A Ca ²⁺ signaling pathway regulates a K ⁺ channel for low-K response in Arabidopsis

Abstract

Nutrient sensing is critical for plant adaptation to the environment. Because of extensive farming and erosion, low content of mineral nutrients such as potassium (K ؉ ) in soils becomes a limiting factor for plant growth. In response to low-K conditions, plants enhance their capability of K ؉ uptake through an unknown signaling mechanism. Here we report the identification of a Ca 2؉ -dependent pathway for low-K response in Arabidopsis. We are not aware of any other example of a molecular pathway for a nutrient response in plants. Earlier genetic analyses revealed three genes encoding two Ca 2؉ sensors (CBL1 and CBL9) and their target protein kinase (CIPK23) to be critical for plant growth on low-K media and for stomatal regulation, indicating that these calcium signaling components participate in the low-K response and turgor regulation. In this study, we show that the protein kinase CIPK23 interacted with, and phosphorylated, a voltage-gated inward K ؉ channel (AKT1) required for K ؉ acquisition in Arabidopsis. In the Xenopus oocyte system, our studies showed that interacting calcium sensors (CBL1 and CBL9) together with target kinase CIPK23, but not either component alone, activated the AKT1 channel in a Ca 2؉ -dependent manner, connecting the Ca 2؉ signal to enhanced K ؉ uptake through activation of a K ؉ channel. Disruption of both CBL1 and CBL9 or CIPK23 gene in Arabidopsis reduced the AKT1 activity in the mutant roots, confirming that the Ca 2؉ -CBL-CIPK pathway functions to orchestrate transporting activities in planta according to external K ؉ availability.calcium signaling ͉ potassium channel ͉ nutrient sensing ͉ potassium uptake ͉ protein kinase P lants and microbes are able to adapt to widely varying environmental conditions, including the availability of inorganic nutrients. The response to K ϩ is especially relevant because it is the major inorganic osmoticum that contributes to turgor pressure and, hence, to cellular growth and development (1, 2). Both plants and fungi maintain internal K ϩ near 100 mM even when extracellular K ϩ varies between 1 M and 100 mM (i.e., over five orders of magnitude in free concentration). A large collection of transporters is required for this homeostatic mechanism (3-6), yet little is known about the mechanism underlying regulation and integration of the transport activities in concert with the extracellular K ϩ levels. Some studies suggest that plants enhance their capability of K ϩ uptake by activating some K ϩ transporters in response to K ϩ -deficient conditions (7-10). A signaling process exists for the plants to ''monitor'' external K ϩ concentration and ''respond'' to the low-K condition by enhancing the capability for K ϩ acquisition. This signaling pathway represents a typical nutrient sensing and response process in plants about which our knowledge is extremely limited. One recent study indicates that low-K status in the soil triggers elevated production of H 2 O 2 that may serve as a signaling molecule to alter the expression of certain genes (11). Becaus...

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?…”

“…These results add to our understanding with regard to the role of CBL-type calcium sensors in plant responses to low-K + stress. While CBL1 and CBL9 function at plasma membrane to regulate K + uptake from the soil through activation of AKT1 [21,22,45], CBL2 and CBL3 may control intracellular K + homeostasis through regulating K + transport across tonoplast.…”

“…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].…”

The role of calcium sensor-interacting protein kinases in plant adaptation to potassium-deficiency: new answers to old questions

“…Wu and colleagues [4] and Luan and colleagues [5] identified a calcineurin B-like protein (CBL)-interacting protein kinase CIPK23 and two upstream elements, CBL1 and CBL9, as regulators of AKT1. AKT1 is a Shaker-type voltage-gated ion channel that mediates the uptake of K + at hyperpolarized membrane voltages [2].…”