Plant hormone-mediated regulation of stress responses

Abstract

Background: Being sessile organisms, plants are often exposed to a wide array of abiotic and biotic stresses. Abiotic stress conditions include drought, heat, cold and salinity, whereas biotic stress arises mainly from bacteria, fungi, viruses, nematodes and insects. To adapt to such adverse situations, plants have evolved well-developed mechanisms that help to perceive the stress signal and enable optimal growth response. Phytohormones play critical roles in helping the plants to adapt to adverse environmental conditions. The elaborate hormone signaling networks and their ability to crosstalk make them ideal candidates for mediating defense responses.

Results: Recent research findings have helped to clarify the elaborate signaling networks and the sophisticated crosstalk occurring among the different hormone signaling pathways. In this review, we summarize the roles of the major plant hormones in regulating abiotic and biotic stress responses with special focus on the significance of crosstalk between different hormones in generating a sophisticated and efficient stress response. We divided the discussion into the roles of ABA, salicylic acid, jasmonates and ethylene separately at the start of the review. Subsequently, we have discussed the crosstalk among them, followed by crosstalk with growth promoting hormones (gibberellins, auxins and cytokinins). These have been illustrated with examples drawn from selected abiotic and biotic stress responses. The discussion on seed dormancy and germination serves to illustrate the fine balance that can be enforced by the two key hormones ABA and GA in regulating plant responses to environmental signals.

Conclusions: The intricate web of crosstalk among the often redundant multitudes of signaling intermediates is just beginning to be understood. Future research employing genome-scale systems biology approaches to solve problems of such magnitude will undoubtedly lead to a better understanding of plant development. Therefore, discovering additional crosstalk mechanisms among various hormones in coordinating growth under stress will be an important theme in the field of abiotic stress research. Such efforts will help to reveal important points of genetic control that can be useful to engineer stress tolerant crops.

Keywords: Abiotic stress; Abscisic acid (ABA); Biotic stress; Crosstalk; Gibberellins (GA); Jasmonates (JA); Plant hormones; Salicylic acid (SA).

Fig. 1

An overview of plant hormone signaling networks and their crosstalk in stress responses. ABA, SA, JA and ET are major players in stress response, with ABA mainly regulating osmotic stresses. SA, JA and ET control biotic stress responses. ABA and GA signaling pathways interact, with DELLAs serving as a crosstalk point, to influence the balance between seed dormancy and germination. SA and JA pathways are antagonistically regulated by several transcription factors. JA-ET crosstalk synergistically. Auxins, GAs and CKs participate in biotic stress responses via SA signaling pathway. CKs also crosstalk with ABA and function in drought and salinity stress responses. Arrows represent positive regulation (accumulation of transcripts, proteins or hormones), and blocked arrows represent negative regulation. For abbreviations refer to text