A novel chloride channel in Vicia faba guard cell vacuoles activated by the serine/threonine kinase, CDPK.

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Abstract
Calcium‐Dependent Protein Kinases (CDPKs) in higher plants contain a C‐terminal calmodulin‐like regulatory domain. Little is known regarding physiological CDPK targets. Both kinase activity and multiple Ca2+‐dependent signaling pathways have been implicated in the control of stomatal guard cell movements. To determine whether CDPK or other protein kinases could have a role in guard cell signaling, purified and recombinant kinases were applied to Vicia faba guard cell vacuoles during patch‐clamp experiments. CDPK activated novel vacuolar chloride (VCL) and malate conductances in guard cells. Activation was dependent on both Ca2+ and ATP. Furthermore, VCL activation occurred in the absence of Ca2+ using a Ca2+‐independent, constitutively active, CDPK* mutant. Protein kinase A showed weaker activation (22% as compared with CDPK). Current reversals in whole vacuole recordings shifted with the Nernst potential for Cl‐and vanished in glutamate. Single channel recordings showed a CDPK‐activated 34 +/− 5 pS Cl‐ channel. VCL channels were activated at physiological potentials enabling Cl‐ uptake into vacuoles. VCL channels may provide a previously unidentified, but necessary, pathway for anion uptake into vacuoles required for stomatal opening. CDPK‐activated VCL currents were also observed in red beet vacuoles suggesting that these channels may provide a more general mechanism for kinase‐dependent anion uptake.

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