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Shoot and root systems have evolved numerous specialized structures, including stomata and lateral roots. Stomata, which are essential for mediating gas exchange across the shoot epidermis during photosynthesis, consist of two bilaterally symmetrical guard cells arranged around an epidermal pore. The symmetry displayed by mature stomata is essential for stomatal function. Stomata develop via a dedicated pathway defined by two key divisions, an asymmetric division, and a symmetric division which initiates stomatal bilateral symmetry. The symmetric division is followed by pore and guard cell morphogenesis, which maintains the previously established bilateral symmetry. Lateral roots increase the ability of root systems to acquire nutrients and water from the surrounding environment and lateral root development is also dependent on stage specific divisions. Interestingly, several genes regulating symmetric divisions have also been shown to function in lateral root development. The Leucine-Rich Repeat Receptor-Like Kinase, MUSTACHES (MUS), which belongs to a family of four closely related MUS-LIKE kinases (MUSLs), is required to enforce bilateral symmetry post-symmetric division. Mutants in mus display pore and guard cell morphogenesis defects, as well as defects in microtubule array organization and the polarity of microtubule movement. MUS is expressed in both stomatal lineage and root epidermal cells, suggesting that MUS also functions outside stomatal development. Thus, MUS represents an ideal candidate through which mechanisms influencing pore and guard cell morphogenesis, as well as the impact of post-symmetric division stomatal genes on lateral root development, may be explored. Mutant analysis and time-lapse studies demonstrated that MUS enforces bilateral symmetry by ensuring symmetrical positioning of microtubule organizing centres post-symmetric division. Time lapse studies indicated that microtubule organizing centre delocalization occurred before, and is likely responsible for, the subsequently occurring alterations in microtubule polarity and guard cell morphogenesis defects observed in mus. As well, this work revealed that MUS, the cytoplasmic kinase NIMA-RELATED KINASE 6 and the microtubule associated protein CLIP-170 ASSOCIATED PROTEIN are required for stomatal microtubule organizing centre formation. Additionally, a model where pore placement is regulated by opposing MUS and MUSL1 signaling pathways was developed. Finally, a redundant role for MUS and MUSL1 in lateral root development was confirmed.

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.