Screening Universal Stress-Response Terpenoids and Their Biosynthetic Genes via Volatile and Transcriptomic Profiling in Citrus

Volatile terpenoids accumulate in citrus and play important roles in plant defense against various stressors. However, the broad-spectrum response of terpenoid biosynthesis to ubiquitous stressors in citrus has not been comparatively investigated. In this study, volatile terpenoids were profiled under six stressors: high temperature, citrus miner, citrus red mite, citrus canker, Alternaria brown spot, and huanglongbing (HLB). Significant content changes in 15 terpenoids, including β-ocimene, were observed in more than four of the six stressors, implying their possibly universal stress-response effects. Notably, the emission of terpenoids, including β-caryophyllene, β-ocimene, and nerolidol glucoside, was significantly increased by HLB in HLB-tolerant “Shatian” pomelo leaves. The upregulation of CgTPS1 and CgTPS2 and their characterization in vivo identified them as mono- or sesquiterpenoid biosynthetic genes. This study provides a foundation for determining stress resistance mechanisms in citrus and biopesticide designations for future industrial applications.