Transformation kinetics of 6-methoxybenzoxazolin-2-one in soil

Abstract

Wheat (Triticum aestivum L.) and other cereals produce allelochemicals as natural defense compounds against weeds, fungi, insects and soil-borne diseases. The main benzoxazinoid allelochemical of wheat is 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), bound as beta-glucoside and released upon plant injury. When leached from wheat to soil, DIMBOA is microbially transformed to 6-methoxy-benzoxazolin-2-one (MBOA). Exploiting benzoxazinoids and their degradation products as substitutes for synthetic pesticides depends on knowledge of transformation pathways and kinetics. In an MBOA degradation experiment at a concentration of 2400 nmol g(-1) soil, the previously identified transformation products 2-amino-7-methoxy-phenoxazin-3-one (AMPO) and 2-acetylamino-7-methoxy-phenoxazin-3-one (AAMPO) were quantified. Three different kinetic models were applied to MBOA transformation kinetics; single first-order (SFO), first-order multi-compartment, and double first-order in parallel. SFO proved to be adequate and was subsequently applied to the transformations of MBOA, AMPO and AAMPO. Degradation endpoints, expressed as degradation time (DT), were calculated for MBOA, AMPO and AAMPO to test whether the maximum values for synthetic pesticides set by the European Commission and the Danish Environmental Protection Agency were exceeded. DT(50) values for MBOA and AMPO were 5.4 d and 321.5 d, respectively, and DT(90) values were 18.1 d and 1068 d, respectively. The DT(50) value for AMPO exceeded the maximum value. The persistence, concentrations and toxicity of metabolites such as AMPO should be considered when breeding cereal crops with increased levels of benzoxazinoids.

Soil Degradation of Parthenin—Does it Contradict the Role of Allelopathy in the Invasive Weed Parthenium hysterophorus L.?

“…The same has been observed previously for other allelochemicals (Borek et al 1995;Etzerodt et al 2008), and is suggested to apply for most current-generation agrochemicals (Aldworth and Jackson 2008). Degradation of higher initial parthenin concentrations proved to be characterized by a broad lag-phase preceding the onset of degradation.…”

“…Characteristic features of the hormetic response were derived by estimation from fitted curves. The kinetics of parthenin degradation in soil as modeled according to Equation 1 was compared further to a commonly used first-order kinetics model [y=d*exp(-kx) with k as the proportionality coefficient (Aldworth and Jackson 2008;Etzerodt et al 2008)] in order to find the most appropriate model describing the degradation. All modeling was done with SPSS®, and the quality of curve fitting was assessed by F test for lack-of-fit based on an analysis of variance (P=0.05) or by graphical assessment of the distribution of data around regression lines.…”

Degradation of the benzoxazolinone class of phytoalexins by Fusarium pseudograminearum, a pathogenic fungus of wheat

“…As the Fusarium crown rot disease is established by infection of wheat seedlings during emergence (Knight & Sutherland, 2013), seedling tissues that have heightened phytoalexin production may provide some protection against disease. As has been previously discussed (Etzerodt et al, 2008;Niemeyer & Jerez, 1997), manipulation by either genetic modification or breeding for altered quantity or identity of benzoxazolinone compounds (Jonczyk et al, 2008) …”

“…In spite of the inability of F. graminearum to metabolize MBOA, the primary cause of Fusarium head blight worldwide, altering levels of different benzoxazolinones in wheat may be a route to increased disease resistance. However, such alterations should be tempered with other agricultural considerations such as persistence, toxicity and concentration of benzoxazinoids and metabolites in soil (Etzerodt et al, 2008). Of note, the wheat enzymes and respective genes responsible for methoxylation of benzoxazolinone precursor DIBOA-β-D-glucoside, which could be used to increase the abundance of MBOA, are yet to be identified (Sue et al, 2011).…”

Degradation of biochanin A in soil

“…All modelling was performed using the method of Etzerodt et al (2008), and in accordance with the Forum for the Coordination of Pesticide Fate Models and their Use (FOCUS) Committee guidelines (Boesten et al 2006).…”