The Arabidopsis leucine-rich repeat receptor-like kinase MUSTACHES enforces stomatal bilateral symmetry in Arabidopsis.

Stomata display a mirror-like symmetry that is adaptive for shoot/atmosphere gas exchange. This symmetry includes the facing guard cells around a lens-shaped and bilaterally symmetric pore, as well as radially arranged microtubule arrays that primarily originate at the pore and then grow outwards. Mutations in MUSTACHES (MUS), which encodes a leucine-rich repeat receptor-like kinase, disrupt this symmetry, resulting in defects ranging from skewed pores and abnormally focused and depolarized radial microtubule arrays, to paired guard cells that face away from each other, or a severe loss of stomatal shape. Translational MUSproMUS:tripleGFP fusions are expressed in cell plates in most cells types in roots and shoots, and cytokinesis and cell plates are mostly normal in mus mutants. However, in guard mother cells, which divide and then form stomata, MUS expression is notably absent from new cell plates, and instead is peripherally located. These results are consistent with a role for MUS in enforcing wall building and cytoskeletal polarity at the centre of the developing stoma via signalling from the vicinity of the guard cell membrane. Significance StatementWe show that the Leucine-Rich Repeat Receptor-Like Kinase MUSTACHES (MUS) is essential for stomatal development by enforcing bilateral symmetry after a symmetric division. The loss of function of this gene leads to skewed stomata and twisted pores, as well as associated defects in the position and symmetry of microtubule arrays. The identification of MUS might eventually lead to defining other genes and proteins required for stomatal development.