During the past years, we established a positive feedback loop between BASL and a mitogen-activated protein kinase (MAPK) signaling cascade that promotes cell polarity. During stomatal development, the canonical MAPK signaling cascade, comprising of the MAPKK Kinase YODA (YDA), the MAPK Kinase 4/5, and MAPK 3/6, plays pivotal roles in stomatal development and patterning. We found that MPK3/6 phosphorylates BASL, causing BASL to exit the nucleus and become polarized on the cell cortex, where BASL acts as a scaffold protein to recruit YDA and MPK3/6 into the cortical polarity site. We demonstrated that the polarized YDA-MPK3/6 cascade provides a key mechanism for cell specification, as it is asymmetrically inherited into the large daughter cell that undergoes pavement cell differentiation. MPK3/6 directly phosphorylate the master stomatal lineage transcription factor SPCH, leading to asymmetric degradation of SPCH, thus asymmetric daughter cell fates (Zheng et al., Dev Cell 2015; Zhang et al., Curr Bio 2016).