Physiological and transcriptomic analyses reveal mechanisms of exogenous strigolactones to regulate cold tolerance in litchi fruit

Strigolactones (SLs) are a class of carotenoid-derived phytohormones that play diverse biological functions in plants. In the current study, we conducted physiological and transcriptomic analysis to explore the effects of two different concentrations (0.1 and 1.0 mu M) of GR24, a synthetic analogue of SLs, on chilling injury (CI) and related mechanisms in litchi fruit during refrigeration at 4 degrees C. The results showed that 0.1 mu M GR24 (GR24-0.1) dipping treatment retarded the advancement of browning and red loss in litchi pericarp during refrigeration. GR24-0.1 treatment ameliorated oxidative stress and restrained membrane deterioration, as manifested by lowered reactive oxygen species (ROS) production, relative electrolyte leakage (REL) and malondialdehyde (MDA) content. GR24-0.1 treatment inhibited the enhancement of polyphenol oxidase (PPO) and peroxidase (POD) activities, which was concomitant with diminished contents of total phenolics, flavonoids and anthocyanins. Transcriptomic analysis revealed that GR24-0.1 treatment up-regulated the expression of genes associated with abscisic acid (ABA) biosynthesis and signal transduction, C-repeat binding factor (CBF) signaling pathway, anthocyanin synthesis and transport, antioxidant system, oxidative protein repair system as well as energy metabolism, contributing to the enhanced levels of some metabolites in these biochemical pathways. In contrast to GR24-0.1 treatment, reverse transcriptional and physiological effects in parallel with aggravated CI severity occurred in GR24-1.0 treated fruit, indicating a dosage-dependent regulatory effect of GR24 on litchi CI. The present findings provide insights into the potential mechanisms by which SLs regulate cold tolerance in postharvest litchi fruit.