Research On The Mechanism For The Differential Accumulation Of Carotenoids In Red-and White-fleshed Loquat

Loquat (Eriobotrya japonica Lindl.), a member of Rosaceae, accumulates carotenoids as the the main pigments in mature fruit. The fruit can be divided into two groups, red-and white-fleshed, according to the colour of the flesh. The goal of the present study was to investigate the mechanism leading to the diversity of carotenoid patterns in Luoyangqing (LYQ, red-fleshed) and Baisha (BS, white-fleshd) cultivars. The main results are as follows:1. The differential accumulation of carotenoids in the peel and flesh of seven red-fleshed and seven white-fleshed cultivars were analyzed. The average total carotenoid contents in the peel (68.04～137.8μg/g FW) and flesh (9.16～20.52μg/g FW) of red-fleshed cultivars was about2.9and26.6times of those in the peel (16.12～61.81μg/g FW) and flesh (0.21～1.09μg/g FW) of white-fleshed ones, respectively. The percentage of β-carotene among total carotenoids was higher in both the peel (63.92%) and the flesh (31.85%) of red-fleshed cultivars as compared with those (50.57%for the peel and11.92%for the flesh) of white-fleshed ones. On the contray, the percentage of lutein was lower in both the peel (5.3%) and flesh (0.47%) of red-fleshed cultivars as compared with those (14.72%for the peel and9.06%for the flesh) of white-fleshed ones.2. No carotenoid metabolism genes were found differentially expressed in library of LYQ and BS mature flesh by suppression subtractive hybridization (SSH). The mRNA level of14target carotenoigenesis-related genes corresponds well to the carotenoid accumulation during LYQ fruit ripening. The mRNA levels of phytoene synthase1(EjPSY1) and chromoplast-specific lycopene P-cyclase (EjCYCB) were about2times higher in the peel, and EjCYCB and β-carotene hydroxylase(EjBCH) mRNAs were also about2times higher in the flesh of LYQ, compared with BS. This is not sufficient to explain the almost50-fold higher carotenoid content in LYQ flesh compared with BS, and a different explanation is required.3. The sequence of EjPSYl in the BS was found reduced in length, and333bp shorter than in the the normal sequence. And the mutation of the EjPSYl gene was happened in all the seven white-flehed cultivars. In addition, the expression level and tissue-specific was not affected for the sequence mutation. Interestingly, besides the normal sequence of EjPSYl, the mutation of the sequence of EjPSYl was also exsit in the red-fleshed cultivar of ’Dawuxing’,’Baozhu’, and ’Dahongpao’, which suggested the normal EjPSYl gene is dominant to the mutation one. Another two family members of PSY was found in the loquat. According to the tissue-specific expression, the EjPSY2mainly responsible for the carotenoid accumulation in the green organ especially at leaf tissue, for the transcripts of EjPSY2in the flesh and peel tissues are about1%in the leaf. The family member of EjPSY3might play the role on the peel, leaf, stem, and root for the carotenoid synthase, for in the BS peel at mature stage could accumulate carotenoids.4. The microscopic and ultrastructural data showed the structure of the plastids was very different between the fruit of LYQ and BS. The number and the degree of development of the globules were more in the LYQ plastids than in the BS at mature stage. Few globules were discovered in the plastids of the BS flesh cell at ripen stage. The mRNA level of plastid lipid-associated protein (PAP) in the peel and flesh of LYQ increased after the breaker stage, but in BS the expression level of PAP was much lower and remained constant during fruit ripening. Decrease in the mRNA level of PAP may contribute to reduce the number and development of globules in the plastids.In conclusion, lacking of carotenoids in flesh of white-fleshed cultivars is due to the production of averrant EjPSY1transcripts which are unlikely to encode a functional phytoene synthase enzyme. The expression of EjPSY2and EjPSY3play the role on the accumulation of carotenoids in BS of leaf and peel. The lower expression levels of PAP in BS related the reduced number and the development of the globules in the plastids.