A potential intrinsic mechanism for asymmetric cell division in the Arabidopsis stomatal lineage [electronic resource]
Keywords: Biology.

Asymmetric cell divisions produce daughter cells of divergent fate. In the context of multicellular developmental processes such as axis specification, stem-cell renewal, and organ formation, such divisions allow for the generation of cell-type diversity in patterns that confer biological function. Intrinsic forms of asymmetric division involve the establishment of mother-cell polarity, orientation of the mitotic spindle, and the differential distribution or activity of genetically encoded cell-fate determinants among nascent daughter cells. Study of such divisions in an array of animal systems has underscored the preeminence of a mechanism for polarity establishment involving conserved molecular components. While plants lack homologs for the genes that encode this machinery, work in other contexts has revealed that disparate molecular species effect cell polarization through the fulfillment of common regulatory motifs such as positive feedback. Thus the extent to which intrinsic mechanisms of asymmetric division are accomplished similarly in plants and animals remains to be determined. Over the past two decades, the stomatal cell lineage of Arabidopsis thaliana has served as a model system for addressing fundamental problems in developmental biology in a plant context, including molecular genetic mechanisms of asymmetric cell division. A forward genetic screen led to the isolation of BASL (Breaking of Asymmetry in the Stomatal Lineage), a novel, plant-specific gene required for the specification of a divergent cell fate among daughter cells of the stomatal lineage (Dong et al., 2009). In basl mutants, daughter cells assume a similar fate, resulting in a hyperproliferative epidermal phenotype and the aberrant distribution of stomatal lineage cells. In vivo analysis of a GFP-tagged form of BASL revealed a dynamic pattern of localization involving polarization at the cell membrane prior to mitosis and unequal segregation between daughter cells. The finding that BASL polarizes and is reliably inherited by the cell type for which its function is required suggests that BASL may operate as a polarity-generating factor and/or cell fate determinant in the mode of proteins characterized in animal systems. To further elucidate the role of BASL in asymmetric cell division, a yeast two-hybrid screen was performed to identify candidate proteins with which it can interact physically. Among candidates identified in the screen were members of two plant-specific gene families known as "BRX" and "PRAF". Presented in this dissertation are genetic and cell biological data which implicate participation of the Arabidopsis BRX gene family in an intrinsic mechanism of polarity establishment and asymmetric cell division that involves BASL. Genetic analyses suggest that BRX genes are redundantly required for asymmetric division in the stomatal lineage and that they function in a common pathway with BASL. In corresponding mutants, the loss of asymmetry between daughter cells is manifested in the equalized expression of cell-fate determinants and subsequent defects in cell differentiation and expansion. Cell biological approaches indicate that BRX family proteins polarize and are segregated between daughter cells in the same manner as BASL, and that BASL and BRX colocalize, physically interact, and affect one another's localization in vivo. Whereas BRX family proteins are dependent on BASL for polarization, BRX genes appear to promote BASL membrane accumulation. These results are consistent with BASL and BRX proteins comprising a plant-specific polarity module that accomplishes asymmetric cell divisions via the unequal segregation of intrinsic cell-fate determinants in a manner analogous to animal systems. Experiments involving the manipulation of BASL and BRX protein localization suggest that both function at the cell periphery, but discrete roles in polarity generation and/or cell-fate determination have yet to be defined for either, and the mechanism of cell differentiation towards which these factors contribute remains a mystery. The final chapter includes a consideration of future experimental approaches that could help to resolve these matters.