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The ABA signaling pathway plays a central role in drought responses in plants ( Cutler et al., 2010). The primary stress signal caused by drought is hyperosmotic stress (or osmotic stress for short), which can result in rapid accumulation of abscisic acid (ABA) in plants ( Zhu, 2002, 2016). These reactions occur in a disciplined spatiotemporal order and are interrelated, forming a systematic drought response mechanism. In plants, drought stress results in a network of physiological and biochemical reactions, such as limitation of photosynthesis, enhanced stomatal closure, changes in cellular components, stimulation of osmolyte production, and accumulation of reactive oxygen species (ROS Miao et al., 2006 Cutler et al., 2010 Krasensky and Jonak, 2012). Therefore, it is of considerable importance for the sustainable development of global agriculture to breed drought-tolerant cultivars and elucidate the mechanisms of abiotic stress responses. Drought stress, in particular, has become a critical problem worldwide as temperatures rise and arid lands expand, seriously affecting the growth and productivity of the most important crops ( Fedoroff et al., 2010 Gilbert, 2012). With global climate change, plant growth and development are often subject to diverse environmental stresses, such as drought, heat, cold, and high salinity ( Zhu, 2016). Collectively, these findings provide insight into the mechanism by which stress-inducible MdHSFA8a modulates flavonoid synthesis to regulate drought tolerance. In addition, we demonstrated that MdHSFA8a participates in abscisic acid-induced stomatal closure and promotes the expression of abscisic acid signaling-related genes. However, under drought stress, the MdHSP90-MdHSFA8a complex dissociated and the released MdHSFA8a further interacted with the APETALA2/ETHYLENE RESPONSIVE FACTOR family transcription factor RELATED TO AP2.12 to activate downstream gene activity. A chaperone, HEAT SHOCK PROTEIN90 (HSP90), interacted with MdHSFA8a to inhibit its binding activity and transcriptional activation. In this study, we demonstrated that a drought-responsive HSF, designated MdHSFA8a, promotes the accumulation of flavonoids, scavenging of reactive oxygen species, and plant survival under drought conditions.
Heat shock factors (HSFs) have well-documented functions in stress responses, but their roles in flavonoid synthesis and the flavonoid-mediated drought response mechanism remain elusive. Drought is an important environmental factor affecting the growth and production of agricultural crops and fruits worldwide, including apple ( Malus domestica).