Transcriptome analysis and physiological response to heat and cold stress in flax (Linum usitatissimum L) at the seedling stage

Exposure to low temperatures renders flax seedlings vulnerable to oxidative damage, leading to delayed flowering, while high temperatures hinder seedling growth and negatively impact pollen viability. The seedling stage is particularly sensitive to environmental stressors, which can result in decreased flax yield and compromised seed oil quality. In this study, phenotypic assessments, along with physiological, biochemical, and transcriptomic analyses, were conducted on flax plants subjected to both high and low temperature stress, followed by 6 d recovery period. Results showed that the length and weight of seedlings and shoots were greatest in flax plants that had recovered from low temperature stress. After exposure to low temperature stress, flax seedlings exhibited the highest relative water content (RWC) and relative water loss (RWL) measuring 66.56 % and 93.34 %, respectively. The levels of Pro, SOD, CAT, POD, and nine phytohormones were significantly elevated compared to the control, whereas MDA levels were notably declined. A total of 43,471 genes were identified in the transcriptome data, Among these 32,319 exhibited significant differential expression. GO analysis highlighted enrichment in biological processes, cellular components, and molecular functions. KEGG analysis showed enrichment in plant hormone signal transduction and secondary metabolite biosynthesis. Moreover, differentially expressed genes associated with phytohormone synthesis and signal transduction were analyzed. The expression level of salicylic acid (SA) genes was significantly upregulated under high temperature stress, whereas jasmonic acid (JA) genes were significantly up-regulated under low temperature stress. These findings will elucidate the intricate regulatory mechanisms of osmoregulatory factors, antioxidant enzymes, and endogenous plant hormones in flax's response to high and low temperature stress, offering valuable insights for the selection and breeding of resistant flax varieties and the enhancement of genetic resources.