Attenuation of impact of rimsulfuron in maize by priming of grains with branched-chain amino acids or dressing with naphthalic anhydride

The toxicity of rimsulfuron (Rim) on acetohydroxyacide synthase (AHAS), nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), catalase (CAT) and ascorbate peroxidase (APX) in maize was evaluated examining their kinetic parameters in vitro on second leaves of 10-day-old maize seedlings. Data showed that Rim decreased Vmax and Kcat of AHAS without changing Km; on the contrary, Vmax and Kcat of the other enzymes were unchanged while Km increased. The lowest IC50 value was found for AHAS and the highest for GOGAT. Application of Rim to 10-dayold seedlings significantly inhibited the activities of all enzymes over the next 10 days besides reducing growth, proteins, amino acids, ascorbate (AsA) and glutathione (GSH). Malondialdehyde (MDA), H2O2 and oxidized glutathione (GSSG) were instead increased. Priming of grains with branched-chain amino acids [valine, isoleucine and leucine (VIL)] or dressing with naphthalic anhydride (NA) mitigated these effects. These findings suggest that Rim caused non-competitive inhibition with AHAS due to the decreased Vmax and unchanged Km; however, the enhanced Km of the other enzymes with unchanged Vmax shows an induction of competitive inhibition. Moreover, decreases of amino acids, especially branched-chain acids, and activities of NR, NiR, GS, and GOGAT could have delayed N incorporation into proteins, thus slowing their formation. The decreases of AsA, GSH, CAT, and APX concurrent with the increase of MDA and H2O2 support the induction of oxidative stress. Application of VIL or NA can protect maize from Rim causing an attenuation of the phytotoxic effects of the herbicide and enhancing maize tolerance by compensating endogenous amino acid levels, ROS homeostasis and/or herbicide detoxification.