Carotenoid oxidation products are stress signals that mediate gene responses to singlet oxygen in plants

O-1(2) (singlet oxygen) is a reactive O-2 species produced from triplet excited chlorophylls in the chloroplasts, especially when plants are exposed to excess light energy. Similarly to other active O-2 species, O-1(2) has a dual effect: It is toxic, causing oxidation of biomolecules, and it can act as a signal molecule that leads to cell death or to acclimation. Carotenoids are considered to be the main O-1(2) quenchers in chloroplasts, and we show here that light stress induces the oxidation of the carotenoid beta-carotene in Arabidopsis plants, leading to the accumulation of different volatile derivatives. One such compound, beta-cyclocitral, was found to induce changes in the expression of a large set of genes that have been identified as O-1(2) responsive genes. In contrast, beta-cyclocitral had little effect on the expression of H2O2 gene markers. beta-Cyclocitral-induced reprogramming of gene expression was associated with an increased tolerance to photooxidative stress. The results indicate that beta-cyclocitral is a stress signal produced in high light that is able to induce defense mechanisms and represents a likely messenger involved in the O-1(2) signaling pathway in plants.