Abstract
High temperature negatively impacts the yield and quality of fruit
crops. Exogenous melatonin (MT) application has shown the capability to
enhance heat tolerance, but the response of endogenous MT to heat
stress, particularly in perennial fruit trees, remains elusive. This
study investigated the effects of high temperatures on transgenic apple
plants overexpressing the MT biosynthetic gene N-acetylserotonin
methyltransferase 9 (MdASMT9 ). Endogenous MT protected transgenic
plants from heat stress, scavenging reactive oxygen species (ROS) and
increasing soluble carbohydrates and amino acids levels.MdASMT9 -overexpressing plants also maintained higher
photosynthetic activity by protecting the chloroplasts from damage.
Transcriptome sequencing indicates that MdASMT9 overexpression
promoting the expression of HSFA1d , HSFA2-like , andHSFA9b , and inhibiting the transcription of HSFB1 andHSFB2b . Application of MT and overexpression of MdASMT9reduced abscisic acid (ABA) accumulation through promoting
MdWRKY33-mediated transcriptional inhibition of MdNCED1 andMdNCED3 , thus promoting stomatal opening for better heat
dissipation. Furthermore, melatonin enhanced autophagic activity through
promoting MdWRKY33-mediated transcriptional enhancement ofMdATG18a under heat stress. These findings provide new
sight into the regulation of endogenous MT and its role in improving
heat tolerance in perennial fruit trees.
Key words: MdASMT9 , melatonin, high temperature,