The physiological importance of GSK3-like kinases in plants emerged when the functional role of plant GSK3-like kinases represented by BIN2 was first elucidated in the brassinosteroid (BR)-regulated signal transduction pathway. While early studies focused more on understanding how GSK3-like kinases regulate BR signaling, recent studies have implicated many novel substrates of GSK3-like kinases that are involved in a variety of cellular processes as well as BR signaling. Plant GSK3-like kinases play diverse roles in physiological and developmental processes such as cell growth, root and stomatal cell development, flower development, xylem differentiation, light response, and stress responses. Here, we review the progress made in recent years in understanding the versatile functions of plant GSK3-like kinases. Based on the relationship between GSK3-like kinases and their newly identified substrates, we discuss the physiological and biochemical relevance of various cellular signaling mediated by GSK3-like kinases in plants.

SummaryBrassinosteroids (BRs) are essential regulators of plant architecture. Understanding how BRs control plant height and leaf angle would facilitate development of new plant type varieties by biotechnology. A number of mutants involved in BR biosynthesis have been isolated but many of them lack detailed genetic analysis. Here, we report the isolation and characterization of a severe dwarf mutant, chromosome segment deleted dwarf 1 (csdd1), which was deficient in BR biosynthesis in rice.We isolated the mutant by screening a tissue culture-derived population, cloned the gene by mapping, and confirmed its function by complementary and RNAi experiments, combined with physiological and chemical analysis.We showed that the severe dwarf phenotype was caused by a complete deletion of a cytochrome P450 gene, CYP90D2/D2, which was further confirmed in two independent T-DNA insertion lines in different genetic backgrounds and by RNA interference. Our chemical analysis suggested that CYP90D2/D2 might catalyze C-3 dehydrogenation step in BR biosynthesis.We have demonstrated that the CYP90D2/D2 gene plays a more important role than previously reported. Allelic mutations of CYP90D2/D2 confer varying degrees of dwarfism and leaf angle, thus providing useful information for molecular breeding in grain crop plants.

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Plant GSK3-like kinases are key regulators that modulate a broad range of physiological processes such as cell growth, stomatal and flower development, responses for abiotic and biotic stress, and carbohydrate metabolism. Arabidopsis Shaggy/GSK3-like kinases (AtSK) consist of ten members that are classified into four subfamilies (I~IV). Only one of these Arabidopsis GSK3s, BIN2 (also named AtSK21), has been characterized by biochemical and genetic studies. BIN2 acts as a negative regulator in brassinosteroid (BR) signaling that controls cell growth and differentiation. Recent studies suggest that at least seven AtSKs are involved in BR signaling. However, specificities for the substrates and the functional differences of each member of the family remain to be determined. Here we report structural characteristics and distinct function of AtSK12 compared with BIN2. AtSK12 has a longer N-terminal extension, which is absent in BIN2. Transgenic plants overexpressing the AtSK12 mutant carrying deletion of Nterminal region display more severe dwarf phenotypes than those of the wild-type AtSK12. Microscopic analysis reveals that N-terminal-deleted AtSK12 accumulates in the nucleus. This implies that structural difference in the Nterminal region of AtSK members contributes to their subcellular localization. In contrast to BIN2, overexpression of AtSK12 does not cause a stomatal cluster. Furthermore, we show that YODA MAPKKK, which controls stomatal development, interacts with BIN2 but not with AtSK12. Our results suggest that AtSK12 mediates BR-regulated cell growth but not stomatal development while BIN2 regulates both processes. Our study provides evidence that different GSK3 members can have overlapping but non-identical functions.

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