MUSTACHES regulates bilateral symmetry in stomata

Arabidopsis stomata develop from specialized stem cells in a dedicated cell lineage which harbours both asymmetric and symmetric divisions. This pathway terminates in the production of two bilaterally symmetrical guard cells (GCs) surrounding a pore, which are essential for shoot gas exchange. Morphogenesis of each GC continues in parallel after the symmetric division, indicating that development is tightly coordinated in time and space. Morphogenesis involves key events, such as the organization of distinctive microtubule arrays and the deposition of ordered cellulose microfibrils, which generate the bilateral symmetry of the mature stomatal valve. Although several genes are known to be required for the symmetric division of the guard mother cell (GMC), the genes and events required for coordinated GC development are largely unknown. MUSTACHES (MUS) is a leucine-rich repeat receptor-like kinase (LRR-RLK) required for stomatal morphogenesis that acts primarily after the two young GCs form. Analysis of the range of phenotypic defects in mature stomata in several loss-of-function mus alleles revealed disruptions in the mirror-like symmetry of microtubule arrays, GC walls, and the stomatal pore. Radial microtubule arrays in developing and mature stomata display a polarity with respect to the trajectories of the microtubule End Binding protein, EB1. In wild-type GCs, most EB1 trajectories move away from the stomatal pore. However in a mus background, many more EB1 comets move towards the stomatal pore, suggesting that the outbound to inbound ratio of microtubule trajectories might regulate or be part of a feedback loop that generates stomatal symmetry. A ProMUS:MUS-GFP translational fusion, transformed into wild-type and mus mutant lines, exhibited GFP localization in cell plates during both symmetric and asymmetric divisions throughout the plant. The only cell type observed that did not show MUS distribution in the cell plate was the GMC where GFP distribution was instead confined to the cell periphery, a distribution that persisted in young stomata. Because the division of the GMC appears to be normal in mus, it is likely that MUS function from the cell periphery is required to generate an appropriately shaped, bilaterally symmetric, and functional stomatal valve.Science, Faculty ofBotany, Department ofGraduat