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

ABSTRACT

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, CitACLα1, CitACLα2, and CitACLß1, in the citrus genome database. Both CitACLα1 and CitACLα2 encode putative ACL α subunits with 82.5 % amino acid identity, whereas CitACLß1 encodes a putative ACL ß subunit. Gene structure analysis showed that CitACLα1 and CitACLα2 had 12 exons and 11 introns, and CitACLß1 had 16 exons and 15 introns. CitACLα1 and CitACLß1 were predominantly expressed in flower, and CitACLα2 was predominantly expressed in stem and fibrous roots. As fruits ripen, the transcript levels of CitACLα1, CitACLß1, and/or CitACLα2 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 CitACLα2, CitACLß1, and/or CitACLα1 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 CitACLß1 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.