Abstract

Heat stress is a major environmental factor that limits plant growth, yield, and quality, and can even cause plant death. Although the homeodomain-leucine zipper transcription factor family plays broad roles in regulating plant stress responses, their underlying mechanisms in apple heat stress remain unclear. Here, we show that MdHB7 positively regulates basal thermotolerance in apple by promoting reactive oxygen species (ROS) scavenging and lignin accumulation. Transcriptome sequencing and functional annotation identified two differentially expressed genes, MdPRX66 (Peroxidase) and MdLac17 (Laccase), as potential MdHB7 targets involved in heat tolerance. Transcriptional analyses revealed that MdHB7 directly binds to the promoters of MdPRX66 and MdLac17 to activate their expression. Functional assays and physiological measurements further indicated two regulatory mechanisms of MdHB7-mediated thermotolerance: MdHB7 enhances peroxidase (POD) activity by activating MdPRX66, thereby promoting ROS scavenging; and it increases laccase activity by activating MdLac17, thereby facilitating lignin accumulation. Together, these findings demonstrate that MdHB7 mediates apple heat stress responses by regulating ROS scavenging and lignin accumulation, providing new insights into the molecular mechanisms of plant thermotolerance.