Authors: Era Vaidya Malhotra, Anshul Watts, Nimmy M. S.
Plants are subjected to a large number of environmental stresses throughout their life cycle. To protect themselves, and to ensure their survival, plants have developed a very efficient immune system, involving induced defense responses brought about by the activation of several defense related genes and production of defense related secondary metabolites. The stress response mechanism in plants comprises of a network of integrated pathways. Various plant hormones control a vast array of physiological processes, and by virtue of their intricately interconnected signalling pathways, they are the key players in mediating a stress response.
Exposure of plants to different abiotic stresses including drought, salinity and extreme temperatures result in ABA (Abscicic acid) accumulation in plants. On the other hand, salicylic acid (SA) and jasmonates (JA) are involved in plant response to biotic stress conditions. Along with these, the other phytohormones, i.e., auxins, gibberellins (GA) and cytokinins (CK) have also been found to have some role in defense responses by interacting with ABA, SA and JA when the plants are under any type of stress.
Regulation of ABA during abiotic stresses
ABA has been widely studied as an abiotic stress response hormone. It induces certain stress responsive genes. When the plant is in any harsh environment, the ABA level increases, the increased ABA binds to its receptor thereby initiating a signal transduction cascade leading to cellular responses to stresses. The genes responsible for ABA biosynthesis, namely, zeaxanthin epoxidase (zep; also known aslow expression of osmotic stress-responsive gene 6 (los6/aba1); aldehyde oxidase (aao3);9-cis-epoxycarotenoid dioxygenase gene (nced3);molybdenum cofactor sulfurase (mcsu; also known as los5/aba3), are upregulated under osmotic conditions, i.e. salinity and drought, leading to ABA accumulation. There are also certain genes, designated as ABA-responsive genes, which contain ABA responsive elements (ABREs) in their promoter regions. ABA mediates the phosphorylation of ABRE binding factors leading to their activation, and these activated factors then bind to the ABREs resulting in upregulation of the ABA-responsive genes. ABA also regulates the function of many other transcription factors, the most important ones being the dehydration responsive element (DRE)-binding protein (DREB) transcription factors.
Regulation of SA and JA during biotic stresses
Salicylic Acid (SA) plays a role in defense pathways acting against biotrophic and hemi-biotrophic pathogens. SA is a major component of the systemic acquired resistance (SAR). Normally, the SA pathway is activated in plants at the site of pathogen attack and the signal is spread throughout the plant such that a defense response in initiated in other parts of plant so as to protect the undamaged parts. A spike in SA levels results in activation of pathogenesis related (PR) genes, which encode proteins having antimicrobial properties. The PR genes are activated by the regulatory element Non-Expressor of PR Gene1 (NPR1), whose activity is regulated by SA. On perception of SA, NPR1 is deoligomerized into its active monomeric forms, and these monomers in turn facilitate PR gene expression by interacting with certain transcription factors.
Jasmonic acid (JA) and its derivatives, have been found to play an important role in plant defense against insects, pathogen infection, wounding, UV radiation damage, and drought. The transcription factor Jasmonate Insensitive 1/MYC2 (JIN1/MYC2) is mainly responsible for JA responsive gene expression during defense response, along with several members of the Apetala2/Ethylene-Responsive Factor (AP2/ERF) family. An important role in JA mediated defense response is played by a repressor protein Jasmonate-Jim-Domin (JAZ). Under normal conditions, JAZ proteins inhibit the expression of JA-responsive genes by binding to JIN1/MYC2 transcription factor. But under stress conditions when JA begins to accumulate in the plant, CORONATINE INSENSITIVE1 (COI1) protein binds to the accumulated JA and triggers JAZ degradation. On JAZ degradation, JIN1/MYC2 becomes free and leads to upregulation of the expression levels of JA responsive genes. Another gene involved in the JA dependent defense pathway is the JA-associated VQ motif gene 1 (JAV1). JAV1 has been observed to be degraded by JA-COI1 signalling system, leading to the activation of various downstream regulators during a defense response to insect attack.
Hormonal cross talk during stress response
Apart from these specific hormonal responses, there is always a co-ordinated network active between different hormones to regulated gene expression during a stress response. As proper plant growth is required during any stress response, there is a continuous cross talk between the stress hormones and the other growth promoting hormones, namely auxins, GAs and CKs. The SA and JA pathways intersect at many nodes and SA and JA regulate biotic stress responses antagonistically. NPR1, which is a key regulatory element in the SA activated pathway is mainly responsible for suppression of JA responsive genes. Transcription factor, WRKY 70 and MAP kinase 4 (mpk4) also mediate this antagonistic relationship. Overexpression of either one of them leads to constitutive expression of SA responsive PR genes, and repression of JA-responsive PDF1.2 gene. But few instances of SA – JA synergistic interactions have been observed at low concentrations of both the hormones, and when both the defenses are simultaneously induced.
Auxins modify root architecture by interacting with ethylene during drought and salinity stress. During any elicited defense response auxin signalling and auxin response are suppressed, as auxin has been found to promote disease susceptibility. Cytokinins have been found to promote SA defense response. In many cases, CKs activate some transcription factors such as TGA3 which promote SA defense responses. In case of interaction with ABA, CKs act as both, positive as well as negative regulators. Some CK pathway genes are suppressed by ABA, eg.Isopentenyl Transferase, Cytokinin Oxidases, few Histidine Kinases, while genes for some proteins like Histidine Phosphotransfer Proteins are suppressed in a drought response leading to activation of ABA responsive genes.
To protect seeds from drought, high ABA levels are maintained in seeds leading to dormancy by activating some Late Embryogenesis Abundant (LEA) genes. When favourable conditions set in, repressive effects of ABA are overcome by GAs, which promote seed germination. GA and ABA share an antagonistic relationship, and a class of proteins known as DELLA proteins integrate the GA-ABA pathways. DELLA proteins suppress GA responses, and stimulate ABA signalling. GA and JA pathways also share an antagonistic relation, mediated by the DELLA proteins.
Plants have elaborate pathways to respond to any biotic/abiotic stress, among which activation of the phytohormone signalling cascade is also an important branch. The changes in phytohormone levels in response to stress forms the part of early defense response of plants. The phytohormone signalling networks allow the plant to adjust according to the environment, by modifying their growth responses and initiating defense responses. The molecular mechanisms governing this hormone cross talk are being deciphered, and have found to be essential factors in governing plant response to stresses.
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About Author / Additional Info:
Scientist at National Research Centre on Plant Biotechnology (NRCPB), Pusa
Campus, New Delhi