Functional diversification of two UGT80 enzymes required for steryl glucoside synthesis in Arabidopsis

Steryl glucosides (SG) are abundant steroid conjugates in plant membranes. Beyond structural roles in lipid bilayers, functions in sugar transport, storage, and/or signalling are predicted. UDP-glucose:sterol glucosyltransferase 80A2 (UGT80A2) and UGT80B1, which share similarity to fungal counterparts, are implicated in SG synthesis in Arabidopsis thaliana. A third related enzyme, which seems specific to the plant lineage, is encoded by UGT713B1/At5g24750. Genetic and biochemical approaches were employed to determine the role of each UGT gene in the production of specific SGs and acyl SGs (ASGs). Using direct infusion electrospray ionization tandem mass spectrometry (ESI-MS/MS), SG and acyl SG (ASG) contents of ugt80 and ugt713 mutants, and triple and double mutants were profiled in seeds. In vitro enzyme assays were performed to assay substrate preferences. Both UGT80A2 and UGT80B1, but not UGT713B1 were shown to be coordinately down-regulated during seed imbibition when SG levels decline, consistent with similar functions as UGT80 enzymes. UGT80A2 was found to be required for normal levels of major SGs in seeds, whereas UGT80B1 is involved in accumulation of minor SG and ASG compounds. Although the results demonstrate specific activities for UGT80A2 and UGT80B1, a role for UGT713B1 in SG synthesis was not supported. The data show that UGT80A2, the more highly conserved enzyme, is responsible for the bulk production of SGs in seeds, whereas UGT80B1 plays a critical accessory role. This study extends our knowledge of UGT80 enzymes and provides evidence for specialized functions for distinct classes of SG and ASG molecules in plants.