllotaxis

Abstract

The shoot apical meristem (SAM) gives rise to all aerial plant organs. Cell walls are thought to play a central role in this process, translating molecular regulation into dynamic changes in growth rate and direction, although their precise role in morphogenesis during organ formation is poorly understood. Here, we investigated the role of xyloglucans (XyGs), a major, yet functionally poorly characterized, wall component in the SAM of Arabidopsis (Arabidopsis thaliana). Using immunolabeling, biochemical analysis, genetic approaches, microindentation, laser ablation, and live imaging, we showed that XyGs are important for meristem shape and phyllotaxis. No difference in the Young's modulus (i.e. an indicator of wall stiffness) of the cell walls was observed when XyGs were perturbed. Mutations in enzymes required for XyG synthesis also affect other cell wall components such as cellulose content and pectin methylation status. Interestingly, control of cortical microtubule dynamics by the severing enzyme KATANIN became vital when XyGs were perturbed or absent. This suggests that the cytoskeleton plays an active role in compensating for altered cell wall composition.

The synthesis of xyloglucan, an abundant plant cell wall polysaccharide, requires CSLC function

“…Consistent with these observations is that XyG might represent a "spacer" molecule to limit interactions between cellulose fibrils and thus maintain their functionality. More recently, the xxt1 xxt2 mutant was used to determine that XyG and microtubules function synergistically to maintain meristem geometry and phyllotaxis in Arabidopsis (58).…”

Mechanical feedback-loop regulation of morphogenesis in plants

“…In addition to auxin, modification to the chemical status of the cell wall polymers, either by application of pectin methyl-esterase or by mutations to xyloglucan biosynthetic enzymes, also impacts microtubule organization at the SAM (Armezzani et al, 2018;Zhao et al, 2019). Perturbation to global amounts of cellulose results in significant reduction of cell wall stiffness at the SAM, yet does not result in cell size defects or tissue level morphological abnormalities, other than the overall reduction in organ size (Sampathkumar et al, 2019).…”

Exploring the relation between apical growth, organ formation and cell wall mechanics across land plant species

“…The formation of new organs at the meristem of the model plant Arabidopsis thaliana (hereafter referred to as Arabidopsis) was reported to depend on auxin-induced increases in cell wall elasticity [7,15]. A similar correlation between cell wall elasticity and differential cell expansion was observed in hypocotyl, roots, leaves and flower organs [16][17][18][19][20]. This relation is quite puzzling as elasticity is a material property corresponding to the ability to undergo a reversible deformation in contrast to cell expansion, which reflects a biologically-driven irreversible deformation of the cell wall [12].…”