Oranges are an important nutritional source for human health and have immense economic value. Here we present a comprehensive analysis of the draft genome of sweet orange (Citrus sinensis). The assembled sequence covers 87.3% of the estimated orange genome, which is relatively compact, as 20% is composed of repetitive elements. We predicted 29,445 protein-coding genes, half of which are in the heterozygous state. With additional sequencing of two more citrus species and comparative analyses of seven citrus genomes, we present evidence to suggest that sweet orange originated from a backcross hybrid between pummelo and mandarin. Focused analysis on genes involved in vitamin C metabolism showed that GalUR, encoding the rate-limiting enzyme of the galacturonate pathway, is significantly upregulated in orange fruit, and the recent expansion of this gene family may provide a genomic basis. This draft genome represents a valuable resource for understanding and improving many important citrus traits in the future.

A novel, pleiotropic sweet orange (Citrus sinensis L. Osbeck) mutant, ‘Hong Anliu’, is described. This mutation causes carotenoid accumulation, high sugar, and low acid in the fruits. Gas chromatographic analysis revealed that high sugar and low acid in the fruit were caused by the accumulation of sucrose and the deficiency of citric acid. The dominant carotenoid accumulated in albedo, segment membranes, and juice sacs is lycopene, which can reach levels that are a 1000-fold higher than those in comparable wild-type fruits. This mutation does not affect the carotenoid composition of leaves. Carotenoid concentration and biosynthetic gene expression of albedo, segment membranes, and juice sacs were dramatically altered by the mutation. Lycopene accumulation in the juice sacs was regulated by co-ordinate expression of carotenoid biosynthetic genes. However, in albedo and segment membranes, the expression of downstream carotenogenic genes seems to be feedback induced by lycopene accumulation. This implies that there must be at least two modes regulating lycopene accumulation in ‘Hong Anliu’ fruit. Taken together, these results suggest that massive amounts of lycopene might be synthesized in the juice sacs and then transported to the segment membrane and the albedo, which leads to lycopene accumulation there.

Background: From field harvest to the consumer’s table, fresh citrus fruit spends a considerable amount of time in shipment and storage. During these processes, physiological disorders and pathological diseases are the main causes of fruit loss. Heat treatment (HT) has been widely used to maintain fruit quality during postharvest storage; however, limited molecular information related to this treatment is currently available at a systemic biological level. Results: Mature ‘Kamei’ Satsuma mandarin (Citrus unshiu Marc.) fruits were selected for exploring the disease resistance mechanisms induced by HT during postharvest storage. Proteomic analyses based on two-dimensional gel electrophoresis (2-DE), and metabolomic research based on gas chromatography coupled to mass spectrometry (GC-MS), and liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QToF-MS) were conducted. The results show resistance associated proteins were up-regulated in heat treated pericarp, such as beta-1, 3-glucanase, Class III chitinase, 17.7 kDa heat shock protein and low molecular weight heat-shock protein. Also, redox metabolism enzymes were down-regulated in heat treated pericarp, including isoflavone reductase, oxidoreductase and superoxide dismutase. Primary metabolic profiling revealed organic acids and amino acids were down-regulated in heat treated pericarp; but significant accumulation of metabolites, including tetradecanoic acid, oleic acid, ornithine, 2-keto-d-gluconic acid, succinic acid, turanose, sucrose, galactose, myo-inositol, glucose and fructose were detected. Noticeably, H₂O₂ content decreased, while, lignin content increased in heat treated pericarp compared to the control, which might increase fruit resistibility in response to external stress. Also, flavonoids, substances which are well-known to be effective in reducing external stress, were up-regulated in heat treated pericarp. Conclusions: This study provides a broad picture of differential accumulation of proteins and metabolites in postharvest citrus fruit, and gives new insights into HT improved fruit disease resistance during subsequent storage of ‘Kamei’ Satsuma mandarin. Interpretation of the data for the proteins and metabolites revealed reactive oxygen species (ROS) and lignin play important roles in heat treatment induced fruit resistance to pathogens and physiological disorders.