Abstract Reactive oxygen species (ROS) are inevitable by-products of photosynthesis. In particular, singlet oxygen (
1O
2) is intimately associated with photosynthesis since it is produced from the chlorophyll molecules acting as photosensitizers. The ROS produced in the chloroplasts have a signaling function leading to changes in the expression of nuclear genes to adjust the cellular metabolism to the levels of absorbed light energy. The changes in gene expression associated with
1O
2 signaling can lead either to cell death or to acclimation to photooxidative stress, depending on the
1O
2 production levels. Because of its high reactivity and short lifetime,
1O
2 uses mediators for its signaling. Produced in the chloroplast, the b-carotene derivative b-cyclocitral has been identified as an upstream signal molecule in the
1O
2 signaling pathway. This talk will summarize our results on the function and mode of action of this apocarotenoid in the acclimation of plants to environmental stresses.
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