Characterization of Carotenoid Accumulation and Carotenogenic Gene Expression During Fruit Ripening in Red Colored Pulp of 'Siam Red Ruby' Pumelo (Citrus grandis) Cultivated in Thailand

The 'Siam Red Ruby' (Citrus grandis) is a new lycopene-accumulating pumelo variety, which is grown only in the Pakpanang area of the Nakhon Si Thammarat province of Thailand. In this study, we harvested 'Siam Red Ruby' fruit at five ripening stages, and the changes in the accumulation of carotenoid and carotenogenic gene expression were investigated in the pulp. During the ripening process, the contents of phytoene, zeta-Carotene, lycopene, beta-carotene, beta-cryptoxanthin, and all-trans-violaxanthin increased gradually, while the contents of alpha-carotene and lutein decreased in the pulp of 'Siam Red Ruby'. In the mature fruit of 'Siam Red Ruby', a high amount of lycopene was accumulated in the pulp, accounting for more than 93.26% of the total carotenoid content. The massive accumulation of lycopene led to an attractive red color of the 'Siam Red Ruby' pulp. In addition, gene expression results showed that the increases in the expression of upstream genes (CitPSY, CitPDS, CitZDS, CitZISO, and CitCRTISO), and decreases in the expression of genes downstream (CitLCYb1, CitLCYb2, CitLCYe, CitHYb, and CitHYe) was the main mechanism of lycopene accumulation in the pulp of 'Siam Red Ruby' pumelo.