Dev. Cell 33, 136-149 (2015)

The structural organization needed for efficient multicellularity derives from asymmetric cell divisions during development, which create functionally diverse and spatially oriented cells and tissues. A model pathway of such symmetry breaking is the stomatal lineage in Arabidopsis. Juan Dong and colleagues have discovered a new mechanism in this signalling cascade, connecting the polarization of molecular players with the fate of daughter cells.

The protein BASL was previously identified as critical for establishing polarity prior to asymmetrical division in the stomatal lineage. BASL has a striking crescent-shaped subcellular localization, and mutants lacking BASL lost the ability to divide asymmetrically during stomata patterning.

The presence of MAP kinase (MAPK) docking sites helped to integrate BASL in the already known pathway controlling cellular transitions. Specifically BASL is a target of the MAPK cascade containing YODA, MKK4/5 and MPK3/6. Once phosphorylated, BASL adopts its polarized location and acts as a scaffold protein, recruiting YODA and MPK3/6 to the cortical side of the dividing cell. Thus a reinforcing feedback loop is created, inhibiting stomatal fate in one of the daughter cells.

This self-organizing loop ensures the establishment of polarity and connects polarity with cell fate. Moreover, as MAPKs have many substrates, subcellular sequestering of a whole signalling cascade could play a role in the cascade's functional specificity.