Apocarotenoids biosynthesis, signaling regulation, crosstalk with phytohormone, and its role in stress tolerance

Apocarotenoids are essential metabolites found in all photosynthetic and heterotrophic organisms. Apocarotenoids include phytohormones, signaling molecules, pigments, volatiles, and many other molecules used in flavor, aroma, food, and cosmetics. Apocarotenoids are a crucial component of horticulture crops' quality and contribute to increasing their health benefits. Apocarotenoids are derived from the oxidative breakdown of carotenoids; the reaction can be enzymatic by CCDs and non-enzymatic (non-site specific) by ROS or enzymatic (site specific) by lipoxygenases and peroxidases. The enzymes belong to the family of carotenoid cleavage dioxygenase (CCDs) that includes nine different types of CCDs along with recently identified CCD10, and each plays a decisive role in catalyzing different apocarotenoids. Recent and novel apocarotenoids like β-cyclocitral, anchorene, zaxinone, and ionone were discussed. In all taxa, apocarotenoids perform crucial biological functions. They have a significant role in controlling plants' growth, development, and reaction to biotic and abiotic environmental stimuli and mediating interactions between plants and other lifeforms. This review covers biosynthesis, isolation, characterization, and types of apocarotenoids. It also discusses the regulation of biosynthesis and CCDs genes at different levels, such as transcription (TFs, promoter, and post-transcription), post-translation, and epigenetic modifications. This review limelights the crucial role of apocarotenoids in defense against abiotic and biotic stresses. The crosstalk of apocarotenoids (SLs and ABA) with other phytohormones is also discussed.