Inter‐tissue and inter‐organ signaling in drought stress response and phenotyping of drought tolerance

Plant response to drought stress includes systems for intracellular regulation of gene expression and signaling, as well as inter‐tissue and inter‐organ signaling, which helps entire plants acquire stress resistance. Plants sense water‐deficit conditions both via the stomata of leaves and roots, and transfer water‐deficit signals from roots to shoots via inter‐organ signaling. Abscisic acid is an important phytohormone involved in the drought stress response and adaptation, and is synthesized mainly in vascular tissues and guard cells of leaves. In leaves, stress‐induced abscisic acid is distributed to various tissues by transporters, which activates stomatal closure and expression of stress‐related genes to acquire drought stress resistance. Moreover, the stepwise stress response at the whole‐plant level is important for proper understanding of the physiological response to drought conditions. Drought stress is sensed by multiple types of sensors as molecular patterns of abiotic stress signals, which are transmitted via separate parallel signaling networks to induce downstream responses, including stomatal closure and synthesis of stress‐related proteins and metabolites. Peptide molecules play important roles in the inter‐organ signaling of dehydration from roots to shoots, as well as signaling of osmotic changes and reactive oxygen species/Ca(2+). In this review, we have summarized recent advances in research on complex plant drought stress responses, focusing on inter‐tissue signaling in leaves and inter‐organ signaling from roots to shoots. We have discussed the mechanisms via which drought stress adaptations and resistance are acquired at the whole‐plant level, and have proposed the importance of quantitative phenotyping for measuring plant growth under drought conditions.