Genome-wide identification of citrus ATP-citrate lyase genes and their transcript analysis in fruits reveals their possible role in citrate utilization

摘要

ATP-citrate lyase (ACL, EC4.1.3.8) catalyzes citrate to oxaloacetate and acetyl-CoA in the cell cytosol, and has important roles in normal plant growth and in the biosynthesis of some secondary metabolites. We identified three ACL genes, CitACL alpha 1, CitACL alpha 2, and CitACL beta 1, in the citrus genome database. Both CitACL alpha 1 and CitACL alpha 2 encode putative ACL alpha subunits with 82.5 % amino acid identity, whereas CitACL beta 1 encodes a putative ACL beta subunit. Gene structure analysis showed that CitACL alpha 1 and CitACL alpha 2 had 12 exons and 11 introns, and CitACL beta 1 had 16 exons and 15 introns. CitACL alpha 1 and CitACL beta 1 were predominantly expressed in flower, and CitACL alpha 2 was predominantly expressed in stem and fibrous roots. As fruits ripen, the transcript levels of CitACL alpha 1, CitACL beta 1, and/or CitACL alpha 2 in cultivars 'Niuher' and 'Owari' increased, accompanied by significant decreases in citrate content, while their transcript levels decreased significantly in 'Egan No. 1' and 'Iyokan', although citrate content also decreased. In 'HB pummelo', in which acid content increased as fruit ripened, and in acid-free pummelo, transcript levels of CitACL alpha 2, CitACL beta 1, and/or CitACL alpha 1 increased. Moreover, mild drought stress and ABA treatment significantly increased citrate contents in fruits. Transcript levels of the three genes were significantly reduced by mild drought stress, and the transcript level of only CitACL beta 1 was significantly reduced by ABA treatment. Taken together, these data indicate that the effects of ACL on citrate use during fruit ripening depends on the cultivar, and the reduction in ACL gene expression may be attributed to citrate increases under mild drought stress or ABA treatment.