Mechanisms of hormone regulation for drought tolerance in plants

Drought stress limits the growth and productivity of plants through significant changes at the physiological, cellular, biochemical, and molecular levels. Considerable progress has been made elucidating on how plant hormones contribute to or influence whole-plant drought responses. Advancements in transcriptomics coinciding with mutant analysis have suggested that specific hormones regulate processes such as leaf senescence, antioxidant metabolism, carbon balance, and gas exchange during periods of drought stress. Hormones including abscisic acid, auxins, cytokinins, gibberellins, ethylene, salicylates, and jasmonates may independently regulate these plant responses through regulation of transcription factors and subsequent downstream induction or repression of stress-responsive genes. Alternatively, hormone-to-hormone or hormone-to-sugar cross talk may facilitate the drought responses. This chapter provides an overview of the major physiological processes regulated by plant hormones and the roles that different hormones serve during physiological responses to drought stress. Current knowledge of hormone-to-hormone or hormone-to-sugar cross talk regulating physiological responses to drought stress is also discussed. Suggestions for ongoing and future research are provided, such as which molecules perceive a specific hormone to then initiate signal transduction cascades during drought stress. Cross talk signals derived from hormone-to-hormone or hormone-to-sugar or secondary messengers (i.e., calcium or ROS) are not yet clear and also need to be investigated further. Research addressing the unanswered questions of hormone-signaling perception and cross talk among hormones and other metabolites will provide further insights into molecular factors controlling hormone regulation of plant tolerance to drought stress.