Plant Hormones in Stress Signaling, Responses and Tolerance
Plant growth, development and survival is largely influenced by the environment which enforce biotic (pathogens) and abiotic (drought, salt, temperature, heat, cold) stresses. Stresses are complex and involve multigenic traits contributing to plant performance. From the agriculture point of view, these kinds of stresses lead to significant economic losses in terms of crop productivity. As plants being sessile (immobile) organisms, they have to fight out a variety of stresses. Therefore, they have gradually evolved numerous adaptive mechanisms by tolerating stresses through physical adaptation and/or integrated cellular and molecular responses. In order to face these environmental challenges, stress signal perception and its transduction is a very important step to ensure plant survival by turning on the adaptive responses to various stresses. In general, functional or regulatory genes gets activated to direct stress tolerance or through signal transduction pathway downstream.
Plant hormones are called “Phytohormones”, the regulators of plant developmental processes play a crucial role in the perception of signals and signaling stress conditions to trigger defense responses. Although these small signaling molecules are produced in very low amounts during multiple stresses, they respond to even the most serious one and begin to build signaling networks. These naturally occurring phytohormones belong to nine known major classes, including Auxins (Indole-3-acetic acid; IAA), Gibberellins (GA), Cytokinins (CT), Ethylene (ET), Abscisic acid (ABA), Salicylic acid (SA), Jasmonic acid (JA), Brassinosteroids (BR) and Strigolactones (ST). All of these hormones elicit different responses to biotic/abiotic stresses. Apart from stress hormones, there are various other receptors/sensors like phytochromes, G-coupled receptors, histidine kinases and receptor-like kinases are involved in different signal transduction pathways. Subsequent to primary signals, secondary signaling molecules such as Reactive Oxygen Species (ROS), ABA and inositol phosphatase are generated.
Modulation of intracellular Ca2+ level takes place in the presence of secondary molecules, which switches on the protein phosphorylation cascades, including mitogen-activated protein kinases (MAPKs), calcium-dependent protein kinases (CDPKs), protein phosphatases, protein kinases, transcription factors (TFs) and stress responsive genes. Transgenic over-expression of the transcription factor HvCBF4 (CBF/DREBs) from barley has demonstrated to enhance tolerance to drought, salt and low temperature stresses without affecting plant growth in rice. Hence, an understanding of the physiological, morphological and molecular characteristics of plant functions under stress conditions followed by a good knowledge on the specificity of signaling pathways are absolutely necessary to engineer stress tolerance traits in plants. However, it’s important to simultaneously focus on the genetic enhancement by introducing diverse physiological traits on to an elite cultivar for obtaining a complete improvement of adaptation to environmental stresses in plants.
In the recent decades, Jasmonic acid (JA) and Salicylic acid (SA) have shown to be potential tools to combat biotic and abiotic stresses. JA is a ubiquitous signaling molecule that plays a regulatory role in distinct developmental processes. JA and its derivatives are majorly associated with plant defense and resistance to pathogen infection, insect damage, wounding, drought, salinity and low temperature stresses. For example, a simple mechanical wounding in tomato plants can turn on the JA signalling pathway upon interaction of the systemin (a plant peptide hormone produced by the Solanaceae family) with the cell surface receptor via the apoplastic pathway (cell to cell movement) to eventually activate the JA responses. Most importantly, Jasmonic acid regulate the expression of several stress responsive genes and TFs such as ERFs, WRKYs, MYBs, MYCs and NACs that are involved in the synthesis of JAs, plant defense, development and secondary metabolite production in response to biotic and abiotic stresses.
Another function of JA is that it crosstalks with other phytohormones to regulate plant responses to stresses, thus may involve in balancing plant growth and defense resistance. Salicylic acid, on the other hand, is a phenolic compound that is synthesized in plants via the phenylpropanoid pathway. The role of SA has been well established in pathogenesis-related (PR) gene expression, hypersensitive response (HR) to pathogen infection and systemic acquired resistance (SAR). There have been contradictory conclusions with regard to the role of SA in response to abiotic stresses. In other words, high levels of SA increase the susceptibility to abiotic stresses in plants. SA generates ROS which prevents from oxidative damage under salt and osmotic stress conditions in plant tissues, therefore participates in the development of stress symptoms.
Also, SA levels majorly contribute to promoting stress tolerances by enhancing glutathione content. Adequate concentrations of SA improve the anti-oxidative capacity of plants and directly influence to induce genes responsible for the synthesis of protective compounds like polyamines. Regardless of the distinct roles of SA and JA in response to stresses, they act coordinatively in regulating disease resistance against pathogens.