From plastids to the cytosol: redefining boundaries for provitamin A biofortification in plants.

Carotenoids are essential molecules found in plants that fulfill critical functions in light capture, photoprotection, and pigmentation. Carotenoids also act as precursors for apocarotenoid signaling compounds including phytohormones such as abscisic acid and strigolactones. Beyond plants, carotenoids play a significant role in humans; particularly, carotenoids possessing unsubstituted β-ionone rings can be converted into retinal, the precursor to vitamin A, making their dietary consumption important to human health ( Watkins and Pogson, 2020 Watkins J.L. Pogson B.J. Prospects for Carotenoid Biofortification Targeting Retention and Catabolism. Trends Plant Sci. 2020; 25: 501-512https://doi.org/10.1016/j.tplants.2019.12.021 Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar ; Zheng et al., 2020 Zheng X. Giuliano G. Al-Babili S. Carotenoid biofortification in crop plants: Citius, altius, fortius. Biochim. Biophys. Acta. Mol. Cell Biol. Lipids. 2020; 1865158664https://doi.org/10.1016/j.bbalip.2020.158664 Crossref Scopus (54) Google Scholar ). Increasing the accumulation of these carotenoid compounds, known as provitamin A carotenoids (PACs), is an important aim in the global endeavor to cultivate biofortified crops with enhanced nutritional quality. The availability of germplasm with variation in carotenoid accumulation is crucial for successful provitamin A biofortification initiatives that use both traditional plant-breeding methods and recently developed molecular biology techniques ( Zheng et al., 2020 Zheng X. Giuliano G. Al-Babili S. Carotenoid biofortification in crop plants: Citius, altius, fortius. Biochim. Biophys. Acta. Mol. Cell Biol. Lipids. 2020; 1865158664https://doi.org/10.1016/j.bbalip.2020.158664 Crossref Scopus (54) Google Scholar ). However, in certain plant systems and tissues, natural variation alone may be inadequate to accomplish this objective. As a result, researchers have explored alternative approaches, such as incorporating enzymes from other organisms, to effectively engineer plants. The expression of bacterial carotenoid biosynthetic enzymes has been employed in provitamin A biofortification for several decades, exemplified by the development of Golden Rice ( Ye et al., 2000 Ye X. Al-Babili S. Klöti A. Zhang J. Lucca P. Beyer P. Potrykus I. Engineering the Provitamin A (β-Carotene) Biosynthetic Pathway into (Carotenoid-Free) Rice Endosperm. Science. 2000; 287: 303-305https://doi.org/10.1126/science.287.5451.303 Crossref PubMed Scopus (1784) Google Scholar ).