Discovery of the apiosyltransferase, celery UGT94AX1 that catalyzes the biosynthesis of a flavone glycoside, apiin

Apiose is a unique branched-chain pentose found in plant glycosides and a key component of the cell wall-polysaccharide pectin and other specialized metabolites. More than 1,200 plant-specialized metabolites contain apiose residues, represented by apiin, a distinctive flavone glycoside found in celery and parsley in the family Apiaceae. The physiological functions of apiin remain obscure, partly due to our lack of knowledge on apiosyltransferase during apiin biosynthesis. Here, we identified celery UGT94AX1 (AgApiT) as a novel apiosyltransferase, responsible for catalyzing the last sugar-modification step in apiin biosynthesis. AgApiT showed strict substrate specificity for the sugar donor, UDP-apiose, and moderate specificity for acceptor substrates, thereby producing various apiose-containing flavone glycosides in celery. Homology modeling of AgApiT with UDP-apiose, followed by site-directed mutagenesis experiments, identified unique Ile139, Phe140, and Leu356 residues in AgApiT, which are seemingly crucial for the recognition of UDP-apiose in the sugar donor pocket. Sequence comparison and molecular phylogenetic analysis of celery glycosyltransferases paralogous to AgApiT suggested that AgApiT is the sole apiosyltransferase-encoding gene in the celery genome. This is the first report on the identification of a plant apiosyltransferase gene that will enhance our understanding of the physio-ecological functions of apiose and apiose-containing compounds.