Advance study of pigment-carotenoids in Citrus fruits

Abstract: The varieties of Citrus fruits such as Citrus sinensis (orange) , Citrus reticulata (mandarin) , Citrus limon (lemon) , Citrus grandis (pomelo) and Citrus paradise (grapefruit) gained massive popularity around the world. The United Nations Commodity Trade Statistics Database revealed that the international consumption of wide-skinned citrus increased from less than 9 million tons to 37 million tons from 2000 to 2016. Nevertheless, 2011, the total export of China was 6157.975 million dollars, only accounting for 5.52 percent of the international citrus market, which is extremely disproportionate to the place of China named major producer of citrus. After entering WTO, the international and domestic markets have put forward higher requirements for fresh citrus fruits quality. The commercial value of citrus fruit primarily involves external quality, including color and rind conditions, and internal quality that was determined by the content of sugar and organic acids, the ratio of sugar/acid, texture and digesting residue degree and others nutritional components. Now“beautiful”is not only a single adjective for people, but also the first factor to be considered of gourmet fruits. Especially in developed country, appearance and color are the most valued qualities by consumers, hence the research of citrus peels pigment shows great significance and urgency. The critical pigments in the peels of immature citrus fruit are caused by the presence of chlorophylls, which exhibit green color, whereas the predominant colorations of bright yellow, orange and red in the peels of mature citrus fruit which closely related with carotenoids composition and proportion. The carotenoids are terpenoids synthesized in plastids of leaves, endosperm, and roots, which include hydrocarbons (carotenes) and oxygenated derivatives (xanthophylls) . The former consists of ξ, α, β, δ-carotene, lycopene and their isomers, while the later include lutein, zeaxanthin, violaxanthin and β-cryptoxanthin and β-citraurin. In different growing conditions of citrus fruits, those compositions of carotenoids mixed in varying proportions that create richer color. In colored fruits, epoxy and hydroxylated carotenoids are the major components and account for up to80% of total carotenoids. Therefore, “why accumulate?”and“what's the function of carotenoids for plants?”. In higher plants, carotenoids are essential components of the photosynthetic apparatus, when exposed to high irradiance, it's the supernumerary protector to dissipate excess light energy absorbed by the antenna pigments, harvest light for photosynthesis, and the colorful flower and fruits for carotenoids attract pollinators and agents of seed dispersal. Actually, many carotenoids are beneficial to human and animals, for its antioxidants, decreasing the risk of cancer, protecting vision. There is a new proof that ample dose of carotenoids applied to pregnant woman in daily foods can reduce anemia risk.However, the color of citrus fruits is not stable, there are many factors impact citrus pigments formation in maturation stages, mainly dividing into three parts: inhesion, outer and others. The inhesion related to citrus plant development stages, the structure gene expression in the pathway of carotenoids, the genes of regulation and light signal transduction expression level, the formation and number of plastids that pigments biosynthesis developed and the contents of endogenous hormones. The outer factors of carotenoids synthesis contain: light intensity, temperature, humidity, soil nutrition and infection of bacteria and fungi. And the third was expanded by people when they did different treatments on fruits in different stages to find out the influence factors or metabolization, which include proteins synthesis inhibitor applied, such as CPTA and MPTA (lycopene cyclase inhibitor) , different sugar solution daubing, and different exogenous hormone spraying. Until now, more than 150 carotenoid synthase genes have been isolated from nature, which encode more than 20 key enzymes in the biosynthesis pathway, respectively.The pathway own three branches: lycopene, carotene and xanthophylls biosynthesis. The first committed step in the carotenoid pathway is the synthesis of phytoene, catalyzed by the enzyme PSY, which is the key limiting enzyme to control the synthesis speed. In the citrus plant, PSY has three transcripts and show tissue specificity, the PSY1 mainly expressed in fruits, the PSY2 increased expressed in green tissue, the PSY3 only expressed in root. In the next steps, proteins PDS and ZDS catalyzed the colorless phytoene into red lycopene. The carotene formation carried out by protein cyclase LCYb and LCYe, which catalyzed precursors lycopene into α, β, -carotene. There is a proof that no α-carotene accumulation for 14 bp nucleotides deletion of LCYe at DNA molecular level in Cara Cara. Well, the xanthophylls biosynthesis pathway contains three kinds of enzymes: hydroxylase (HYb, HYe) , carotenoid cleavage dioxygenases (CCDs) and 9-cis-epoxycarotenoid dioxygenases (NCEDs) . Those enzymes catalyzed and formed richer carotenoids together. Besides that, the biosynthesis in plants is connected with the plant regulator abscisic acid (ABA) , which produced through C15 intermediates after oxidative cleavage of specific xanthophylls. Based on previous studies, we found these aspects will still attract our eyes, which include mutating key site of the structure genes to explore why the different citrus plants show distinct function; the interaction between transcript factors with structure genes; epigenetic regulation mechanism of carotenoids accumulation; the ligation with anthocyanin formation; and related omics of citrus fruits. Whereas, the related flavor and aroma released by carotenoids degradation products that impact fruit value in citrus are still unclear, maybe the work should be spotlighted.