Benzoxazolin-2(3H)-one (BOA) is an allelochemical most commonly associated with monocot species, formed from the O-glucoside of 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one by a two-step degradation process. The capacity of Arabidopsis to detoxify exogenously supplied BOA was analyzed by quantification of the major known metabolites BOA-6-OH, BOA-6-O-glucoside, and glucoside carbamate, revealing that detoxification occurs predominantly through O-glucosylation of the intermediate BOA-6-OH, most likely requiring the sequential action of as-yet-unidentified cytochrome P450 and UDP-glucosyltransferase activities. Transcriptional profiling experiments were also performed with Arabidopsis seedlings exposed to BOA concentrations, representing I 50 and I 80 levels based on root elongation inhibition assays. One of the largest functional categories observed for BOA-responsive genes corresponded to protein families known to participate in cell rescue and defense, with the majority of these genes potentially associated with chemical detoxification pathways. Further experiments using a subset of these genes revealed that many are also transcriptionally induced by a variety of structurally diverse xenobiotic compounds, suggesting they comprise components of a coordinately regulated, broad specificity xenobiotic defense response. The data significantly expand upon previous studies examining plant transcriptional responses to allelochemicals and other environmental toxins and provide novel insights into xenobiotic detoxification mechanisms in plants.

The allelopathic potency of rye (Secale cereale L.) is due mainly to the presence of phytotoxic benzoxazinones-compounds whose biosynthesis is developmentally regulated, with the highest accumulation in young tissue and a dependency on cultivar and environmental influences. Benzoxazinones can be released from residues of greenhouse-grown rye at levels between 12 and 20 kg/ha, with lower amounts exuded by living plants. In soil, benzoxazinones are subject to a cascade of transformation reactions, and levels in the range 0.5-5 kg/ha have been reported. Starting with the accumulation of less toxic benzoxazolinones, the transformation reactions in soil primarily lead to the production of phenoxazinones, acetamides, and malonamic acids. These reactions are associated with microbial activity in the soil. In addition to benzoxazinones, benzoxazolin-2(3H)-one (BOA) has been investigated for phytotoxic effects in weeds and crops. Exposure to BOA affects transcriptome, proteome, and metabolome patterns of the seedlings, inhibits germination and growth, and can induce death of sensitive species. Differences in the sensitivity of cultivars and ecotypes are due to different species-dependent strategies that have evolved to cope with BOA. These strategies include the rapid activation of detoxification reactions and extrusion of detoxified compounds. In contrast to sensitive ecotypes, tolerant ecotypes are less affected by exposure to BOA. Like the original compounds BOA and MBOA, all exuded detoxification products are converted to phenoxazinones, which can be degraded by several specialized fungi via the Fenton reaction. Because of their selectivity, specific activity, and presumably limited persistence in the soil, benzoxazinoids or rye residues are suitable means for weed control. In fact, rye is one of the best cool season cover crops and widely used because of its excellent weed suppressive potential. Breeding of benzoxazinoid resistant crops and of rye with high benzoxazinoid contents, as well as a better understanding of the soil persistence of phenoxazinones, of the weed resistance against benzoxazinoids, and of how allelopathic interactions are influenced by cultural practices, would provide the means to include allelopathic rye varieties in organic cropping systems for weed control.

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In conventional agriculture, weed control by herbicides is an expensive practice and can also have a negative effect on the environment. Allelopathy permits sustainable weed management while reducing the impact of agriculture on the environment. We studied the content of 2,4-dihydroxy-1,4 (2H)-benzoxazin-3-one (DIBOA) and benzoxazolin-2(3H)-one (BOA), indicated as benzoxazinoids and considered effective for weed control, in 8 cultivars of rye and 1 of triticale grown in a greenhouse. We also tested the ability of mulches to inhibit the germination of four warm-season weeds. Our results show that all rye cultivars produced DIBOA, while BOA was found only in four of them. Benzoxazinoids were absent in triticale. Total benzoxazinoid content ranged from 177 to 545 µg g −1 and was statistically different among cultivars. Rye mulches were not able to suppress velvetleaf and common lambsquarters seedlings, while redroot pigweed and common purslane were significantly affected. Weed suppression ranged from 40% to 52% for redroot pigweed and from 40% to 74% for common purslane. The inhibitory activity of triticale mulch was observed only for common purslane, with a suppression percentage of 33%. No correlation was found between total benzoxazinoid content and the number of weed seedlings suppressed, with R 2 of 0.076 for redroot pigweed and R 2 of 0.003 for common purslane, indicating that benzoxazinoids are not the only source of phytotoxicity. allelopathy / BOA / DIBOA / integrated weed control / mulch / rye / triticale

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