Down-regulation of sweetpotato lycopene β-cyclase gene enhances tolerance to abiotic stress in transgenic calli

Lycopene beta-cyclase (LCY-beta) is a key enzyme involved in the synthesis of alpha- and beta-branch carotenoids such as alpha-carotene and beta-carotene through the cyclization of lycopene. IbLCY-beta had a length of 1,506 bp and approximately 80 % nucleotide sequence identity with that of tomato LCY-beta. IbLCY-beta was strongly expressed in leaves, and expression was enhanced by salt-stress and osmotic-stress conditions. To characterize the LCY-beta gene (IbLCY-beta) of sweetpotato (Ipomoea batatas), it was isolated and transformed into calli of white-fleshed sweetpotato using an IbLCY-beta-RNAi vector. Transgenic IbLCY-beta-RNAi calli had yellow to orange color and higher antioxidant activity compared to that of white, nontransgenic (NT) calli. Transgenic cells had significantly higher contents of total carotenoids, although lycopene was not detected in transgenic or NT cells. All transgenic calli had strongly activated expression of carotenoid biosynthetic genes such as beta-carotene hydroxylases (CHY-beta), cytochrome P450 monooxygenases (P450), and carotenoid cleavage dioxigenase 1 (CCD1). Transgenic cells exhibited less salt-induced oxidative-stress damage compared to that of NT cells, and also had greater tolerance for polyethylene glycol (PEG)-mediated drought compared to that of NT cells, due to the higher water content and reduced malondialdehyde (MDA) content. The abscisic acid content was also higher in transgenic cells. These results show that a study of IbLCY-beta can facilitate understanding of the carotenoid biosynthetic pathway in sweetpotato. IbLCY-beta could be useful for developing transgenic sweetpotato enriched with nutritional carotenoids and with greater tolerance to abiotic stresses.