Advances in Molecular Mechanisms Regulating Heat Stress Response and Thermotolerance Variation in Woody Fruit Trees

As global warming continues, extreme high temperature events have become a growing threat to the growth and production of fruit trees. Heat stress damages plant photosynthetic functions, disturbs normal energy metabolism, causes the build-up of harmful reactive oxygen species, and affects reproductive growth. These negative changes ultimately limit tree growth, reduce fruit yield and lower fruit quality. Fruit trees show a series of active and changing responses to high temperature, including reduced photosynthetic efficiency, early sensing of stress signals, activation of gene regulation pathways, adjustment of metabolic patterns, and stabilization of internal oxidation balance. Key transcription factors, such as HSF, WRKY, bZIP and NAC proteins, as well as their regulatory systems, play important roles in collecting stress signals and controlling downstream defense reactions. In addition, natural genetic differences and non-coding regulatory sequences lead to differences in heat resistance among different fruit tree varieties. Although more and more studies have focused on how fruit trees respond to heat stress, the overall understanding of heat tolerance mechanisms and effective genetic improvement methods is still not complete. To provide clear theoretical support and useful technical guidance for breeding heat-resistant fruit tree varieties, this paper reviews recent research progress on the physiological and molecular mechanisms of fruit tree heat responses; summarizes the physiological, molecular and metabolic changes induced by heat stress; introduces the key regulatory factors and genetic characteristics of heat tolerance; and discusses future directions for molecular breeding of heat-resistant fruit trees.