Modulation of carotenoid biosynthesis during tomato fruit ripening through phytochrome regulation of phytoene synthase activity

Abstract

The accumulation of carotenoids greatly influences the quality of ripe tomato ( Solanum lycopersicum) fruit because these compounds contribute to their color and functional food value. The regulatory properties and gene expression of phytoene synthase (PSY), a key enzyme involved in carotenoid biosynthesis, was analyzed in tomato fruit. PSY activity was detected in the soluble fraction showing a time- and a protein-dependent increase in the formation of phytoene from geranylgeranyl pyrophosphate (GGPP). The enzyme was stimulated by Mg 2+ and ATP, and was inhibited by Tween 20 and Triton X-100. The enzyme followed typical Michaelis–Menten kinetics with increasing GGPP. During in vivo studies, pericarp discs from breaker stage tomatoes were ripened in darkness (D), or D interrupted by daily pulses of red (R) light, or R followed by far red (FR) light. After 14 days incubation, R-treated discs had accumulated 12 mg carotenoids/100 g fresh weight; nearly a 50% increase over D- and R/FR-treated discs. This R/FR reversibility of carotenoid accumulation was also observed in PSY activity 8 days post breaker, where it showed peak activity. However, this R/FR regulation of PSY activity was not reflected in PSY1 transcript levels, suggesting that PSY may be subject to post-translational regulation.