Analysis of apocarotenoid volatiles from lettuce ( Lactuca sativa ) induced by insect herbivores and characterization of carotenoid cleavage dioxygenase gene

Plant apocarotenoids have been shown to have a diverse biological role in herbivore–plant interactions. Despite their importance, little is known about herbivores’ effect on apocarotenoid emissions in Lactuca sativa . In this study, we examined changes in apocarotenoid emissions in lettuce leaves after infestation by two insects, viz., Spodoptera littoralis larvae and Myzus persicae aphids. We found that β -ionone and β -cyclocitral showed higher concentrations than the other apocarotenoids, with a significant increase as per the intensity of infestation of both herbivore species. Furthermore, we performed functional characterization of Lactuca sativa carotenoid cleavage dioxygenase 1 ( LsCCD1 ) genes. Three LsCCD1 genes were overexpressed in E. coli strains, and recombinant proteins were assayed for cleavage activity on an array of carotenoid substrates. The LsCCD1 protein cleaved β- carotene at the 9,10 (9′,10′) positions producing β -ionone. The transcript analysis of LsCCD1 genes revealed differential expression patterns under varying levels of herbivores’ infestation, but the results were inconsistent with the pattern of β -ionone concentrations. Our results suggest that LsCCD1 is involved in the production of β -ionone, but other regulatory factors might be involved in its induction in response to herbivory. These results provide new insights into apocarotenoid production in response to insect herbivory in lettuce.