Biodegradation of topramezone by a Trichoderma isolate in soil.

Abstract

Topramezone, a pyrazolone compound, has been introduced in many countries as a post-emergence herbicide. It inhibits 4-hydroxyphenylpyruvate dioxygenase (HPPD), a key enzyme in carotenoid pigment biosysnthesis, in susceptible plants, and can effectively control annual grasses and broadleaf weeds in maize (Zea mays L.), sweet corn (Zea mays convar. Saccharata var. rugosa) and popcorn (Zea mays var. everta). Topramezone is a highly persistent herbicide, which has high mobility in soil, posing a risk of leaching to ground and surface water. Despite its increasing use, not much is known about topramezone degradation and the potential impact of its persistence in the agricultural environment. We investigated the interaction between the herbicide and soil microorganisms in topramezone-treated soil, in order to test its bio-remediation potential particularly by soil fungi, and to elucidate the possible degradation pathways. One microbial strain, capable of transforming topramezone, was isolated from soils treated with the herbicide and identified as a species of Trichoderma, a well-known, common soil organism. The isolate survived in the minimal broth, incorporated with topramezone, at a concentration of 1000 mg/L of the medium. In sterilized soil, spiked with the herbicide, the Trichoderma isolate degraded 85% of the applied topramezone within 30 days of incubation, which is much faster than the reported, standard half-life of the herbicide (about 120 days). Based on the eight breakdown products (I to VIII), which were identified by liquid chromatography-mass spectroscopy (LC-MS) analyses, we propose that the herbicide was degraded by the fungus through various biochemical reactions, viz. demethylation, desulfonylation followed by hydroxylation of the herbicides, alkyl hydroxylation, hydrolysis of the carbonyl group of ketone, methoxylation, and hetero ring hydroxylation. Our results add to previous research that Trichoderma species and its strains are capable of degrading some pesticides, including herbicides in soil. The degradation products identified strongly imply the presence of a substrate recognition mechanism and a corresponding metabolic response system in the Trichoderma isolate, which can effectively degrade topramezone in the agricultural soil.