Comparative transcriptome analysis reveals the role of CsABA8ox1 in cucumber under high-temperature stress.

High temperature (HT) is a critical abiotic factor that restricts plant growth and development. The role of abscisic acid (ABA) in stress tolerance is well established, and ABA 8'-hydroxylase (ABA8ox), a key enzyme in ABA degradation, is crucial for plant responses to abiotic stress. In this study, a CsABA8ox1-deficient mutant, yf-343, and its wild-type counterpart, BY, were subjected to continuous HT treatment to assess phenotypic, physiological, and transcriptomic changes. Under HT, ABA accumulation increased in BY and yf-343, with significantly higher levels in the yf-343 mutant. Exogenous ABA application accelerated leaf yellowing in BY and triggered pronounced leaf senescence and cell death in yf-343. HT treatment also increased the activities of superoxide dismutase and peroxidase, elevated ABA and malondialdehyde content, and simultaneously inhibited catalase activity and photosynthetic rate. Comparative RNA sequencing (RNA-seq) revealed that genes associated with plant hormone signaling, secondary metabolite biosynthesis, starch and sucrose metabolism, phenylalanine metabolism, and the mitogen-activated protein kinase signaling pathway were differentially expressed between yf-343 and BY under 42 °C HT treatment. Among these, two genes, heat shock proteins 70 (CsHSP70) and wall-associated receptor kinase 2 (CsWAKL2), were validated through virus-induced gene silencing. Knockdown of CsHSP70 and CsWAKL2 enhanced susceptibility to HT, confirming the reliability and significance of the candidate genes involved in HT stress response identified by RNA-seq. These findings establish a strong foundation for elucidating the role of ABA8ox in cucumber resistance to abiotic stress.