Understanding cell polarity and asymmetric cell division in the context of stomatal development in brachypodium and other plants
Keywords: Biology.

Stomata are epidermal valves found in stereotyped patterns on the leaves of land plants. Their function is to facilitate water and gas exchange between the plant and atmosphere. In most plants stomatal complexes consist of two epidermal cells that flank a pore, but there are variations among different plant species. Production of mature stomata requires a series of regulated asymmetric and symmetric cell divisions, and these divisions contribute to overall leaf size and pattern. Complex biological processes like nuclear movement, cytoskeleton dynamics, cell cycle regulation and fate determination are involved in these divisions. The molecular mechanism of stomatal formation has been extensively studied in the model dicot Arabidopsis thaliana (thale cress) and to a lesser extent in Maize. From these two species, plant-specific regulators essential for cell polarity, associated with asymmetric cell divisions (ACDs) that form stomata were identified. The mechanisms enabling restricted subcellular localization of these "polarity factors" and their function guiding ACDs is just beginning to be understood. Moreover, how the polarity of individual cells is coordinated within diverse body plans and how this is regulated across taxa through evolutionary history are completely open questions. Like in Maize, rice, and other well-characterized grasses, cells in the epidermis of the Brachypodium distachyon leaf undergo two distinct asymmetric divisions and a symmetric division to form a four-celled stomatal complex. The first of these asymmetric divisions is oriented perpendicular to the long axis of a growing leaf, but the second division is oriented parallel to the axis and is closely associated with a specific cell type. These divisions provide a framework to investigate the genetic and cell biological regulation of ACDs. I utilized three polarity factors—BASL, BRX and PAN1—identified in Arabidopsis and Maize (Cartwright et al., 2009; Dong et al., 2009; Rowe et al., 2019) to monitor polarity in Brachypodium. I observed their subcellular behavior during stomatal development in leaves of both wild type and plant with mutations that affected various stomatal lineage divisions. My observations of reporters of these polarity factors reveal molecular differences between the two rounds of ACD during stomata formation in Brachypodium as well as differences and similarities between grass and Arabidopsis stomatal development. None of the plant polarity factors are found in animals, but their distribution among plant lineages can vary. For example, BRX family members are found in nearly all extant (sequenced) plants from mosses to flowering plants. Homologues of BASL, however, cannot be found outside of the eudicots. To understand the evolution of BASL in plants, I took a functional approach to test whether a putative BASL homologue in Solanum (SlBASL) was polarized and/or functioned like Arabidopsis BASL. When expressed in Arabidopsis, SlBASL was not polarized, nor could it fully rescue the Arabidopsis basl mutant. This suggests that either Solanum has different mechanisms to guide its ACDs, or that another protein takes on the functionality. Therefore, I speculate on the evolutionary history of BASL, BRX, their partnership and polarity in plant cells.