The role of CBL-CIPK signaling in plant responses to biotic and abiotic stresses

Plants have a variety of regulatory mechanisms to perceive, transduce, and respond to biotic and abiotic stress. One such mechanism is the calcium-sensing CBL-CIPK system responsible for the sensing of specific stressors, such as drought or pathogens. CBLs perceive and bind Calcium (Ca2+) in response to stress and then interact with CIPKs to form an activated complex. This leads to the phosphorylation of downstream targets, including transporters and ion channels, and modulates transcription factor levels and the consequent levels of stress-associated genes. This review describes the mechanisms underlying the response of the CBL-CIPK pathway to biotic and abiotic stresses, including regulating ion transport channels, coordinating plant hormone signal transduction, and pathways related to ROS signaling. Investigation of the function of the CBL-CIPK pathway is important for understanding plant stress tolerance and provides a promising avenue for molecular breeding.

Ca2+-CBL-CIPK: a modulator system for efficient nutrient acquisition

Calcium (Ca²⁺) is a universal second messenger essential for the growth and development of plants in normal and stress situations. In plants, the proteins, CBL (calcineurin B-like) and CIPK (CBL-interacting protein kinase), form one of the important Ca²⁺ decoding complexes to decipher Ca²⁺ signals elicited by environmental challenges. Multiple interactors distinguish CBL and CIPK protein family members to form a signaling network for regulated perception and transduction of environmental signals, e.g., signals generated under nutrient stress conditions. Conservation of equilibrium in response to varying soil nutrient status is an important aspect for plant vigor and yield. Signaling processes have been reported to observe nutrient fluctuations as a signal responsible for regulated nutrient transport adaptation. Recent studies have identified downstream targets of CBL-CIPK modules as ion channels or transporters and their association in signaling nutrient disposal including potassium, nitrate, ammonium, magnesium, zinc, boron, and iron. Ca²⁺-CBL-CIPK pathway modulates ion transporters/channels and hence maintains a homeostasis of several important plant nutrients in the cytosol and sub-cellular compartments. In this article, we summarize recent literature to discuss the role of the Ca²⁺-CBL-CIPK pathway in cellular osmoregulation and homeostasis on exposure to nutrient excess or deprived soils. This further establishes a link between taking up the nutrient in the roots and its distribution and homeostasis during the generation of signal for the development and survival of plants.

VDACs: An Outlook on Biochemical Regulation and Function in Animal and Plant Systems

The voltage-dependent anion channels (VDACs) are the most abundant proteins present on the outer mitochondrial membrane. They serve a myriad of functions ranging from energy and metabolite exchange to highly debatable roles in apoptosis. Their role in molecular transport puts them on the center stage as communicators between cytoplasmic and mitochondrial signaling events. Beyond their general role as interchangeable pores, members of this family may exhibit specific functions. Even after nearly five decades of their discovery, their role in plant systems is still a new and rapidly emerging field. The information on biochemical regulation of VDACs is limited. Various interacting proteins and post-translational modifications (PTMs) modulate VDAC functions, amongst these, phosphorylation is quite noticeable. In this review, we have tried to give a glimpse of the recent advancements in the biochemical/interactional regulation of plant VDACs. We also cover a critical analysis on the importance of PTMs in the functional regulation of VDACs. Besides, the review also encompasses numerous studies which can identify VDACs as a connecting link between Ca2+ and reactive oxygen species signaling in special reference to the plant systems.

Characterization of CBL-CIPK signaling complexes and their involvement in cold response in tea plant

Calcineurin B-like (CBL) proteins, a class of Ca²⁺-binding proteins, play vital roles in calcium signal transduction by interacting specifically with CBL-interacting protein kinases (CIPKs), and these two gene families and their interacting complexes are involved in regulating plant responses to various environmental stimuli. In the present study, eight CBL and 25 CIPK genes were identified in tea plant and divided into four and five subfamilies, respectively. Analysis of the expression of these genes in response to abiotic stresses (mature leaves treated with cold, salinity, and PEG and young shoots treated with cold) revealed that CsCBL1/3/5 and CsCIPK1/4/5/6a/7/8/10b/10c/12/14a/19/23a/24 could be induced by at least two stresses. Under cold stress, CsCBL9 and CsCIPK4/6a/6b/7/11/14b/19/20 were upregulated in both mature leaves and young shoots, CsCBL1/3/5 and CsCIPK1/8/10a/10b/10c/12/14a/23a/24 were induced only in mature leaves, and CsCIPK5/25 were induced only in young shoots. Yeast two-hybrid analysis showed that CsCBL1 could interact with CsCIPK1/10b/12 but not with CsCIPK6a/7/11/14b/20. CsCBL9 was found to interact with CsCIPK1/10b/12/14b but not with CsCIPK6a/7/11/20. These results suggest divergent responses to cold stress regulated by CBL-CIPK complexes between tea plant and Arabidopsis, as well as between mature leaves and young shoots in tea plant. A model of Ca²⁺-CsCBL-CsCIPK module-mediated abiotic stress signaling in tea plant is proposed.

