Eucalyptol serves a signaling function to enhance Cinnamomum camphora thermotolerance

Terpenes serve important functions in enhancing plant thermotolerance, but the thermotolerance mechanism is still unknown. Cinnamomum camphora releases an abundance of monoterpenes to tolerate high temperature, and is subdivided into five chemotypes, such as camphor chemotype, eucalyptol chemotype, linalool chemotype, borneol chemotype, and iso-nerolidol chemotype. To uncover the thermotolerance mechanism of the uppermost monoterpenes in C. camphora and promote their development as anti-high temperature agents, the thermotolerance functions of eucalyptol in the corresponding chemotype of C. camphora were investigated. In contrast to normal temperature (28^oC), reactive oxygen species (ROS), thiobarbituric acid reactive substance (TBARS) levels, and antioxidant enzyme activities increased under 38^oC, and further increased in the treatment with fosmidomycin (Fos), inhibiting monoterpene synthesis at 38^oC (Fos+38^oC) due to alterations in the expression of the genes related to non-enzymatic and enzymatic antioxidant formations. Compared with Fos+38^oC treatment, Fos+38^oC treatment with eucalyptol fumigation (Fos+38^oC+eucalyptol) lowered ROS levels and antioxidant enzyme activities for increased non-enzymatic antioxidant gene expression and decreased enzymatic antioxidant gene expression. High temperature at 38^oC reduced the chlorophyll and carotenoid content as well as photosynthetic abilities by reducing the expression of the genes associated with photosynthetic pigment biosynthesis, light reaction, and carbon fixation. Fos+38^oC treatment aggravated the reduction. In contrast to Fos+38^oC treatment, Fos+38^oC+eucalyptol treatment increased photosynthetic pigment content and improved photosynthetic abilities by up-regulating related gene expression. Therefore, the findings first uncover that the uppermost monoterpenes should serve important signaling functions in enhancing C. camphora thermotolerance, which provides a new thought for uncovering the thermotolerance mechanism of terpenes.