The Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries

Abstract

In many land plants, asymmetric cell divisions (ACDs) create, and pattern differentiated cell types on the leaf surface. In the Arabidopsis stomatal lineage, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) regulates ACD division plane placement and cell fate enforcement. Polarized subcellular localization of BASL is initiated before ACD and persists for many hours after the division in one of the two daughters. Untangling the respective contributions of polarized BASL before and after division is essential to gain a better understanding of its roles in regulating stomatal lineage ACDs. Here we combine quantitative imaging and lineage tracking with genetic tools that provide temporally restricted BASL expression. We find that pre-division BASL is required for division orientation, whereas BASL polarity post-division ensures proper cell fate commitment. These genetic manipulations allowed us to uncouple daughter-cell size asymmetry from polarity crescent inheritance, revealing independent effects of these two asymmetries on subsequent cell behavior. Finally, we show that there is coordination between the division frequencies of sister cells produced by ACDs, and this coupling requires BASL as an effector of peptide signaling.

A cell size threshold triggers commitment to stomatal fate inArabidopsis

“…Previously, we identified CONSTITUTIVE TRIPLE RESPONSE1 ( CTR1) as a positive regulator of meristemoid proliferation (Gong, Alassimone, Varnau, et al, 2021), although the detailed mechanism underlying this effect was unknown. Notably, Vaseva et al (2018) reported that epidermal cell expansion was inhibited in the ctr1 mutant, leading us to investigate whether the lack of cell expansion and consequently, a reduction in cell size, could trigger premature differentiation.…”

“…Previously reported mutants and transgenic lines include: pSPCH∷SPCH-YFP pATML1∷RCI2A-mCherry in spch-3 (Lopez-Anido et al, 2021), pMUTE∷MUTE-YFP pATML1∷RCI2A-mCherry (Davies & Bergmann, 2014), pBRXL2∷BRXL2-YFP pATML1∷RCI2A-mCherry (Gong, Varnau, et al, 2021; Rowe et al, 2019), pBRXL2∷BRXL2-YFP pATML1∷RCI2A-mCherry in ctr1 (Gong, Alassimone, Varnau, et al, 2021), tetraploid Col-0 (Robinson et al, 2018), p35S∷PIP2A-RFP in basl-2 (Rowe et al, 2019), pBRXL2∷BRXL2-YFP pATML1∷RCI2A-mCherry in myoxi-i (Muroyama et al, 2020), pATML1∷mCherry-RCI2A pBASL∷BRX-CFP (Rowe et al, 2019), and pATML1∷H2B-mTFP pATML1∷mCit-RCI2A (Robinson et al, 2018). We created crwn1-1 pATML1∷H2B-mTFP pATML1∷mCit-RCI2A by crossing crwn1-1 (Dittmer et al, 2007) with pATML1∷H2B-mTFP pATML1∷mCit-RCI2A (Robinson et al, 2018).…”

“…All fluorescence imaging, time-lapse, and time-course experiments were performed as described in (Gong, Alassimone, Muroyama, et al, 2021). To quantify SPCH protein levels (Figure 1D), we captured a time-lapse of a 3 dpg cotyledon expressing the translational reporter pSPCH::gSPCH-YFP.…”

“…During leaf growth, stem cells of the stomatal lineage—meristemoids and stomatal lineage ground cells (SLGCs)—undergo a variable number of self-renewing asymmetric cell divisions (ACDs) before committing to terminal differentiation as stomata or pavement cells, respectively (Figure 1A). Notably, both meristemoid and SLGC proliferation can be tuned by hormonal, nutrient and environmental inputs (Balcerowicz et al, 2014; Engineer et al, 2014; Gong, Alassimone, Varnau, et al, 2021; Han et al, 2020; Lau et al, 2018; Le et al, 2014; Lee et al, 2017; Vaten et al, 2018). This flexible developmental program is deeply conserved across land plants and functions to optimize mature leaf physiology to an individual’s unique local environment (Chater et al, 2017; McKown & Bergmann, 2020).…”

Cortical polarity ensures its own asymmetric inheritance in the stomatal lineage to pattern the leaf surface

“…After it is inherited by the SLGC through the ACD, cortical BASL/BRXf enhances MITOGEN ACTIVATED PROTEIN KINASE (MAPK) signaling to suppress meristemoid identity ( 15, 17 ). Despite its central role in coordinating ACD entry and daughter-cell identity post-division ( 18 ), how BASL/BRXf asymmetric inheritance is regulated is unknown (Fig. 1A).…”