POLAR-guided signalling complex assembly and localization drive asymmetric cell division.

Area of Science:

• Plant cell biology
• Molecular plant science
• Developmental biology

Background:

• Asymmetric cell division (ACD) is crucial for plant survival and development, exemplified by stomatal cell lineage.
• The GLYCOGEN SYNTHASE KINASE3 (GSK3)/SHAGGY-like kinase BRASSINOSTEROID INSENSITIVE 2 (BIN2) has dual roles in regulating stomatal ACD.
• Mechanisms balancing BIN2's opposing effects on mitogen-activated protein kinase (MAPK) signaling and SPEECHLESS (SPCH) activity are unknown.

Purpose of the Study:

• To elucidate the regulatory mechanisms controlling asymmetric cell division in plant stomatal lineage.
• To identify factors involved in balancing the dual roles of GSK3-like kinases in stomatal development.
• To understand how substrate specificity of GSK3 is achieved in plant cells.

Main Methods:

• Identification of the plant-specific protein POLAR as a key regulator in stomatal lineage.
• Investigating the role of POLAR as a scaffold for GSK3-like kinases.
• Analyzing the effects of POLAR and BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) on kinase localization and activity.

Main Results:

• POLAR confines GSK3-like kinases to the cytosol and transiently polarizes them with BASL before ACD.
• This localization attenuates MAPK signaling, allowing SPCH to promote ACD in the nucleus.
• POLAR turnover is regulated by GSK3-mediated phosphorylation, indicating a feedback mechanism.

Conclusions:

• The scaffolding protein POLAR ensures GSK3 substrate specificity, providing a novel mechanism for regulating ACD.
• POLAR's function in controlling kinase activity and localization offers a paradigm for understanding GSK3 regulation in plants.
• This study reveals critical insights into the molecular control of cell fate decisions during plant development.