Overview of the roles of calcium sensors in plants’ response to osmotic stress signalling.

Calcium signals serve an important function as secondary messengers between cells in various biological processes due to their robust homeostatic mechanism, maintaining an intracellular free Ca2+ concentration. Plant growth, development, and biotic and abiotic stress are all regulated by Ca2+ signals. Ca2+ binding proteins decode and convey the messages encoded by Ca2+ ions. In the presence of high quantities of Mg2+ and monovalent cations, such sensors bind to Ca2+ ions and modify their conformation in a Ca2+ -dependent manner. Calcium-dependent protein kinases (CPKs), calmodulins (CaMs), and calcineurin B-like proteins are all calcium sensors (CBLs). To transmit Ca2+ signals, CPKs, CBLs, and CaMs interact with target proteins and regulate the expression of their genes. These target proteins may be protein kinases, metabolic enzymes, or cytoskeletal-associated proteins. Beyond its role in plant nutrition as a macroelement and its involvement in the plant cell wall structure, calcium modulates many aspects of development, growth and adaptation to environmental constraints such as drought, salinity and osmotic stresses. This review summarises current knowledge on calcium sensors in plant responses to osmotic stress signalling.

A soybean calcineurin B-like protein-interacting protein kinase, GmPKS4, regulates plant responses to salt and alkali stresses.

Calcineurin B-like protein-interacting protein kinases (CIPKs) are key elements of plant abiotic stress signaling pathways. CIPKs are SOS2 (Salt Overly Sensitive 2)-like proteins (protein kinase S [PKS] proteins) which all contain a putative FISL motif. It seems that the FISL motif is found only in the SOS2 subfamily of protein kinases. In this study, the full-length cDNA of a soybean CIPK gene (GmPKS4) was isolated and was revealed to have an important role in abiotic stress responses. A qRT-PCR analysis indicated that GmPKS4 expression is upregulated under saline conditions or when exposed to alkali, salt-alkali, drought, or abscisic acid (ABA). A subcellular localization assay revealed the presence of GmPKS4 in the nucleus and cytoplasm. Further studies on the GmPKS4 promoter suggested it affects soybean resistance to various stresses. Transgenic Arabidopsis thaliana and soybean hairy roots overexpressing GmPKS4 had increased proline content as well as high antioxidant enzyme activities but decreased malondialdehyde levels following salt and salt-alkali stress treatments. Additionally, GmPKS4 overexpression activated reactive oxygen species scavenging systems, thereby minimizing damages due to oxidative and osmotic stresses. Moreover, upregulated stress-related gene expression levels were detected in lines overexpressing GmPKS4 under stress conditions. In conclusion, GmPKS4 improves soybean tolerance to salt and salt-alkali stresses. The overexpression of GmPKS4 enhances the scavenging of reactive oxygen species, osmolyte synthesis, and the transcriptional regulation of stress-related genes.