Plant Apocarotenoids: From Retrograde Signaling to Interspecific Communication.

Carotenoids are isoprenoid compounds synthesized by all photosynthetic and some non-photosynthetic organisms. They are essential for photosynthesis and contribute to many other aspects of a plant's life. The oxidative breakdown of carotenoids gives rise to the formation of a diverse family of essential metabolites called apocarotenoids. This metabolic process either takes place spontaneously through reactive oxygen species (ROS) or is catalyzed by enzymes generally belonging to the carotenoid cleavage dioxygenase (CCD) family. Apocarotenoids include the phytohormones abscisic acid (ABA) and strigolactones (SLs), signaling molecules, and growth regulators. ABA and SLs are vital in regulating plant growth, development, and stress response. SLs are also an essential component in plants'rhizospheric communication with symbionts and parasites. Other apocarotenoid small molecules, such as blumenols, mycorradicins, zaxinone, anchorene, β-cyclocitral, β-cyclogeranic acid, β-ionone, and loliolide, are involved in plant growth and development, and/or contribute to different processes, including arbuscular mycorrhiza (AM) symbiosis, abiotic stress response, plant-plant and plant-herbivore interactions, and plastid retrograde signaling. There are also indications for the presence of structurally unidentifiedlinearcis-carotene-derived apocarotenoids (LCDAs), which are presumed to modulateplastid biogenesis and leaf morphology, among other developmental processes. Here, we provide an overview on the biology of old, recently discovered, and supposed plant apocarotenoid signaling molecules, describing their biosynthesis, developmental and physiological functions, and role as a messenger in plants' communication.