Glycogen synthase kinases in model and crop plants - From negative regulators of brassinosteroid signaling to multifaceted hubs of various signaling pathways and modulators of plant reproduction and yield

Abstract

Glycogen synthase kinases, also known as SHAGGY-like Kinases (GSKs/SKs), are highly conserved serine/threonine protein kinases present both in animals and plants. Plant genomes contain multiple homologs of the GSK3 genes which participate in various biological processes. Plant GSKs/SKs, and their best known representative in Arabidopsis thaliana - Brassinosteroid Insentisive2 (BIN2/SK21) in particular, were first identified as components of the brassinosteroid (BR) signaling pathway. As phytohormones, BRs regulate a wide range of physiological processes in plants - from germination, cell division, elongation and differentiation to leaf senescence, and response to environmental stresses. The GSKs/SKs proteins belong to a group of several highly conserved components of the BR signaling which evolved early during evolution of this molecular relay. However, recent reports indicated that the GSKs/SKs proteins are also implicated in signaling pathways of other phytohormones and stress-response processes. As a consequence, the GSKs/SKs proteins became hubs of various signaling pathways and modulators of plant development and reproduction. Thus, it is very important to understand molecular mechanisms regulating activity of the GSKs/SKs proteins, but also to get insights into role of the GSKs/SKs proteins in modulation of stability and activity of various substrate proteins which participate in the numerous signaling pathways. Although elucidation of these aspects is still in progress, this review presents a comprehensive and detailed description of these processes and their implications for regulation of development, stress response, and reproduction of model and crop species. The GSKs/SKs proteins and their activity are modulated through phosphorylation and de-phosphorylation reactions which are regulated by various proteins. Importantly, both phosphorylations and de-phosphorylations may have positive and negative effects on the activity of the GSKs/SKs proteins. Additionally, the activity of the GSKs/SKs proteins is positively regulated by reactive oxygen species, whereas it is negatively regulated through ubiquitylation, deacetylation, and nitric oxide-mediated nitrosylation. On the other hand, the GSKs/SKs proteins interact with proteins representing various signaling pathways, and on the basis of the complicated network of interactions the GSKs/SKs proteins differentially regulate various physiological, developmental, stress response, and yield-related processes.

Keywords: brassinosteroids; crosstalk; glycogen synthase kinases; plant reproduction; plant yield; signaling; stress response.

FIGURE 1

Maximum-likelihood phylogenetic tree representing the glycogen synthase kinases (GSK) family proteins from Arabidopsis thaliana (At), Brassica rapa (Bra), Hordeum vulgare (Hv), Medicago sativa (Ms), Nicotiana tabacum (Nt), Oryza sativa (Os), and Triticum aestivum (Ta). All kinases were clustered into four clades: I (yellow), II (blue), III (green) and IV (pink). Phylogenetic analysis was performed using MEGA X software. Bootstrap support values over 50%, calculated from 1000 replicates, are shown under branches. Visualization of the phylogenetic tree was designed by the iTOL v6 tool.

FIGURE 2

Role of the GSK proteins in regulation of the BR signaling. Green arrows indicate stimulation, whereas red lines with the bullet points represent inhibition. Transcription factors are shown as ovals, other proteins are depicted as rectangles. Double black line represents plasma membrane. Detailed description is given in the text. BRI1, Brassinosteroid-Insensitive1; BAK1, BRI1-Associated receptor Kinase1; BKI1, BRI1 Kinase Inhibitor1; BSKs, BR-Signaling Kinases; CDG1, Constitutive Differential Growth1; BSU1, bri1 suppressor1 phosphatase; BZR1, Brassinazole-resistant1; BES1, bri1-EMS suppressor1.

FIGURE 3

Three-dimensional structure of the AtSK21/BIN2 protein (UniProt Entry: Q39011) of Arabidopsis. Positions of the amino-acid residues: Cys⁵⁹, Cys⁹⁵, Cys⁹⁹, Cys¹⁶², Ser¹⁸⁷, Lys¹⁸⁹, Tyr²⁰⁰, Ser²⁰³, Thr²⁶¹, Arg²⁶², Glu²⁶³, and Glu²⁶⁴ which are modified during regulation of the BIN2 protein activity are shown together with modifying factors. Ac, Acetyl group; P, Phosphate group. Visualization and analysis of the three-dimensional structure of the AtSK21/BIN2 protein was performed using the SWISS-MODEL database and UCSF Chimera program (version 1.10.1).

