Chemical signaling under abiotic stress environment in plants

Abstract

Many chemicals are critical for plant growth and development and play an important role in integrating various stress signals and controlling downstream stress responses by modulating gene expression machinery and regulating various transporters/pumps and biochemical reactions. These chemicals include calcium (Ca(2+)), cyclic nucleotides, polyphosphoinositides, nitric oxide (NO), sugars, abscisic acid (ABA), jasmonates (JA), salicylic acid (SA) and polyamines. Ca(2+) is one of the very important ubiquitous second messengers in signal transduction pathways and usually its concentration increases in response to the stimuli including stress signals. Many Ca(2+) sensors detect the Ca(2+) signals and direct them to downstream signaling pathways by binding and activating diverse targets. cAMP or cGMP protects the cell with ion toxicity. Phosphoinositides are known to be involved both in transmission of signal across the plasma membrane and in intracellular signaling. NO activates various defense genes and acts as a developmental regulator in plants. Sugars affect the expression of many genes involved in photosynthesis, glycolysis, nitrogen metabolism, sucrose and starch metabolism, defense mechanisms and cell cycle regulation. ABA, JA, SA and polyamines are also involved in many stress responses. Cross-talk between these chemical signaling pathways is very common in plant responses to abiotic and bitotic factors. In this article we have described the role of these chemicals in initiating signaling under stress conditions mainly the abiotic stress.

Keywords: ABA; Ca2+ binding proteins; abiotic stress; calcium signaling; cyclic nucleotides; nitric oxide; phosphoinositides signaling; signal transduction; sugar signaling.

Figure 1

Generic pathway for plant response to stress. The extracellular stress signal is first perceived by the membrane receptors and then activate large and complex signaling cascade intracellularly including the generation of secondary signal molecules. The stress signal can first activates phospholipase C (PLC), which hydrolyses PIP₂ to generate IP₃ and DAG resulting in an increase in the level of Ca²⁺ ions in the cytosol, which is sensed by calcium sensor. The signal cascade results into the expression of multiple stress responsive genes, the products of which can provide the stress tolerance directly or indirectly. Overall, the stress response could be coordination action of many genes, which may cross-talk with each others.

Figure 2

The units of the calcium signaling network. A major protein that regulates a number of calcium mediated signals is calmodulin, which has four calcium binding sites (A). On receiving stimulii, calcium mobilizing signals act on various ON mechanisms that lead to an increase in the intracellular Ca⁺⁺. The increase in calcium is sensed by various proteins like calmodulin or directly by calcium dependent protein kinases (CDPKs). The CDPKs have a catalytic domain and calcium binding domain like calmodulin (B). There are other calcium binding proteins also which can sense calcium concentration. All these proteins can trigger downstream processes which can elicit specific response (C). The response is terminated by OFF mechanisms that restore Ca⁺⁺ to the resting level.

Figure 3

NO as a signaling molecule. The NO is synthesized from arginine (A). In plants the NO is synthesized by an enzyme that has homology with the p-protein of alanine decarboxylase in response to pathogen. It can also be synthesized by the activity of nitrate reductase as was shown in ABA mediated stomata closure response (B).