9 - Calcium sensing and signaling in plants during metal/metalloid stress

Abstract

Since plants are not mobile organisms, they have extended instant signals to be consistent with external stress conditions. Toxic levels of heavy metals (HMs) in plants inhibit various metabolic processes such as photosynthesis and respiration by interacting with sulfhydryl groups, which adversely affect the structure of the proteins and enzymes and disrupt plasma membrane integrity. Calcium is known to be an important messenger in adaptation and improvement of the plant functions. Stable state of the Ca2+ channels has been shown to occur during the exposure to the excess rates of the HMs that resulted in calcium flow to the cell. Temporary increase in the concentration of the cytosol is recognized by the extremely sensitive proteins that sense calcium and contribute to change this chemical signal into biological responses. Free calcium ions found in intracellular spaces function as a second messenger to signal stress responses caused by the HMs leading to regulation of the downstream genes’ expressions containing the transportation, tolerance, and metabolism of the HMs. For perceiving environmental signals during the stress, CaM-like proteins (CMLs), calcium-modulated proteins or calmodulins (CaMs), and calcineurin B-like proteins (CBLs) are identified to be primary elements of calcium sensing. These proteins sense and decode the alterations in cytosolic Ca2+ concentration through binding to Ca2+ and induce various downstream signaling pathways. Higher knowledge about HM stress signaling and related responses can alleviate or remove loses in crop performance in countries with rising populations.