Content Changes Of The Main Bitter Compounds And Related Genes Expression In Citrus

Bitterness is one of the important citrus qualities. Bitter compounds not only affect citrus flavor, but also have many biological functions, which will benefit a lot for human health. In order to understand the metabolism of the bitter compounds and govern the bitter compounds in citrus, this study was carried out from views of physiological and molecular level. The main results are as following:1. The main bitter compounds (limonin, nomilin and naringin) in the fruit tissues of ’Guoqing No.1’Satsuma mandarin(Citrus unshiu Marc.) during fruit development in2010-2011and’Oroblanco’grapefruit (C. paradisi Macf.) in2010were determined. The results were analyzed in combination with the data obtained from2008-2009. Firstly, although fluctuating largely throughout the fruit development, the contents of the three bitter compounds in fruit tissues mostly displayed a declining trend. Secondly, the contents of three bitter compounds in different years were significantly different, and there were growing season dependent. Thirdly, the bitter compounds unevenly distributed in the fruit tissues. Flavedo contained the lower level of naringin, while the segment membrane accumulated large amount of limonin and nomilin. Finally, contents of the three main bitter compounds in the albedo were significantly related with those in the corresponding segment membrane of both citrus cultivars and content of naringin was interestingly related with that of limonin or nomilin at the significant level.2. Relative expression of limonoid glucosyltransferase gene (LGT) and1,2-rhamonsyltransferase gene(1,2RhaT) were analyzed via Quantitative Real time-PCR. Limonoid glucosyltransferase and1,2-rhamonsyltransferase are two important enzymes in the metabolic pathway of bitter compounds so far known to the researchers. The expression of the two genes was not well consistent with the accumulation of the corresponding bitter compounds.3. The limonoid glucosyltransferase gene in’Guoqing No.l’Satsuma mandarin,’Oroblanco’grapefruit,’Huanong bendizao’tangerine (C. reticulata Blanco),’Feicui’ pomelo (C. grandis Osbeck),’Cara Cara’navel orange (C. sinensis Osbeck), and’Seike’ navel orange (C. sinensis Osbeck) was cloned and aligned. The results revealed that there were two types of cDNA clones encoding limonoid glucosyltransferase in each investigated cultivar. The results of southern blot for LGT in above six cultivars indicated that, only one copy of LGT in’Guoqing No.1’Satsuma mandarin,’Huanong bendizao’ and’Feicui’ pomelo, but two copies were found in’Oroblanco’grapefruit,’Cara Cara’ navel orange and’Seike’ navel orange. Furthermore, for one of the transcripts, comparing with other cultivars, there was a missing of two consecutive amino acids in’Oroblanco’ grapefruit,’Cara Cara’ navel orange and ’Seike’ navel orange, and an early stop codon was also introduced.