Salicylic acid and transcriptional activation of phytohormone signalling potentially mediate chilling response in cucumber fruit peel

Cucumber ( Cucumis sativus L.) fruit is susceptible to chilling injury (CI) during low-temperature storage; however, the mechanism behind the chilling response of cucumber fruit is not fully understood. Cellular responses to low-temperature conditions should be determined to identify the factors behind CI occurrence and chilling tolerance in postharvest fruit. In this study, the CI incidence in cucumber fruit stored at 13 °C (non-chilling temperature) and 2 °C (chilling temperature) were compared to determine the key factors involved in the chilling response. Transcriptome analysis revealed that different metabolite and phytohormone signalling pathways were activated in the two storage temperatures. Thus, metabolites and phytohormones were profiled to gain a deeper understanding of their roles in the chilling response. Sucrose and amino acids such as isoleucine, serine, valine, and threonine significantly accumulated in the fruit stored at 13 °C compared with those stored at 2 °C. Among the analysed phytohormones, more abscisic acid accumulated at 13 °C than at 2 °C as an early response to non-chilling temperature. Meanwhile, salicylic acid levels and expressions of its signalling genes increased with persistent exposure to 2 °C as a response to chilling stress. Phytohormones such as brassinolide, ethylene, indole-3-acetic acid, jasmonic acid did not significantly accumulate at 2 °C. However, their signalling genes were upregulated in the fruit stored at 2 °C compared to 13 °C. These results suggest that the transcriptional activation of phytohormone signalling pathways, rather than their biosynthesis, plays a crucial role in the chilling response. These findings provide insights into chilling response and complex interplay of various molecular components during the low-temperature storage of cucumber fruit. • Severe chilling injury symptoms in cucumber were observed at 2 °C but not at 13 °C. • Transcriptomes and metabolic processes were differentially affected by temperatures. • Abscisic acid, sucrose, and amino acid levels increased as early response to 13 °C. • Salicylic acid levels increased as a late response to prolonged chilling stress. • Transcriptional activation of phytohormone signalling may play a key role at 2 °C.