Two calcineurin B‐like calcium sensors, interacting with protein kinase CIPK23, regulate leaf transpiration and root potassium uptake in Arabidopsis

Abstract

SummaryCalcium signalling involves sensor proteins that decode temporal and spatial changes in cellular Ca 2+ concentration. Calcineurin B-like proteins (CBLs) represent a unique family of plant calcium sensors that relay signals by interacting with a family of protein kinases, designated as CBL-interacting protein kinases (CIPKs). In a reverse genetic screen for altered drought tolerance, we identified a loss-of-function allele of CIPK23 as exhibiting a drought-tolerant phenotype. In the cipk23 mutant, reduced transpirational water loss from leaves coincides with enhanced ABA sensitivity of guard cells during opening as well as closing reactions, without noticeable alterations in ABA content in the plant. We identified the calcium sensors CBL1 and CBL9 as CIPK23-interacting proteins that targeted CIPK23 to the plasma membrane in vivo. Expression analysis of the CIPK23, CBL1 and CBL9 genes suggested that they may function together in diverse tissues, including guard cells and root hairs. In addition, expression of the CIPK23 gene was induced by low-potassium conditions, implicating a function of this gene product in potassium nutrition. Indeed, cipk23 mutants displayed severe growth impairment on media with low concentrations of potassium. This phenotype correlates with a reduced efficiency of K + uptake into the roots. In support of the conclusion that CBL1 and CBL9 interact with and synergistically serve as upstream regulators of CIPK23, the cbl1 cbl9 double mutant, but not the cbl1 or cbl9 single mutants, exhibit altered phenotypes for stomatal responses and low-potassium sensitivity. Together with the recent identification of the potassium channel AKT1 as a target of CIPK23, these results imply that plasma membrane-localized CBL1-and CBL9-CIPK23 complexes simultaneously regulate K + transport processes in roots and in stomatal guard cells.

Tonoplast calcium sensors CBL2 and CBL3 control plant growth and ion homeostasis through regulating V-ATPase activity in Arabidopsis

“…The fact that almost all CBLs are associated with either plasma membrane or tonoplast suggests that CBLs and their target kinases (CIPKs) build up an extensive network for the regulation of membrane transport processes. Indeed, findings of CBL1, CBL4 and CBL9 as critical regulators of plasma membrane transporters such as SOS1, AKT1, and CHL1 support this hypothesis [17,18,[21][22][23][24]. Do tonoplastassociated CBLs coordinate with the plasma membrane CBLs in regulating cellular ionic homeostasis?…”

“…While CBL1 and CBL9 harbor motifs for N-myristoylation and S-acylation that anchor them to the plasma membrane and target their function towards regulation of transporters [21][22][23]30], several other CBL members, including CBL2, CBL3, CBL6 and CBL10, contain variable N-terminal extensions that target the proteins specifically to the vacuolar membranes [20,28]. As discussed above, substantial progress has been made in characterizing plasma membrane-targeted CBL proteins such as CBL1, CBL4 and CBL9.…”

“…Further studies indicated that another CBL member, CBL10, also interacts with CIPK24/SOS2 to protect Arabidopsis shoots from salt stress [19,20]. In response to low-K + status, CIPK23 is activated and recruited to the plasma membrane by Ca 2+ sensors CBL1 and CBL9, and the CBL-CIPK23 complexes activate the shaker-like K + channel AKT1 for optimal K + uptake under limiting K + supply conditions [21][22][23]. Apart from controlling K + transport in roots, the CBL1-CIPK23 and CBL9-CIPK23 complexes also regulate the dual-affinity nitrate (NO 3 − ) transporter CHL1 by phosphorylation that serves as a molecular switch between low-and high-affinity NO 3 − transport modes [24].…”

Dual Fatty Acyl Modification Determines the Localization and Plasma Membrane Targeting of CBL/CIPK Ca2+ Signaling Complexes in Arabidopsis

“…CBLs determine the cellular localization of their interacting protein kinases (D'Angelo et al, 2006;Xu et al, 2006;Cheong et al, 2007). The resulting CBL/CIPK complexes exert important functions at the plasma membrane by regulating the activity of ion channels and H þ -ATPases (Xu et al, 2006;Fuglsang et al, 2007).…”

“…Reverse genetics analyses have uncovered important functions of these proteins in abiotic stress responses and in the regulation of ion homeostasis (Albrecht et al, 2003;Zhu, 2003;Xu et al, 2006). The calcium sensors CBL1 and CBL9 have been shown to localize to the plasma membrane, where they form alternative complexes with their target kinases (D'Angelo et al, 2006;Cheong et al, 2007). Here, both calcium sensors function synergistically to activate the kinase CIPK23, which regulates the activity of the potassium channel Arabidopsis K þ Transporter1 (Xu et al, 2006).…”