FIGURE 4

Mechanisms regulating stability and activity of the GSK proteins. Green arrows indicate stimulation, whereas red lines with the bullet points represent suppression. Solid ovals containing the letter “P” denote phosphorylation/dephosphorylation events, whereas “Ac” and “Ub” represent acetyl functional group and ubiquitylation, respectively. Positive and negative effects of these chemical modifications on the GSK proteins are represented by green and red arrows, respectively. Some of the regulatory mechanisms were reported in: a, Indian dwarf wheat; b, rice; c, sorghum. Detailed description is given in the text. KIB1, Kink Suppressed In bzr1-1D1; BRF, Brassinosteroid F-box; COP1/SPA, CONSTITUTIVELY PHOTOMORPHOGENIC1/SUPPRESSOR OF phyA-105; BSU1, bri1 suppressor1 phosphatase; PIF3, PHYTOCHROME INTERACTING FACTOR3; HY5, Elongated Hypocotyl5; OsPPKL1, Protein Phosphatase Kelch-Like1; ABI1/2, ABA-Insensitive1 and 2 protein phosphatases; S6K2, Ribosomal protein S6 kinase2; POLAR, Polar Localization During Asymmetric Division and Redistribution; BASL, Breaking of Asymmetry in the Stomatal Lineage; OPS, OCTOPUS; GW5, Grain Width5; TTL3, Tetratricopeptide Thioredoxin-like3; BSK3, BR-Signaling Kinase3; HSP90, Heat shock protein90; DW1, Dwarfing1; SOS3, Salt Overly Sensitive3; SCaBP8, SOS3-like Calcium Binding Protein8.

FIGURE 5

Influence of GSKs on proteins participating in various developmental, hormonal, and stress response pathways. Green arrows indicate stimulation, whereas red lines with the bullet points represent suppression. Solid circles containing the letter “P” denote phosphorylation events. Positive and negative effects of the phosphorylations on the target proteins are represented by green and red arrows, respectively. Detailed description is given in the text. CES, CESTA; ICE1, Inducer of CBF Expression1; BZR1, Brassinazole-resistant1; YODA, Mitogen-Activated Protein Kinase; MKK4/5, Mitogen-Activated Protein Kinase kinase 4/5; SPCH, SPEECHLESS; EGL3, Enhancer of Glabra3; GL3, Glabra3; WER, WEREWOLF; TTG1, Transparent Testa Glabra1; GL2, Glabra2; CYC U2, U-type cyclin; D3, Dwarf3 ubiquitin ligase; SL, strigolactone; CO, CONSTANS; WRKY, Trp-Arg-Lys-Tyr domain transcription factor; MYBL2, Myeloblastosis Family Transcription Factor-like2; HAT1, Homeodomain-leucine zipper protein Arabidopsis thaliana1; AIF2, ATBS1-Interacting Factor2; ABA, abscisic acid; PIF, PHYTOCHROME INTERACTING FACTOR; ARF, Auxin Response Factor; ARR, Arabidopsis Response Regulator.

FIGURE 6

Influence of GSKs on proteins participating in various developmental, hormonal, and stress response pathways (continuation). Green arrows indicate stimulation, whereas red lines with the bullet points represent suppression. Solid circles containing the letter “P” denote phosphorylation events. Positive and negative effects of the phosphorylations on the target proteins are represented by green and red arrows, respectively. Detailed description is given in the text. ATG8, Autophagy8; RD26, Responsive to Desiccation26; TINY, transcription factor; DSK2, Dominant Suppressor of KAR2; PUB40, Plant U-Box40 ubiquitin ligase; SnRK2, Sucrose non-fermenting1-Related protein Kinase2; ABI5, Abscisic acid Insensitive5; AKSs, ABA-responsive Kinase Substrates; UPB1, UPBEAT1; GLK1, GOLDEN2-like1; G6PD, Glucose-6-phosphate dehydrogenase; BZR1, Brassinazole-resistant1; LIC, Leaf and Tiller Angle Increased Controller; DLT, DWARF and Low-Tillering; RLA1, Reduced Leaf Angle1; OFP, Ovate Family Protein; JAZ4, Jasmonate ZIM-Domain4; MYC2, Myelocytomatosis2 transcription factor; WRKY53, Trp-Arg-Lys-Tyr domain transcription factor53; SOS2, Salt Overly Sensitive protein kinase2; NSP, Nodulation Signaling Pathway transcription factor.

FIGURE 7

Influence of GSKs on proteins participating in various developmental and reproduction-related processes (continuation). Green arrows indicate stimulation, whereas red lines with the bullet points represent suppression. Solid circles containing the letter “P” denote phosphorylation events. Positive and negative effects of the phosphorylations on the target proteins are represented by green and red arrows, respectively. Some of the regulatory mechanisms were reported in: a, grapevine; b, rapeseed, barley, and cork oak. Detailed description is given in the text. TTG1, Transparent Testa Glabra1; TT2, Transparent Testa2; GL2, Glabra2; OsmiR396, Oryza sativa microRNA 396; OsGRF4, Growth Regulating Factor4; ARF, Auxin Response Factor; OML4, MEI2-like Protein4; OsMAPKK4, Mitogen-Activated Protein Kinase Kinase4; ZmIAA28, auxin response regulator28.