Identification of an apiosyltransferase in the plant pathogen Xanthomonas pisi

The apiosyltransferase celery UGT94AX1 catalyzes the biosynthesis of the 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 (Apium graveolens) and parsley (Petroselinum crispum) 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 UGT94AX1 as an A. graveolens apiosyltransferase (AgApiT) 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 suggested that AgApiT is the sole apiosyltransferase-encoding gene in the celery genome. Identification of this plant apiosyltransferase gene will enhance our understanding of the physioecological functions of apiose and apiose-containing compounds.

Practical preparation of UDP-apiose and its applications for studying apiosyltransferase

UDP-apiose, a donor substrate of apiosyltransferases, is labile because of its intramolecular self-cyclization ability, resulting in the formation of apiofuranosyl-1,2-cyclic phosphate. Therefore, stabilization of UDP-apiose is indispensable for its availability and identifying and characterizing the apiosyltransferases involved in the biosynthesis of apiosylated sugar chains and glycosides. Here, we established a method for stabilizing UDP-apiose using bulky cations as counter ions. Bulky cations such as triethylamine effectively suppressed the degradation of UDP-apiose in solution. The half-life of UDP-apiose was increased to 48.1 ± 2.4 h at pH 6.0 and 25 °C using triethylamine as a counter cation. UDP-apiose coordinated with a counter cation enabled long-term storage under freezing conditions. UDP-apiose was utilized as a donor substrate for apigenin 7-O-β-D-glucoside apiosyltransferase to produce the apiosylated glycoside apiin. This apiosyltransferase assay will be useful for identifying genes encoding apiosyltransferases.