Function and evolution of 'green' GSK3/Shaggy-like kinases

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GSK3 proteins have extremely diverse functions both between kingdoms and within a single species. This suggests that GSK3s have been co-opted into many signalling pathways throughout evolution and highlights their pivotal importance in eukaryotic signalling. Plant GSK3 proteins are required for growth, development and environmental stress tolerance. These are all agronomically important plant traits, making plant GSK3s potential targets for future crop manipulation. So far, most functional studies have been carried out in Arabidopsis or in other dicots. The exact degree of GSK3 functional conservation across land plants is unknown, although rice and cotton BIN2 homologues appear to have conserved function when overexpressed in Arabidopsis [39-40]. Producing dwarf plants was key to the success of the Green Revolution, and cereal semi-dwarf mutants with altered BR signalling have been discovered [63-65]. Perhaps cereal BIN2 homologues can be manipulated to produce semi-dwarf plants. Indeed, the discovery of a GSK3 mutant [48] affecting the phosphorylation of only some substrates could be a powerful tool for future development and use in crop manipulation.

GSK3s play a role in integrating multiple hormonal signals (BR, ABA and auxin). In addition to their role in development, these hormones are involved in responding to a variety of environmental stresses. Therefore, deciphering the molecular mechanism by which GSK3s mediate hormone crosstalk represents an attractive way to modulate plant stress tolerance.

Whether plant GSK3s share conserved molecular functions with GSK3s in other systems is still an open question. Key unanswered questions include (i) whether plant GSK3s provide a direct link between developmental cell signalling and the cytoskeleton, and (ii) whether plant and animal GSK3s share any conserved protein targets and upstream regulators. These should be exciting areas for future research in Arabidopsis and other species.