The CBL-CIPK Pathway in Plant Response to Stress Signals

Abstract

Plants need to cope with multitudes of stimuli throughout their lifecycles in their complex environments. Calcium acts as a ubiquitous secondary messenger in response to numerous stresses and developmental processes in plants. The major Ca²⁺ sensors, calcineurin B-like proteins (CBLs), interact with CBL-interacting protein kinases (CIPKs) to form a CBL-CIPK signaling network, which functions as a key component in the regulation of multiple stimuli or signals in plants. In this review, we describe the conserved structure of CBLs and CIPKs, characterize the features of classification and localization, draw conclusions about the currently known mechanisms, with a focus on novel findings in response to multiple stresses, and summarize the physiological functions of the CBL-CIPK network. Moreover, based on the gradually clarified mechanisms of the CBL-CIPK complex, we discuss the present limitations and potential prospects for future research. These aspects may provide a deeper understanding and functional characterization of the CBL-CIPK pathway and other signaling pathways under different stresses, which could promote crop yield improvement via biotechnological intervention.

Keywords: CBL; CIPK; abiotic stress; biotic stress; signaling.

Figure 1

The schematic diagram and classification of calcium sensors in plants. Calcium sensors combine with the Ca²⁺ by EF-hand, and then activate the binding proteins or themselves to regulate the downstream. calmodulins (CaMs), CaM-like proteins (CMLs), and calcineurin B-like proteins (CBLs) are sensor relays that interact with Ca²⁺-dependent protein, and calcium-dependent protein kinases (CDPKs) are sensor responders which contain the protein kinase domain. The special sensors CBLs interact with CBL-interacting protein kinases (CIPKs) and form the bimolecular sensor responder.

Figure 2

The domain structure and phylogenetic relationships of CBLs and CIPKs in Arabidopsis and rice (Oryza sativa). (A) CBLs contain four EF-hands that are separated by short amino acids with conserved numbers; (C) CIPKs contain conserved NAF motif and protein phosphatase interaction (PPI) motif which interact with CBLs and PP2Cs, respectively; (B,D) The phylogenetic relationships combined the classification of CBLs and CIPKs. The intron-rich CIPKs are distinguished with a red dot.

Figure 2

The domain structure and phylogenetic relationships of CBLs and CIPKs in Arabidopsis and rice (Oryza sativa). (A) CBLs contain four EF-hands that are separated by short amino acids with conserved numbers; (C) CIPKs contain conserved NAF motif and protein phosphatase interaction (PPI) motif which interact with CBLs and PP2Cs, respectively; (B,D) The phylogenetic relationships combined the classification of CBLs and CIPKs. The intron-rich CIPKs are distinguished with a red dot.

Figure 2

The domain structure and phylogenetic relationships of CBLs and CIPKs in Arabidopsis and rice (Oryza sativa). (A) CBLs contain four EF-hands that are separated by short amino acids with conserved numbers; (C) CIPKs contain conserved NAF motif and protein phosphatase interaction (PPI) motif which interact with CBLs and PP2Cs, respectively; (B,D) The phylogenetic relationships combined the classification of CBLs and CIPKs. The intron-rich CIPKs are distinguished with a red dot.

Figure 3

Regulation of the CBL–CIPK network in ion homeostasis in Arabidopsis. The solid lines of exchanger at membrane represent the ion direction and interaction, and the dotted line represents negative regulation and moving. See main text for further details. AKT1: Arabidopsis K⁺ transporter 1; AKT2: Arabidopsis K⁺ transporter 2; AHA2: Arabidopsis H⁺ ATPase 2; AMT1: Ammonium transporters 1; AIP1: AKT1-interacting PP2C 1; CHL1: also named NRT1.1, nitrate transporter 1.1; HAK5 high-affinity K⁺ transporter 5; NHX: Na⁺ (K⁺)/H⁺ antiporters; PAT10: Protein S-Acyl transferase 10; SOS1: Salt Overly Sensitive 1, Na⁺/H⁺ exchanger.

Figure 4

The relationship among Ca²⁺, the CBL–CIPK network, reactive oxygen species (ROS) signaling, abscisic acid (ABA) signaling, and other stresses in Arabidopsis. Stimuli usually increase the concentration of Ca²⁺, the production of ROS and ABA in the cytoplasm. Ca²⁺ and ROS promote each other, and the accumulation of ABA will influence the concentration of Ca²⁺ and K⁺. The dotted lines represent indirect connections. The solid lines represent different signaling. AtCBL1 or AtCBL9 interacts with multiple CIPKs to mediate different target proteins in the regulation of many progresses including K⁺, NO₃^-, NH₄⁺ homeostasis, ROS, and ABA signaling. Other CBL–CIPK complexes regulate their related target to involve in H⁺ and Na⁺ homeostasis. ABI1/2: ABA insensitive 1/2; ABR1: Abscisic acid repressor 1; AMT1: Ammonium transporter 1; AKT1: Arabidopsis K⁺ transporter 1; AKT2: Arabidopsis K⁺ transporter 2; AHA2: Arabidopsis H⁺ ATPase 2; HAK5 high-affinity K⁺ transporter 5; NHX: Na⁺ (K⁺)/H⁺ antiporters; RBOHF: respiratory burst oxidase homologs F; SOS1: Salt Overly Sensitive 1, Na⁺/H⁺ exchanger.

Figure 5

A model of CBL-CIPK pathway during stimuli. CBLs combine with the Ca²⁺ increased by stimuli, and activate the CIPKs in response to relevant stresses. The solid lines represent the classical Ca²⁺/CBL-CIPK signaling. The atypical function of CBLs and the target protein PP2C are shown with dotted lines.