Physiological and transcriptomic comparisons shed light on the high-temperature stress response mechanisms of...

High-temperature (HT) stress poses a significant threat to plant growth and productivity, necessitating a deeper understanding of thermotolerance mechanisms in economically important species like Oncidium orchids. This study investigates the physiological and molecular responses of heat-tolerant (GR) and heat-sensitive (HC) Oncidium cultivars under HT stress to identify key adaptive strategies.

Physiological analyses revealed that GR maintained superior chlorophyll retention, membrane stability, and metabolic flexibility under HT stress, while HC exhibited severe photosynthetic collapse and oxidative damage. Transcriptomic profiling identified 26,683 differentially expressed genes (DEGs) in GR, with pronounced upregulation of heat shock proteins (HSP20, HSP70, HSP90), antioxidant enzymes (glutathione peroxidase), and chloroplast-stabilizing genes. Functional enrichment analyses highlighted GR's coordinated activation of protein homeostasis (GO:0044267), photosynthetic protection (GO:0009522), and metabolic reprogramming (ko01100), including glutathione metabolism (ko00480) and phenylpropanoid biosynthesis (ko00940). Weighted gene co-expression network analysis (WGCNA) further underscored GR's robust transcriptional network, dominated by heat-shock proteins (HSPs) and heat stress transcription factors (HSFs), whereas HC displayed fragmented stress responses.

Collectively, these results demonstrate that the thermotolerant GR cultivar employs a multi-layered defense strategy, including: (1) predominant upregulation of small heat shock proteins (HSP20) rather than canonical HSP70/90; (2) chloroplast protection via oxygen-evolving enhancer proteins; and (3) a well-coordinated gene regulatory network centered on HSFA2. Notably, thylakoid membrane stability emerged as an orchid-specific thermotolerance trait. Comparative analysis demonstrated that GR's multi-layered defense strategy contrasts sharply with HC's fragmented responses, characterized by protein homeostasis collapse and oxidative damage. Our findings provide both fundamental insights into orchid stress physiology and practical targets (HSP20, chloroplast HSP70, phenylpropanoid biosynthesis) for developing climate-resilient orchids through molecular breeding approaches.