Drought Stress Responses and Resistance in Plants: From Cellular Responses to Long-Distance Intercellular Communication

Abstract

The drought stress responses of vascular plants are complex regulatory mechanisms because they include various physiological responses from signal perception under water deficit conditions to the acquisition of drought stress resistance at the whole-plant level. It is thought that plants first recognize water deficit conditions in roots and that several molecular signals then move from roots to shoots. Finally, a phytohormone, abscisic acid (ABA) is synthesized mainly in leaves. However, the detailed molecular mechanisms of stress sensors and the regulators that initiate ABA biosynthesis in response to drought stress conditions are still unclear. Another important issue is how plants adjust ABA propagation, stress-mediated gene expression and metabolite composition to acquire drought stress resistance in different tissues throughout the whole plant. In this review, we summarize recent advances in research on drought stress responses, focusing on long-distance signaling from roots to shoots, ABA synthesis and transport, and metabolic regulation in both cellular and whole-plant levels of Arabidopsis and crops. We also discuss coordinated mechanisms for acquiring drought stress adaptations and resistance via tissue-to-tissue communication and long-distance signaling.

Keywords: abscisic acid; drought stress; metabolites; peptides; protein kinases; tissue-to-tissue communication; transporters.

Figure 1

Conceptual diagram of root-to-shoot, shoot-to-shoot, and local signals in response to drought stress conditions. Mobile signals such as hydraulic pressure, ROS/Ca²⁺ waves, peptides, and phytohormones mediate tissue-to-tissue and long-distance communication for the acquisition of drought stress resistance at the whole-plant level. The red line indicates the root-to-shoot signals such as hydraulic pressure, ROS/Ca²⁺ waves, and peptides signals under dehydration stress conditions. The orange line indicates the shoot-to-shoot signals of ROS/Ca²⁺ waves to mediate stomatal closure under stress conditions. The blue line indicates the local signals of ROS/Ca²⁺ waves, peptide, or ABA signals that mediate stomatal control under stress conditions.

Figure 2

Metabolites and their functions in abiotic stress tolerance, especially under drought, salinity, cold and heat stress. Plants show a variety of metabolic responses to diverse abiotic stresses. Stress-induced accumulation of compatible solutes such as sugars (e.g., raffinose and sucrose), sugar alcohols (e.g., galactinol), and amino acids (e.g., BCAAs, proline, and glycine-betaine) functions in osmotic adjustment, enabling the maintenance of cell turgor for plant growth and survival under stress conditions. These compatible solutes, especially galactinol and raffinose, can act as free radical (ROS) scavengers, protecting against oxidation by removing excess ROS and reestablishing the cellular redox balance. BCAAs can act as regulatory factors in the production of specialized/secondary metabolites as a defense response against biotic stress during abiotic stress. Stress-induced accumulation of antioxidants such as flavonoids (e.g., anthocyanins and flavonols) allows them to act as free radical scavengers to mitigate oxidative and drought stress in plants. Genes involved in metabolite synthesis in diverse abiotic stress responses are useful for application in the metabolic engineering of stress resistance in dry field conditions.