β-Cyclocitral and derivatives: Emerging molecular signals serving multiple biological functions

beta-cyclocitral is a volatile short-chain apocarotenoid generated by enzymatic or non-enzymatic oxidation of the carotenoid beta-carotene. beta-cyclocitral has recently emerged as a new bioactive compound in various organisms ranging from plants and cyanobacteria to fungi and animals. In vascular plants, beta-cyclocitral and its direct oxidation product, beta-cyclocitric acid, are stress signals that accumulate under unfavorable environmental conditions such as drought or high light. Both compounds regulate nuclear gene expression through several signaling pathways, leading to stress acclimation. In cyanobacteria, beta-cyclocitral functions as an inhibitor of competing microalgae and as a repellent against grazers. As a volatile compound, this apocarotenoid plays also an important role in intra-species and inter-species communication. This review summarizes recent findings on the multiple roles of beta-cyclocitral and of some of its derivatives.