Metabolism of salicylic acid in wild-type, ugt74f1 and ugt74f2 glucosyltransferase mutants of Arabidopsis thaliana

Arabidopsis thaliana contains two salicylic acid (SA) glucosyltransferase enzymes designated UGT74F1 and UGT74F2. UGT74F1 forms only SA 2-O-beta-D-glucose (SAG), while UGT74F2 forms both SAG and the SA glucose ester (SGE). In an attempt to determine the in vivo role of each SA glucosyltransferase (SAGT), the metabolism of SA in ugt74f1 and ugt74f2 mutants was examined and compared with that of the wild-type. The three major metabolites formed in wild-type Arabidopsis included SAG, SGE, and 2,5-dihydroxbenzoic acid 2-O-beta-D-glucose (DHB2G). This is the first description of DHB2G as a major metabolite of SA in plants. The major metabolites of SA formed in ugt74f1 mutants were SGE, SAG and 2,5-dihydroxybenzoic acid 5-O-beta-D-glucose (DHB5G). DHB5G was not formed in the wild-type plants. SAG and DHB2G were the main metabolites of SA in ugt74f2 mutants. The ugt74f2 mutant was unable to form SGE. Only SGE could be detected during in vitro SAGT assays of untreated wild-type and ugt74f1 mutants. This activity was because of constitutive UGT74F2 activity. Both SGE and SAG could be formed during in vitro assays of SA-pretreated wild-type and ugt74f1 leaves. Neither SAG nor SGE could be detected during the in vitro SAGT assays of untreated ugt74f2 leaves. Only SAG was formed during the in vitro SAGT assays of SA-pretreated ugt74f2 leaves. The SAG formation was a result of the UGT74F1 activity. This work demonstrates that changes in the activity of either SAGT enzyme can have a dramatic effect on the metabolism of exogenously supplied SA in Arabidopsis.