Conversions of Benzoxazinoids and Downstream Metabolites by Soil Microorganisms

Vadim Schütz,Margot Schulz,Sergey Girel,Dieter Sicker,Laurent Bigler,Laura Laschke

doi:10.3389/fevo.2019.00238

Abstract

Benzoxazinoids, secondary metabolites of several Poaceae, and some benzoxazinoid downstream metabolites are bioactive compounds that act as allelochemicals and natural pesticides. Since a short lifetime of the substances is crucial to avoid long-term environmental effects, total degradation by microorganisms is of exceptional importance. We performed a screening with cultivable microorganisms (Species names and strain numbers: Mycobacterium fortuitum, 7; Bacillus aryabhattai, 34; Bacillus cereus, 59; Bacillus megaterium, 21, 48; Bacillus methylotrophicus, 58; Lysinibacillus xylanilyticus, 56; Paenibacillus polymyxa, 51; Aminobacter aminovorans, 49; the fungi Papulaspora sepedonioides, 12 and Trichoderma viride, 47) isolated from soil previously used for wheat and Persian clover mixed-culture systems to assess their behavior in the presence of the compounds. The microorganisms were exposed to glucosylated benzoxazinones, the benzoxazinones HBOA, DIBOA and DIMBOA, the benzoxazolinones BOA, BOA-6-OH, and MBOA, and to several downstream products (AP, AAP, oHPMA, glucoside carbamate) in liquid culture to avoid interferences with soil minerals and other organisms. The microorganisms differed strongly in their metabolic activities in terms of growth, compound modification, and degradation. We observed degradation with DIBOA and GDIMBOA but rarely with DIMBOA, whereas BOA and MBOA showed almost no degradation when directly applied. Hydroxylation of BOA and demethylation of MBOA by the plant, resulting in BOA-6-OH, activated the benzoxazolinones for bacterial nitration. The resulting NBOA-6-OH was short-lived but could function temporarily as an allelochemical by inhibiting photosynthesis, e.g., in young seedlings of cress and kohlrabi. The BOA downstream products AP and oHPMA were converted to AAP, which can be nitrated to N-(2-OH-5-nitrophenyl)-acetamide and then degraded by A. aminovorans (49) and P. polymyxa (51). Only P. sepedonioides (12) and P. polymyxa (51) failed in the conversion of HBOA into AAP. While DIBOA, DIMBOA, MBOA, NBOA-6-OH, AP, AAP, and oHPMA reduced the growth of most microorganisms, glucoside carbamate promoted their growth. GDIMBOA had a stimulatory effect towards the fungi and three bacterial species. These findings lead to the hypothesis that in a natural habitat, such as the root surface, microorganisms may cooperate, perhaps by involving the plant, for the successful elimination of benzoxazinoids and their downstream metabolites.

Highlights

Benzoxazinones are bioactive secondary metabolites of several Poaceae, including the crops maize, rye, and wheat, as well as some dicotyledonous species
We performed a screening with cultivable microorganisms (Species names and strain numbers: Mycobacterium fortuitum, 7; Bacillus aryabhattai, 34; Bacillus cereus, 59; Bacillus megaterium, 21, 48; Bacillus methylotrophicus, 58; Lysinibacillus xylanilyticus, 56; Paenibacillus polymyxa, 51; Aminobacter aminovorans, 49; the fungi Papulaspora sepedonioides, 12 and Trichoderma viride, 47) isolated from soil previously used for wheat and Persian clover mixed-culture systems to assess their behavior in the presence of the compounds
The microorganisms were exposed to glucosylated benzoxazinones, the benzoxazinones HBOA, DIBOA, and DIMBOA, the benzoxazolinones BOA, BOA-6-OH, and MBOA, and to several downstream products (AP, AAP, oHPMA, glucoside carbamate) in liquid culture to avoid interferences with soil minerals and other organisms

Summary

Introduction

Benzoxazinones are bioactive secondary metabolites of several Poaceae, including the crops maize, rye, and wheat, as well as some dicotyledonous species. Benzoxazinones, for instance GDIBOA (2-(2,4-dihydroxy-1,4-benzoxazin3-one)-β-D-glucopyranose, 1) (Table S1) and GDIMBOA [(2R)-2-β -D-glucopyranosyloxy-4-hydroxy-7-methoxy-2H -1,4benzoxazin-3(4H)-one, 2], are unstable when deglucosylated. Their aglycones DIBOA (3) and DIMBOA (4) undergo heterocyclic ring contraction, releasing the benzoxazolinones BOA (5) and MBOA (6), which are more stable and, detectable for longer durations in the soil (Sicker and Schulz, 2002; Fomsgaard et al, 2004). Degradation of benzoxazinoids in soil involves most probably microorganisms. Since the degradation of phenolic compounds is influenced by soil minerals (Nannipieri et al, 2002), benzoxazinoids may be degraded or polymerized, perhaps partly, by abiotic processes or by extracellular enzymes adsorbed at soil minerals. In soil, biotic and abiotic degradation steps may be miscellaneous

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