Investigation of physiological roles of UDP-glycosyltransferase UGT76F2 in auxin homeostasis through the TAA-YUCCA auxin biosynthesis pathway

Cellular auxin (indole-3-acetic acid, IAA) levels are coordinately regulated by IAA biosynthesis and inactivation. IAA is synthesized through sequential reactions by two enzymes, TAA1 and YUCCA, in a linear indole-3-pyruvic acid (IPA) pathway. TAA1 converts tryptophan to IPA, and YUCCA catalyzes the oxidative decarboxylation of IPA into IAA. Arabidopsis UDP-glycosyltransferase UGT76F2 (At3g55710) was previously reported to catalyze the glycosylation of IPA and consequently modulate IAA levels. We carefully analyzed the physiological roles of UGT76F2 and its close homolog UGT76F1 (At3g55700) in IAA homeostasis. We generated two independent ugt76f1 ugt76f2 double null Arabidopsis mutants (ugt76f1f2) with a 2.7 kb deletion, along with two independent ugt76f2 single null mutants by CRISPR/Cas9 gene editing technology. Surprisingly, these null mutants exhibited indistinguishable phenotypes from the wild-type seedlings under our laboratory conditions. Our results indicate that UGT76F1 and UGT76F2 do not play important roles in regulating IAA biosynthesis via the IPA glycosylation.