Putative Daucus carota Capsanthin-Capsorubin Synthase (DcCCS) Possesses Lycopene beta-Cyclase Activity, Boosts Carotenoid Levels, and Increases Salt Tolerance in Heterologous Plants

Plant carotenoids are synthesized and accumulated in plastids through a highly regulated pathway. Lycopene beta-cyclase (LCYB) is a key enzyme involved directly in the synthesis of ff-carotene and fi-carotene through the cyclization of trans-lycopene. Daucus carota harbors two LCYB genes, of which DcLCYB2 (annotated as CCS-Like) is mostly expressed in mature storage roots, an organ that accumulates high alpha-carotene and beta-carotene content. In this work, we determined that DcLCYB2 of the orange Nantes variety presents plastid localization and encodes for a functional LCYB enzyme determined by means of heterologous complementation in Escherichia coli. Also, ectopic expression of DcLCYB2 in tobacco (Nicotiana tabacum) and kiwi (Actinidia deliciosa) plants increases total carotenoid content showing its functional role in plants. In addition, transgenic tobacco T2 homozygous plants showed better performance under chronic salt treatment, while kiwi transgenic calli also presented a higher survival rate under salt treatments than control calli. Our results allow us to propose DcLCYB2 as a prime candidate to engineer carotenoid biofortified crops as well as crops resilient to saline environments.

Automated summary

Carotenoids, such as alpha-carotene and beta-carotene, are synthesized in plants through a pathway involving the enzyme lycopene beta-cyclase (LCYB). In this study, DcLCYB2, a gene mainly expressed in mature storage roots of carrot, was determined to encode a functional LCYB enzyme with plastid localization. Ectopic expression of DcLCYB2 in tobacco and kiwi plants increased total carotenoid content and improved their performance under salt treatments. These findings suggest DcLCYB2 as a potential candidate for engineering carotenoid-enriched crops and crops resilient to saline environments.