Abstract
Echinochloa crus-galli var. mitis has rarely been reported for herbicide resistance, and no case of quinclorac resistance has been reported so far. Synthetic auxin-type herbicide quinclorac is used extensively to control rice weeds worldwide. A long history of using quinclorac in Chinese rice fields escalated the resistance in E. crus-galli var. mitis against this herbicide. Bioassays in Petri plates and pots exhibited four biotypes that evolved into resistance to quinclorac ranking as JS01-R > AH01-R > JS02-R > JX01-R from three provinces of China. Ethylene production in these biotypes was negatively correlated with resistance level and positively correlated with growth inhibition. Determination of the related ethylene response pathway exhibited resistance in biotypes that recorded a decline in 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase oxidase activities, and less inducible ACS and ACO genes expressions than the susceptible biotype, suggesting that there was a positive correlation between quinclorac resistance and ethylene biosynthesis inhibition. Cyanides produced during the ethylene biosynthesis pathway mainly degraded by the activity of β-cyanoalanine synthase (β-CAS). Resistant biotypes exhibited higher β-CAS activity than the susceptible ones. Nucleotide changes were found in the EcCAS gene of resistant biotypes as compared to sensitive ones that caused three amino acid substitutions (Asn-105-Lys, Gln-195-Glu, and Gly-298-Val), resulting in alteration of enzyme structure, increased binding residues in the active site with its cofactor, and decreased binding free energy; hence, its activity was higher in resistant biotypes. Moreover, these mutations increased the structural stability of the enzyme. In view of the positive correlation between ethylene biosynthesis inhibition and cyanide degradation with resistance level, it is concluded that the alteration in ethylene response pathway or at least variation in ACC synthase and ACC oxidase enzyme activities—due to less relative expression of ACS and ACO genes and enhanced β-CAS activity, as well as mutation and increased relative expression of EcCAS gene—can be considered as a probable mechanism of quinclorac resistance in E. crus-galli var. mitis.
Highlights
Contributing more than USD 34 billion toward herbicidal control cost [1], weeds are considered to be the most troublesome entity in agricultural systems [2]
This study presented that expression levels of EcACS7, EcACS-like, EcACO1, EcACO-like, and EcACO5-like a showed >10-fold increment in FJ01-S, while relative expression induction was lower in resistant biotypes
Vivid induction of expressions of EcACS7, EcACS-like, EcACO1, EcACO-like, and EcACO5-like genes in quinclorac-susceptible biotype (FJ01-S) of E. crus-galli var. mitis after herbicide treatment gave rise to ACC synthase and ACC oxidase enzyme activities, which resulted in increased ethylene biosynthesis
Summary
Contributing more than USD 34 billion toward herbicidal control cost [1], weeds are considered to be the most troublesome entity in agricultural systems [2]. Weed control by using herbicides dates back to 1945 with the commercial availability of 2,4-dichlorophenoxyacetic acid (2,4-D (synthetic auxin)) [3]. Herbicides greatly contributed to abundant food production worldwide. The evolution of a resistant population is a major threat to the sustained efficacy of these herbicides. Robust selection pressure, which is imposed by herbicides on vast and genetically assorted pest populations, firstly results in high mortality; on the other hand, selection and enrichment of the rare resistant genes present in the population result in the evolution of resistance [4,5]. 256 species containing 500 resistant biotypes are present, which consists of 149 dicotyledon and 107 monocotyledon biotypes [6]
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