Metabolism of the Folate Precursor p-Aminobenzoate in Plants

Abstract

Plants produce p-aminobenzoate (pABA) in chloroplasts and use it for folate synthesis in mitochondria. In plant tissues, however, pABA is known to occur predominantly as its glucose ester (pABA-Glc), and the role of this metabolite in folate synthesis has not been defined. In this study, the UDP-glucose:pABA acylglucosyltransferase (pAGT) activity in Arabidopsis extracts was found to reside principally (95%) in one isoform with an apparent K m for pABA of 0.12 mM. Screening of recombinant Arabidopsis UDP-glycosyltransferases identified only three that recognized pABA. One of these (UGT75B1) exhibited a far higher k cat /K m value than the others and a far lower apparent K m for pABA (0.12 mM), suggesting its identity with the principal enzyme in vivo. Supporting this possibility, ablation of UGT75B1 reduced extractable pAGT activity by 95%, in vivo [ 14 C]pABA glucosylation by 77%, and the endogenous pABAGlc/pABA ratio by 9-fold. The K eq for the pABA esterification reaction was found to be 3 ؋ 10 ؊3 . Taken with literature data on the cytosolic location of pAGT activity and on cytosolic UDPglucose/UDP ratios, this K eq value allowed estimation that only 4% of cytosolic pABA is esterified. That pABA-Glc predominates in planta therefore implies that it is sequestered away from the cytosol and, consistent with this possibility, vacuoles isolated from [ 14 C]pABA-fed pea leaves were estimated to contain >88% of the [ 14 C]pABA-Glc formed. In total, these data and the fact that isolated mitochondria did not take up [ 3 H]pABAGlc, suggest that the glucose ester represents a storage form of pABA that does not contribute directly to folate synthesis.

Does Abiotic Stress Cause Functional B Vitamin Deficiency in Plants?

“…The esterification reaction is reversible and the ester is stored in vacuoles (Eudes et al, 2008). Glycosides (probably b-D-glucosides) of the tetrahydrofolate precursors neopterin and monapterin were found in tomato fruit engineered to overproduce pterins (Díaz de la Garza et al, 2004); it is not known whether such glycosides occur naturally or whether they can be mobilized.…”

Nicotinate O-Glucosylation Is an Evolutionarily Metabolic Trait Important for Seed Germination under Stress Conditions in Arabidopsis thaliana

“…Previous studies showed that UGT74F2 exhibited activity toward SA and o-ABA in planta (Quiel and Bender, 2003;Dean and Delaney, 2008;Eudes et al, 2008;Song et al, 2009). To determine the biological functions of UGT74F2 in other tissues, we also compared the chemical profiling (NA, SA, BA, o-ABA, and their glucosides) in 7-d-old seedlings and pathogeninfected rosette leaves in Col-0 and the ugt74f2-1 mutant.…”

“…In vitro biochemical assays previously showed that UGT74F2 recognized SA, BA, and o-ABA as substrates (Lim et al, 2002;Quiel and Bender, 2003;Dean and Delaney, 2008;Eudes et al, 2008;Song et al, 2009). Although NA is not the best substrate for UGT74F2 in vitro, the high catalytic efficiency of UGT74F2 toward NA still strongly suggests that NA is an important substrate for UGT74F2 in Arabidopsis (Table 1; Supplemental Figure 2).…”

Multiomics in Grape Berry Skin Revealed Specific Induction of the Stilbene Synthetic Pathway by Ultraviolet-C Irradiation

“…Also found were pectinesterase activity (GO:0030599) and aspartyl esterase activity (GO:0045330), which includes pectinesterase2-like (Louvet et al, 2006; Table II). The category transferase activity, transferring hexosyl groups (GO:0016758) includes UDP-glycosyltransferase, related to the formation of cell wall and cell plate (Eudes et al, 2008), while trihydroxystilbene synthase activity includes STS (Table II). STS is a key enzyme of resveratrol synthesis from coumaroyl-CoA and malonyl-CoA (Delaunois et al, 2009).…”