Down-regulation of lycopene epsilon-cyclase expression in transgenic sweetpotato plants increases the carotenoid content and tolerance to abiotic stress

Abstract

Carotenoids are required for many biological processes in plants and humans. Lycopene epsilon-cyclase (LCY-epsilon) catalyzes the conversion of lycopene into lutein via the a-branch carotenoid biosynthesis pathway. Down-regulation of IbLCY-epsilon by RNAi increases carotenoid accumulation and salt stress tolerance in transgenic sweetpotato calli. As the role of IbLCY-epsilon in carotenoid biosynthesis and environmental stress responses in whole plants is poorly understood, transgenic sweetpotato (RLE plants) with reduced expression of IbLCY-e were developed. RLE plants contained higher levels of total carotenoid and beta-carotene, due to an elevated beta-carotene/lutein ratio rather than increased de novo biosynthesis. RLE plants showed high reactive oxygen species/radical-scavenging activity. They also exhibited an enhanced tolerance of both salt and drought stress, which was associated with lower membrane permeability and a higher photosynthetic rate, respectively. Elevated carotenoid accumulation in RLE plants mitigated the reductions in leaf photosystem II efficiency and chlorophyll induced by abiotic stress. Expression of the carotenoid cleavage genes 9-cis-epoxycarotenoid dioxygenase, carotenoid cleavage dioxygenase 1 (CCD1) and CCD4 was higher in RLE plants, as was abscisic acid accumulation. IbLCY-epsilon silencing thus offers an effective approach for developing sweetpotato plants with increased tolerance to abiotic stress that will grow on global marginal lands with no reduction in nutritional value.