Sumedha B S, Bangalore University
Herbicides are substances used in agriculture to kill weeds to promote the growth of desirable crops. A weed called Waterhemp is known for its resistance, so it is very difficult to control its growth. This belongs to the pigweed family and grows in soybean and cornfields.
Herbicides used in corn commonly work by inhibiting 4-hydroxyphenylpyruvate dioxygenase (HPPD). HPPD is involved in the formation of homogentisic acid, the precursor of plastoquinone and vitamin E. SA3 (syncarpic acid-3) is not used commercially because it can kill the crops along with the weeds. Corn can tolerate other herbicides like Callisto as it has a robust detoxification ability. It can neutralize and block harmful chemicals. However, corn’s detoxification systems don’t act on SA3. Weeds such as waterhemp typically have detoxification systems similar to corns. So, it was surprising to find that HPPD-resistant waterhemp can detoxify SA3.
Waterhemp and Detoxification of Herbicides:
Waterhemp is a small-seeded, annual dicot, dioecious weed. It is capable of producing about 1 million seeds per female plant, even in suboptimal growth conditions. Multiple-herbicide-resistant – waterhemp and tolerant corn detoxify the HPPD inhibitors through oxidative metabolism. It involves the hydroxylation of the cyclohexanedione ring of mesotrione and tembotrione.
Typically, herbicide detoxification occurs in four successive phases:
- Phase I– oxidation,
- Phase II – conjugation,
- Phase III -transport and
- Phase IV-compartmentation.
Herbicidal activity of SA3 was previously studied in several grass species and broadleaf weed species, but not in waterhemp.
The recent study:
The goal of this research was to better understand potential detoxification pathways and resistance mechanisms of waterhemp. Researchers used targeted analysis of SA3 metabolism. To uncover the metabolic pathway of resistance, they tested the hypothesis, that SA3 resistance in waterhemp is metabolism-based, by a general metabolomics approach.
Usually, the detoxification of herbicides and other toxic compounds occurs in phases. In the first step, a group of enzymes called p450s, remove electrons from toxic compounds. This makes these compounds less reactive inside the plant cells. It was found that in resistant waterhemp, the opposite happened. The electrons were added to the SA3 molecules. Phase 2 enzymes called GSTs do not get activated for Callisto as p450s does this quickly and efficiently in corn. But for SA3, GSTs were found to be responsible for detoxification.
Significance of the study:
This discovery was surprising because the phase-one reactions were as expected, but also as GSTs were found to be involved for this class of herbicides. It was observed that corn didn’t prepare chemicals for attack by GSTs. This is a very, very rare discovery for herbicides.
The authors believe that this deviation from standard biochemical detoxification pathways signifies something truly novel. However, this can potentially be devastating for crop producers.
Metabolic detoxification provides crop tolerance and weed resistance to herbicides. It targets a wide range of modes of action in plants. Herbicide metabolism resistance in weedy plant species threatens effective and sustainable crop production. So, to design new and effective herbicides, it is essential to identify herbicide detoxification pathways. These herbicides could block the resistance mechanisms of weeds.
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References:
1. Concepcion, J.C.T., Kaundun, S.S., Morris, J.A., Hutchings, S.-J., Strom, S.A., Lygin, A.V. and Riechers, D.E. (2021), Resistance to a nonselective 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide via novel reduction–dehydration–glutathione conjugation in Amaranthus tuberculatus. New Phytol. https://doi.org/10.1111/nph.17708
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