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Protein arrays have been used to identify kinase substrates using purified protein kinases and radiolabeled adenosine triphosphate (ATP) ( 22, 23), and global phosphorylation networks in yeast and MAP kinase modules in Arabidopsis were studied using protein array-based methods ( 23, 24). SOS1 is the only known substrate of SOS2 thus far, and how SOS3-SOS2 regulates salinity responses is not fully understood.ĭespite the importance of phosphorylation-dependent signaling for cellular physiology, the identification of kinase substrates, which is essential for understanding the role of the kinases in signaling pathways, is still very challenging. SOS3 and CBL10 activate SOS2, and SOS2 then phosphorylates and activates the plasma membrane Na +/H + antiporter SOS1 ( 21).
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High salinity increases the cytosolic calcium content, which is sensed by the calcium sensor SOS3 (calcineurin B-like calcium sensor protein 4/CBL4) in roots ( 19), and by SCaBP8/CBL10 in shoots ( 20). Several other kinases, such as calcineurin B-like protein interacting serine/threonine-protein kinase 23 (CIPK23) and casein kinase 1-like 2 (CKL2), are essential for ABA-induced stomatal closure and drought resistance ( 15– 18), and SALT OVERLY SENSITIVE 2 (SOS2), also known as CIPK24, is a key regulator of salinity response in plants ( 19). In addition to SnRK2s, CPKs are also important for ABA signaling ( 14). Moreover, all SnRK2s except SnRK2.9 can be activated by osmotic stress caused by NaCl or mannitol treatment in Arabidopsis ( 12, 13). In Arabidopsis, three of them, SnRK2.2, SnRK2.3, and SnRK2.6, are activated by abscisic acid (ABA), a phytohormone important for plant responses to several abiotic stresses ( 9), and play critical roles in ABA responses ( 10, 11). There are 10 members in the SnRK2 family in Arabidopsis (SnRK2.1 to SnRK2.10) and rice (SAPK1 to SAPK10). Sucrose nonfermenting (SNF1)-related protein kinase 2s (SnRK2s) are central components in plant responses to drought and osmotic stresses. It is well known that protein kinases are central components in the response of plants to abiotic stresses ( 8). These results provide comprehensive information on the role of these protein kinases in the control of cellular activities and could be a valuable resource for further studies on the mechanisms underlying plant responses to environmental stresses. As a result, we identified more than 5,000 putative target sites of osmotic stress-activated SnRK2.4 and SnRK2.6, abscisic acid-activated protein kinases SnRK2.6 and casein kinase 1-like 2 (CKL2), elicitor-activated protein kinase CDPK11 and MPK6, cold-activated protein kinase MPK6, H 2O 2-activated protein kinase OXI1 and MPK6, and salt-induced protein kinase SOS1 and MPK6, as well as the low-potassium-activated protein kinase CIPK23. In this study, we introduce a strategy based on isotope-labeled in vitro phosphorylation reactions using in vivo phosphorylated peptides as substrate pools and apply this strategy to identify putative substrates of nine protein kinases that function in plant abiotic and biotic stress responses.
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Moreover, how these protein kinases regulate downstream biological processes and mediate stress responses is still largely unknown.
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However, only a few targets of these protein kinases have been identified. It is known that protein kinases are central components in plant responses to environmental stresses such as drought, high salinity, cold, and pathogen attack. By adding phosphate groups, protein kinases regulate the activity, localization, protein–protein interactions, and other features of their target proteins. Protein kinases are major regulatory components in almost all cellular processes in eukaryotic cells.