Emerging concepts of potassium homeostasis in plants

Potassium (K+) is an essential cation in all organisms that influences crop production and ecosystem stability. Although most soils are rich in K minerals, relatively little K+ is present in forms that are available to plants. Moreover, leaching and run-off from the upper soil layers contribute to K+ deficiencies in agricultural soils. Hence, the demand for K fertilizer is increasing worldwide. K+ regulates multiple processes in cells and organs, with K+ deficiency resulting in decreased plant growth and productivity. Here, we discuss the complexity of the reactive oxygen species-calcium-hormone signalling network that is responsible for the sensing of K+ deficiency in plants, together with genetic approaches using K+ transporters that have been used to increase K+ use efficiency (KUE) in plants, particularly under environmental stress conditions such as salinity and heavy metal contamination. Publicly available rice transcriptome data are used to demonstrate the two-way relationship between K+ and nitrogen nutrition, highlighting how each nutrient can regulate the uptake and root to shoot translocation of the other. Future research directions are discussed in terms of this relationship, as well as prospects for molecular approaches for the generation of improved varieties and the implementation of new agronomic practices. An increased knowledge of the systems that sense and take up K+, and their regulation, will not only improve current understanding of plant K+ homeostasis but also facilitate new research and the implementation of measures to improve plant KUE for sustainable food production.

Arabidopsis calcineurin B-like proteins differentially regulate phosphorylation activity of CBL-interacting protein kinase 9

Calcium (Ca²⁺) is a versatile and ubiquitous second messenger in all eukaryotes including plants. In response to various stimuli, cytosolic calcium concentration ([Ca²⁺]_cyt) is increased, leading to activation of Ca²⁺ sensors including Arabidopsis calcineurin B-like proteins (CBLs). CBLs interact with CBL-interacting protein kinases (CIPKs) to form CBL-CIPK complexes and transduce the signal downstream in the signalling pathway. Although there are many reports on the regulation of downstream targets by CBL-CIPK module, knowledge about the regulation of upstream components by individual CIPKs is inadequate. In the present study, we have carried out a detailed biochemical characterization of CIPK9, a known regulator of K⁺ deficiency in Arabidopsis, with its interacting CBLs. The present study suggests that CIPK9 specifically interacts with four CBLs, i.e. CBL1, CBL2, CBL3 and CBL9, in yeast two-hybrid assays. Out of these four CBLs, CBL2 and CBL3, specifically enhance the kinase activity of CIPK9, while the CBL1 and CBL9 decrease it as examined by in vitro kinase assays. In contrast, truncated CIPK9 (CIPK9ΔR), without the CBL-interacting regulatory C-terminal region, is not differentially activated by interacting CBLs. The protein phosphorylation assay revealed that CBL2 and CBL3 serve as preferred substrates of CIPK9. CBL2- and CBL3-CIPK9 complexes show altered requirement for metal cofactors when compared with CIPK9 alone. Moreover, the autophosphorylation of constitutively active CIPK9 (CIPK9T178D) and less active CIPK9 (CIPK9T178A) in the presence of CBL2 and CBL3 was further enhanced. Our study suggests that CIPK9 differentially phosphorylates interacting CBLs, and furthermore, the kinase activity of CIPK9 is also differentially regulated by specific interacting CBLs.

Rice grassy stunt virus p5 interacts with two protein components of the plant-specific CBL-CIPK Ca+2 signaling network of rice

Rice grassy stunt virus (RGSV) is a tenuivirus posing a threat to rice production in many South, Southeast, and East Asian countries. To date, no host factor interacting with RGSV has been reported. In this study, we screened a rice cDNA library with the GAL4-based yeast two-hybrid system using RGSV p5 as the bait. One of the candidate host factors interacting with RGSV p5 was found to be CBL-interacting protein kinase 25 (OsCIPK25), a member of the plant-specific CBL-CIPK Ca2+ signaling network. The interaction between RGSV p5 and OsCIPK25, as well as OsCIPK5, which is closely related to OsCIPK25, was confirmed by their cellular co-localization and by a bimolecular fluorescence complementation assay in Nicotiana benthamiana cells. Given the importance of CIPKs in the regulation of ion homeostasis and the resemblance of RGSV symptoms to potassium deficiency in rice, we evaluated potassium content of RGSV-infected rice and found it to be much lower than that in the healthy rice.