Metabolism and Residues of 2,4-Dichlorophenoxyacetic Acid in DAS-40278-9 Maize (Zea mays) Transformed with Aryloxyalkanoate Dioxygenase-1 Gene

Abstract

DAS-40278-9 maize, which is developed by Dow AgroSciences, has been genetically modified to express the aryloxyalkanoate dioxygenase-1 (AAD-1) protein and is tolerant to phenoxy auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D). To understand the metabolic route and residue distribution of 2,4-D in DAS-40278-9 maize, a metabolism study was conducted with C-radiolabeled 2,4-D applied at the maximum seasonal rate. Plants were grown in boxes outdoors. Forage and mature grain, cobs, and stover were collected for analysis. The metabolism study showed that 2,4-D was metabolized to 2,4-dichlorophenol (2,4-DCP), which was then rapidly conjugated with glucose. Field-scale residue studies with 2,4-D applied at the maximum seasonal rate were conducted at 25 sites in the U.S. and Canada to measure the residues of 2,4-D and free and conjugated 2,4-DCP in mature forage, grain, and stover. Residues of 2,4-D were not detectable in the majority of the grain samples and averaged<1.0 and <1.5 μg/g in forage and stover, respectively. Free plus conjugated 2,4-DCP was not observed in grain and averaged <1.0 μg/g in forage and stover.

Comparative Analysis of 2,4-D Uptake, Translocation, and Metabolism in Non–AAD-1 Transformed and 2,4-D–Resistant Corn

“…The excised-leaf assay was also used to determine the metabolism of DCP. DCP is the first metabolite formed in E corn by the AAD-1 enzyme (Wright et al 2010;Zhou et al 2016) and would indicate whether DCP is metabolized in a similar fashion between corn hybrids regardless of the transgene, as determined previously in 2,4-D-resistant and 2,4-D-sensitive cotton (Laurent et al 2006). The assay used 25 μM [ 14 C]DCP (specific activity 1.2 GBq mmol −1 ) to measure metabolism at 1 and 24 HAA.…”

“…Nonextractable 14 C material was not different between 2,4-D only or 2,4-D plus glyphosate at any time point in E corn. Although no further attempts were made to solubilize or characterize these nonextractable compounds in E corn leaves, previous research using extraction under acidic or basic hydrolytic conditions has provided insights into the nature of conjugates formed from 2,4-D in planta (Hamburg et al 2001;Zhou et al 2016). b Parameter corresponding to time (h) for treatment to reach 50% M max .…”

“…However, the metabolite(s) peak at approximately 11 min was not detected when NT corn leaves were supplied with 2,4-D ( Figure 6D), consistent with an inability to form DCP from parent 2,4-D. Three polar metabolites (Figure 6C and D; approximately 4, 5, and 6.5 min) were detected in E and NT corn treated with 2,4-D; a relatively nonpolar metabolite specific to 2,4-D metabolism by NT corn was also detected ( Figure 6D; approximately 24 min). These first three peaks may contain ring-hydroxylated and/ or glycosylated metabolites generated by endogenous 2,4-D metabolic pathways previously characterized in corn, wheat, soybean, tobacco, and several other dicot and monocot plants (Chkanikov et al 1976;Feung et al 1975;Hamburg et al 2001;Montgomery et al 1971;Robertson and Kirkwood 1970;Roberts 1998;Schroder and Collins 2002;Zhou et al 2016), but were not characterized further. E corn metabolized more 2,4-D than NT corn, and glyphosate did not alter 2,4-D metabolism in either hybrid (Table 4).…”

“…DCP does not accumulate but is rapidly metabolized to a polar metabolite ( Figure 6C; approximately 11 min) in E corn that is not detectable when 2,4-D is supplied as substrate in NT corn ( Figure 6D). Recent residue analyses have determined the long-term metabolic fate of 2,4-D choline in various plant tissues and grain beyond 24 HAA in E corn (Zhou et al 2016).…”

“…The primary difference between E and NT corn hybrids is the initial metabolic mechanism(s) in which 2,4-D is detoxified. NT corn can only metabolize 2,4-D initially through ring hydroxylation (Chkanikov et al 1976;Feung et al 1975;Montgomery et al 1971;Roberts 1998;Schroder and Collins 2002) mediated by cytochrome P450 monooxygenases (Siminszky 2006), but E corn can also use the engineered bacterial AAD-1-mediated pathway that metabolizes 2,4-D to nonphytotoxic DCP (Wright et al 2010;Zhou et al 2016). As a result, the use of different metabolic pathways and enzymes will likely result in varying initial rates of metabolism by E corn compared with NT corn hybrids, but a direct comparison of 2,4-D metabolism in E vs. NT corn within a single study has not been reported in the literature.…”