Chlorophyll And Carotenoid Metabolism Based Fruit Color-break In Citrus

Citrus is one of most important economic fruit in china. The fruit color is an important appearance quality, marking its fully ripen, which directly affect its commercial value. Citrus color-change is from green color to yellow or orange color, which is mainly due to the reduction of chlorophyll and accumulation of carotenoid in peel. Based on the previous researches in citrus and other plants, the further study had done to find the key regulation factor in chlorophyll metabolism and carotenoid metabolism in this paper. The detail findings are as follows:1. During fruit development, the transcript level of pheophorbide a oxygenase (CitPaO) and stay-green protein (CitSGR) was stable, and no obvious change of chlorophyll b reductase (CitNYC) mRNA was found. In addition, chlorophyllase (CitChlase) mRNA was decreased, indicating the decline of chlorophyll degradation capacity in this process. Only the reduced expression of Mg-chelatase (CitCHLH) and chlorophyll a/b binding protein (CitCAB1,2) was found to be correlated with the reduction in chlorophyll content. Chlorophyll loss was greatly accelerated by postharvest ethylene fumigation.In this process, the expression of CitCHLH, CitPaO and CitSGR was not affected. However, it greatly increased the expression of CitNYC and CitChlase, and accelerated the decline in CitCABs expression. Taken together, these results indicate that the availability of free chlorophyll, influenced by chlorophyll a/b binding protein is likely to be a key step in chlorophyll reduction during natural or ethylene-induced degreening. However, the increase in CitChlase and CitNYC transcript abundance was only related to accelerated chlorophyll degradation in ethylene-induced degreening.2. During the storage, the color of detached citrus fruit was changed from green to yellow with the increase of CCI value, which was associated with the decline of chlorophylls and increase of carotenoids after six-day light treatment. The carotenoid HPLC analysis indicated that the content of phytoene and β-cryptoxanthin was greatly improved, but the decline of β-carotene. Gene expression suggested that the chlorophyll synthesis ability was inhibited by the gradually reduction of CitPORA expression, even the induction of CitCHLH transcript. In chlorophyll degradation pathway, the CitRCCR was largely induced by light treatment, which resulted in the increase of chlorophyll degradation. In carotegenesis, only CitPSYl and CitIPI were induced by light treatment, seem to be responsible for carotenoid increase. In general, it was suggested that the acceleration of citrus color-break by light treatment was due to the reduction of chlorophylls and accumulation of carotenoids, which is likely due to the expression change of CitRCCR, CitIPI and CitPSYl.3. Three1-deoxy-D-xylulose-5-phos-phate synthases (DXS) and three phytoene synthases (PSY) were identified in citrus, from Affymetrix GeneChip Citrus Genome Array, GenBank and public orange genome databases. Tissue-specific expression analysis of these genes was carried out on fruit peel and flesh, flower and leaf of Satsuma mandarin{Citrus unshiu Marc.) in order to determine their roles in carotenoid accumulation in different tissues. Expression of CitDXSl and CitPSYl was highest in all test tissues, while that of CitDXS2and CitPSY2was lower, and that of CitDXS3and CitPSY3undetectable. The transcript profiles of CitDXSl and CitPSYl paralleled carotenoid accumulation in flesh of Satsuma mandarin and orange (C. sinensis Osbeck) during fruit development, and CitPSYl expression was also associated with carotenoid accumulation in peel, while the CitDXSl transcript level was only weakly correlated with carotenoid accumulation in peel. Similar results were obtained following correlation analysis between expression of CitDXSl and CitPSYl and carotenoid accumulation in peel and flesh of16citrus cultivars. These findings identify CitPSYl and CitDXSl as the main gene members controlling carotenoid biosynthesis in citrus fruit. Furthermore, chromoplasts were extracted from flesh tissue of these citrus, and chromoplasts of different shape (spindle or globular), different size, and color depth were observed in different cultivars, indicating chromoplast abundance, number per gram tissue, size and color depth were closely correlated with carotenoid content in most cultivars. The relationship between carotenoid biosynthesis and chromoplast development was discussed.