Abstract
Plant growth, development, and ultimately crop productivity are largely impacted by the interaction of plants with different abiotic and biotic factors throughout their life cycle. Perception of different abiotic stresses, such as salt, cold, drought, heat, and heavy metals, and interaction with beneficial and harmful biotic agents by plants lead to transient, sustained, or oscillatory changes of [calcium ion, Ca2+]cyt within the cell. Significant progress has been made in the decoding of Ca2+ signatures into downstream responses to modulate differential developmental and physiological responses in the whole plant. Ca2+ sensor proteins, mainly calmodulins (CaMs), calmodulin-like proteins (CMLs), and others, such as Ca2+-dependent protein kinases (CDPKs), calcineurin B-like proteins (CBLs), and calmodulin-binding transcription activators (CAMTAs) have played critical roles in coupling the specific stress stimulus with an appropriate response. This review summarizes the current understanding of the Ca2+ influx and efflux system in plant cells and various Ca2+ binding protein-mediated signal transduction pathways that are delicately orchestrated to mitigate abiotic and biotic stresses. The probable interactions of different components of Ca2+ sensor relays and Ca2+ sensor responders in response to various external stimuli have been described diagrammatically focusing on established pathways and latest developments. Present comprehensive insight into key components of the Ca2+ signaling toolkit in plants can provide an innovative framework for biotechnological manipulations toward crop improvability in near future.
Highlights
Calcium ion (Ca2+) acts as a critically important relay hub for many biological messages
Plant cells maintain a relatively low concentration of free cytosolic calcium ([Ca2+]cyt), of around 100 nM compared to extracellular surroundings and intracellular storage compartments where Ca2+ concentration ranges in the mM scale (Trewavas and Malhó, 1998). [Ca2+]cyt level is strictly regulated at the cellular level and tiny fluctuations in [Ca2+]cyt can be decoded as useful external inklings for activation of downstream signaling components that include various proteins
Since calcineurin B-like 10 (CBL10) and the outer membrane of chloroplasts 34 (TOC34) are expressed in different types of plastids in plants (Cho et al, 2016), it can be speculated that salt stress signaling modulates the functioning of various proteins in the cytoplasm and other organelles
Summary
Calcium ion (Ca2+) acts as a critically important relay hub for many biological messages. These transient boosts in [Ca2+]cyt, known as Ca2+ signatures, depend on Ca2+ channels available in cell membranes or in the sub-cellular compartments within the cell along with the network of Ca2+ transporting proteins (Qudeimat and Frank, 2009) These Ca2+ signatures are eventually sensed, decoded, and transmitted to the downstream signaling components. Calmodulins (CaMs), calmodulin-like proteins (CMLs), and calcineurin B-like proteins (CBLs) are the components of sensor relays that do not possess any enzymatic functions but they possess one or more E and F helices (EF) hands for binding with Ca2+ ions and subsequent modulation in their protein conformation (Meena et al, 2019) Sensor responders, such as calcium-dependent protein kinases (CDPKs or CPKs) and CBL-interacting protein kinases (CIPKs), possess enzymatic functions for interacting with specific downstream proteins through phosphorylation and dephosphorylation (Hashimoto and Kudla, 2011). Taking lessons of the escalating environmental vagaries, attempts have been made to underpin the impact of different Ca2+ signatures and their downstream components toward elevating abiotic stress tolerance in plants
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