Cell and tissue polarity

Several decades of work in animal systems has revealed the molecules and mechanisms that enable proteins to be localized to discrete regions of the cell cortex and how this polarity, in the context of asymmetric cell divisions (ACDs) is harnessed to orient division planes and to ensure the segregation of fate determinants by only one daughter.  Plants also use ACDs in their development, and exhibit cell polarities, but the ways in which plant cell polarity is established and used differ markedly from animals.  The Arabidopsis stomatal lineage is a paradigm for plant asymmetric cell divisions.  It was here that the first plant “polarity protein”, BASL, was identified. Over the years, we and others have made fundamental discoveries about the nature of ACDs in plants and expanded the repertoire of proteins that are segregated during stomatal lineage asymmetric divisions.  Known plant polarity proteins are largely disordered and are likely to serve as scaffolds for the function and regulation of other proteins.   Our recent efforts center on defining the cellular functions of polarity and the client list of interactors that execute those functions. We also have new projects that link the behavior of individual epidermal cell polarities with whole organs (including underlying tissues). 

Using time-lapse imaging, lineage tracing and quantitative polarity measurements, we showed how specific asymmetries—in cell size, division rates, inherited factors, and orientation relative to landmarks—affect cell fate and the overall pattern of the leaf epidermis. By creating genetic reagents to supply polarity proteins at discrete times over the course of asymmetric division, we found that pre-division polarity is critical for division orientation while BASL polarization post-division regulates daughter cell fate asymmetry. Digging deeper into division plane orientation, we showed that BASL/BRXL2 engage the microtubule cytoskeleton to ensure their own segregation. 

We’ve recently found that stomatal lineage cells can exhibit bipolarity, and that this bipolarity regulates plant stem-cell decisions.  Coordination of two separate polarity systems may enable more definitive cell fate decisions, both between sisters resulting from an ACD and during the transitions from stem-cell behavior to differentiation. Interestingly, although polarities may be coordinated to enable robust tissue patterning, we found that the two cellular polarity domains show a great deal of independence, in contrast to expectations from established models of polarity (such as the PAR system) in animals.

Some recent papers on this theme:

Cortical polarity ensures its own asymmetric inheritance in the stomatal lineage to pattern the leaf surface (2022) Andrew Muroyama, lead author [link to PDF]

The Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries (2021) Yan Gong, lead author [link to PDF]

The value of asymmetry: how polarity proteins determine plant growth and morphology (2020) Sophie Wallner, sole author [link to PDF]