Transcriptional Regulation Of Citric Acid Degradation GS Pathway In Citrus Fruits

Organic acids affect fruit quality.The main organic acid in citrus fruits is citric acid.In this study,’Newhall’ navel orange(Citrus sinensis L.Osbeck cv.Newhall)was used as experimental material.The effects of different latitude(Ganzhou,Jiangxi and Songyang,Zhejiang)on the degradation of citric acid in fruits were analyzed,and the target gene CitGS1 was characterized.The role of CitGS1 gene was studied,and a transcription factor,CitGATA7,was observed to regulate citric acid degradation via activating the GS pathway.The main results are as follows:1.The citric acid of navel orange fruits in different latitude is mainly degraded through GS pathway.Citric acid contents of Ganzhou navel oranges were significantly lower than Songyang’s.The expression levels of CitAco3,CitIDH1 and CitGS1 in Ganzhou fruits were higher than Songyang fruits’.GABA pathway-related genes showed no significant difference in two areas.2.Identification and analysis of GS genes.The citrus GS family has two members: CitGS1 and CitGS2.CitGS1 located in cytoplasm and CitGS2 located in chloroplast.The substrate affinity experiments demonstrated the CitGS1 enzyme functioned as glutamine synthetase.GS enzyme activity of Ganzhou fruits was significantly higher than Songyang fruits’,suggesting that CitGS1 may play an important role in citric acid degradation in different habitats.3.Transcriptional regulation analysis of CitGS1.A transcription factor CitGATA7 was obtained.CitGATA7 located in the nucleus,and its expression pattern was consistent with CitGS1.Dual-luciferase assay showed that CitGATA7 could trans-activate the promoter of CitAco3,CitIDH1 and CitGS1.The content of citric acid in CitGATA7 over-expressed transgenic callus was significantly lower than wild-type callus’ s,suggesting that CitGATA7 may be related to citric acid degradation.4.Citric acid degradation mechanism of navel orange fruits in different latitude.The fruits of Ganzhou(lower latitude)had lower acid contents,which were related to the higher expression levels of CitAco3,CitIDH1 and CitGS1.CitGATA7 could promote citric acid degradation via activating the promoter activity of CitAco3,CitIDH1 and CitGS1 to in the GS pathway.