The BASL Polarity Protein Controls a MAPK Signaling Feedback Loop in Asymmetric Cell Division

Abstract

SUMMARY Cell polarization is linked to fate determination during asymmetric division of plant stem cells, but the underlying molecular mechanisms remain unknown. In Arabidopsis, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) is polarized to control stomatal asymmetric division. A MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) cascade determines terminal stomatal fate by promoting the degradation of the lineage determinant SPEECHLESS (SPCH). Here we demonstrate that a positive feedback loop between BASL and the MAPK pathway constitutes a polarity module at the cortex. Cortical localization of BASL requires phosphorylation mediated by MPK3/6. Phosphorylated BASL functions as a scaffold and recruits the MAPKKK YODA and MPK3/6 to spatially concentrate signaling at the cortex. Activated MPK3/6 reinforces the feedback loop by phosphorylating BASL, and inhibits stomatal fate by phosphorylating SPCH. Polarization of the BASL-MAPK signaling feedback module represents a mechanism connecting cell polarity to fate differentiation during asymmetric stem cell division in plants.

The chemical compound bubblin induces stomatal mispatterning in Arabidopsis by disrupting the intrinsic polarity of stomatal lineage cells

“…By contrast, bubblin abolishes the asymmetry of cell division and causes stomatal precursor clustering, suggesting that bubblin acts on cell polarity independently of extrinsic signaling. Meristemoid cell polarity is generated by BASL that localizes to the cell periphery and is predicted to regulate SPCH degradation (Zhang et al, 2015). Bubblin induced stomatal lineage cells to divide without establishing BASL polarity, and this accounted for the retention of SPCH, which resulted in stomatal clustering (Fig.…”

“…Peripheral localization of BASL requires MPK3/ 6-mediated phosphorylation (Zhang et al, 2015). Bubblin might act as a kinase inhibitor to perturb BASL phosphorylation or regulate the targeting of phosphorylated BASL to the cell periphery.…”

“…Recently, SPCH degradation was suggested to be regulated by the polarity factor BASL, which is specifically localized at the cell periphery (Zhang et al, 2015). In the pre-mitotic stomatal precursor cell, BASL is observed in the nucleus and at the cell periphery.…”

Plant Development: Suppression the Key to Asymmetric Cell Fate

“…This slow mobility of BASL was specific to the polar site since BASL localised to non-polar sites on the PM showed greater mobility, demonstrating that distinct mechanisms may direct or retain BASL at polar versus non-polar sites. Targeting to the polar site requires phosphorylation of BASL by MPK3/6 [15]. By mutating the MAPK phosphorylation sites in BASL with phospho-mimicking residues, FRAP analysis demonstrated that BASL phosphorylation actually reduces mobility at the polar PM site, which has significant consequences.…”

“…BASL is phosphorylated by MPK3/6, and this facilitates BASL functioning as a scaffold for YDA at the plasma membrane (PM) [15]. Prior to the ACD of a MMC, BASL localises to both the nucleus and the cell periphery distal to the future division plane of the MMC, and is therefore predictive of the future ACD plane.…”