Action mechanism of a herbicide, thiobencarb

Abstract

The action mechanism of thiobencarb was studied by examining the inhibitory effects of this herbicide on the biosynthesis of very-long-chain fatty acids (VLCFAs). Thiobencarb treatment decreased VLCFAs, such as C20:0, C20:1, C22:0, C24:0, C24:1 and C26:0 fatty acids, and increased long-chain-fatty acids and medium-chain-fatty acids, such as C14:0, C15:0, C18:0 and C18:1 fatty acids, which are precursors of VLCFAs, in barnyard millet cultured cells. Thiobencarb sulfoxide and sulfone potently inhibited VLCFA elongase (VLCFAE) activity in the microsomal fraction of etiolated barnyard millet seedlings, although thiobencarb itself slightly inhibited it. These results suggested that thiobencarb is a VLCFAE-inhibiting herbicide whose active forms are its oxidized metabolites, such as sulfoxide and sulfone. Thiobencarb sulfoxide inhibited the VLCFAE activity of the microsomal fraction of etiolated barnyard millet seedlings in a time-independent manner. This time-independent inhibition proposed a reversible inhibition mechanism of the VLCFAE by thiobencarb sulfoxide, likely with isoxazoline-type herbicides, such as pyroxasulfone, which have been classified into group K3 of the Herbicide Resistance Action Committee (HRAC) in the U.S. It is assumed that the time-independent reversible inhibition of VLCFAE is applicable to other thiocarbamate herbicides presently classified into group N of the HRAC. © Pesticide Science Society of Japan Keywords: thiobencarb, thiobencarb sulfoxide, very-long-chain fatty acid (VLCFA), very-long-chain fatty acid elongase (VLCFAE), time-independent manner, N group. IntroductionThiobencarb, S-(4-chlorobenzyl)-N,N-diethylthiocarbamate, is a herbicide developed by Kumiai Chemical Industry for crops such as rice, wheat, corn and soybean (Fig. 1A).1) This herbicide at the application rate of 1.5-7.5 kg a.i./ha provides good efficacy on both grass and broad leaf weeds, especially needle spikerush and barnyard grass.2) Barnyard grass from pre-emergence up to the three-leaf stage is effectively controlled by this herbicide. This herbicide shows excellent selectivity for rice plants.Differences in the physiological activity between rice and barnyard grass for absorbing and metabolizing this herbicide have been reported. 3-5)Thiobencarb does not inhibit the germination of seeds but potently inhibits the shoot elongation of germinated seeds. This herbicide belongs to the thiocarbamate herbicides and is classified into group N of the Herbicide Resistance Action Committee (HRAC) in the U.S.6) According to the HRAC, the target site of the group N herbicides is described as the lipid synthesis except for acetyl-CoA carboxylase (ACCase), indicating that their mode of action has not been completely elucidated. 7,8) However, it has been reported that sulfoxide of pebulate, which is also a thiocarbamate herbicide, inhibits the incorporation of 14 Clabeled acetic acid into very-long-chain fatty acids (VLCFAs) in some plants. 9,10) Therefore, the thiocarbamate herbicides were presumed to inhibit the biosynthesis of...

S-Methyl Cysteine Protective Effects in Oreochromis Niloticus Fish Contaminated by Thiobencarb Herbicide

“…Chemical herbicides are a group of widely spread pesticides used for control of exotic weeds, which may be selective or nonselective. Thiobencarb is one of the intensively used selective herbicides in Egypt, which used primarily for the weeds emerging in the rice fields (Tanetani et al, 2013). Furthermore, the uncontrolled use of these chemicals during rice cultivation season poses a crucial source of aquatic environments pollution (Sapari and Ismail, 2012), it finds their way to the near water bodies through the surface runoff (Phong et al, 2006;Papadakis et al, 2015) causing adverse impacts on aquatic livings ranging from mild effects to life threatening ones (Bailey, 1993).…”

Thiobencarb Herbicide Reduces Growth, Photosynthetic Activity, and Amount of Rieske Iron‐Sulfur Protein in the Diatom Thalassiosira pseudonana

“…The underlying mechanism responsible for the decreases in the F v / F m ratio and Rieske Fe‐S protein content is not clear, although there is a possibility of a direct action of thiobencarb on the Rieske Fe‐S protein. However, inhibition of the synthesis of fatty acids or proteins by thiobencarb may be able to explain its effects on photosynthetic activity and the Rieske Fe‐S protein content in diatoms. In general, such inhibition is thought to reduce the consumption of NADPH and ATP.…”

“…Thiocarbamate herbicides inhibit the biosynthesis of fatty acids, proteins, isoprenoids, flavonoids, and gibberellins in higher plants . Thiobencarb itself has not been thoroughly investigated, but it has been reported to inhibit the synthesis of fatty acids and proteins through an antagonistic effect on auxins in higher plants . However, because the distribution and functions of auxins in microalgae are studied in limited species , toxicity mechanisms of thiobencarb in microalgae are not clear.…”

A DFT study of the chemical reactivity of thiobencarb and its oxidized derivatives in aqueous phase

“…TB does not inhibit the germination of seeds, but potently inhi- bits shoot elongation of germinated seeds. 4 The resultant syndrome is called delayed phytotoxicity syndrome. 3,5 Additionally, when TB is used on paddy fields, its residues in standing water may harm beneficial flora and fauna.…”

“…8 Additionally, in aqueous phase, TB can easily be oxidized to thiobencarb sulfoxide (TBS) and thiobencarb sulfone (TBSu) under environmental and biological conditions, [9][10][11] since these oxidized forms are the active forms of TB. 4 Although, thiobencarb is considered as photostable, 12 TB degradation in natural water may occur abiotically through photooxidation, 13 and indirect photolysis via hydroxyl radicals. 12,13 If laser flash photolysis is employed, TB degradation involves the photocleavage of the C-S bond.…”