•Allelochemical tricin derived a benzothiazine derivative with HPPD-inhibiting activity.•Tricin and benzothiazine derivative inhibited the growth of penoxsulam-resistant barnyardgrass.•The benzothiazine derivative interfered with photosynthesis of resistant barnyardgrass.•The benzothiazine derivative may be as allelochemical-based bleaching herbicide discovery.Despite increasing knowledge of allelochemicals as leads for new herbicides, relatively little is known about the mode of action of allelochemical-based herbicides on herbicide-resistant weeds. In this study, herbicidal activities of a series of allelochemical tricin-derived compounds were evaluated. Subsequently, a benzothiazine derivative 3-(2-chloro-4-methanesulfonyl)-benzoyl-hydroxy-2-methyl-2H-1,2-benzothiazine-1,1-dioxide with 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) inhibiting activity was identified as a target compound on photosynthetic performance of penoxsulam-resistant versus -susceptible barnyardgrass (Echinochloa crus-galli). Regardless of barnyardgrass biotype, the benzothiazine derivative greatly affected chlorophyll fluorescence parameters (Fv/Fm, ETR1 min and NPQ1 min), reduced the chloroplast fluorescence levels and expression of HPPD gene. In particular, the benzothiazine derivative interfered with photosynthetic performance of resistant barnyardgrass more effectively than the allelochemical tricin itself. These results showed that the benzothiazine derivative effectively inhibited the growth of resistant barnyardgrass and its mode of action on photosynthesis system was similar to HPPD-inhibiting sulcotrione, making it an ideal lead compound for further development of allelochemical-based herbicide discovery.Download high-res image (135KB)Download full-size image

The action of allelochemicals in soil needs their presence in the vicinity of the target plants. Using a soil TLC combined with bioassay approach, the mobility of 10 typical allelochemicals was evaluated. Ferulic, p-hydroxymandelic, p-hydroxybenzoic, and vanillic acids always had the lowest mobility (Rf < 0.1), whereas phenolic aldehyde and lactone (vanillin and coumarin) showed the highest mobility (Rf > 0.5). The Rf values of daidzein, 1α-angeloyloxycarotol, DIMBOA, and m-tyrosine ranged from 0.24 to 0.32. Binary mixtures of these allelochemicals led to an increase in mobility factors for selected combinations. Phospholipid fatty acid profiling indicated that there were different soil microbial communities in the segments containing allelochemicals residues in the developed TLC soil layer. A difference in microbial community structure occurred between two nitrogenous DIMBOA and m-tyrosine and another eight allelochemicals. The results suggest that the soil activity of allelochemicals on bioassay species and microbial communities depends on their mobility in soil.

Three chemicals, veratric acid, maltol, and (−)-loliolide, were isolated from crabgrass and their structures were identified by spectroscopic analysis. The chemicals were detected in crabgrass root exudates and rhizosphere soils, and their concentrations ranged from 0.16 to 8.10 μg/g. At an approximate concentration determined in crabgrass root exudates, all chemicals significantly inhibited the growth of wheat, maize, and soybean and reduced soil microbial biomass carbon. Phospholipid fatty acid profiling showed that veratric acid, maltol, and (−)-loliolide affected the signature lipid biomarkers of soil bacteria, actinobacteria, and fungi, resulting in changes in soil microbial community structures. There were significant relationships between crop growth and soil microbes under the chemicals’ application. Chemical-specific changes in the soil microbial community generated negative feedback on crop growth. The results suggest that veratric acid, maltol, and (−)-loliolide released from crabgrass may act as allelochemicals interfering with crop growth and the soil microbial community.

Despite increasing knowledge of allantoin as a phytochemical involved in rice, relatively little is known about its distribution and function in rice grains. In this study, allantoin was quantified in 15 Chinese rice grains, and its contents varied with grain fraction, cultivar, and genotype. Bran always had the highest allantoin level, followed by brown rice and milled rice. Hull contained the lowest allantoin content. Allantoin in japonica bran ranged from 70 to 171 μg/g but rarely exceeded 100 μg/g in indica bran. There was a positive relationship between allantoin level in grains and seedling survival in seedbeds under low temperature or water deficit. Exogenous allantoin stimulated plant growth, increased soluble sugar and free proline contents, and decreased malondialdehyde content in rice seedlings. However, allantoin did not show any antioxidant activity through free radical-scavenging capacity, reducing power, linoleic acid peroxidation inhibition, and chelating activity. The results suggest that allantoin in rice grains may play some roles in providing plant stress protection but not serving as a beneficial health antioxidant.

The production of secondary metabolites and biomass of plants may be altered by coexistence and competition. Rice has coexisted with barnyardgrass in paddy systems for centuries and represents a model system for understanding plant–plant interactions. Despite increasing knowledge of allelochemicals produced and released by rice involved in coexistence of these two species, relatively little is known about the role and production of beneficial chemicals of rice in response to competing barnyardgrass. The purpose of this study was to quantify the growth-stimulating allantoin (5-ureidohydantoin) and its effect on barnyardgrass growth in a rice–barnyardgrass system. In this system, rice produced and released allantoin but barnyardgrass did not. The production of allantoin by rice responded to competition from barnyardgrass, and its concentration varied among rice cultivars tested. At 1:1 barnyardgrass and rice mixture proportion, allantoin concentration in allelopathic cultivars was obviously lowered by competing barnyardgrass, while no significant variation of allantoin concentration was observed in non-allelopathic cultivars. Barnyardgrass biomass was reduced in mixed-culture with rice cultivars, in particular, allelopathic cultivars. There was a positive relationship between allantoin concentration in rice cultivars and barnyardgrass biomass. Furthermore, allantoin stimulated the growth of barnyardgrass once released from rice or added to soil. The results suggest that rice plants of allelopathic cultivars appear to be able to detect the presence of competing barnyardgrass and respond by decreasing production of growth-stimulating allantoin, regulating the growth of barnyardgrass. In this manner, given rice cultivars may alleviate the competition of barnyardgrass and provide greater benefit to their own growth.

Despite increasing knowledge of jasmonic acid (JA) and salicylic acid (SA) as signaling compounds involved in the defense of rice against attacking microbes and insect predators, relatively little is known about their levels in the growth media and their interactions with other plant competitors. In present study we quantified JA and SA in a rice-barnyardgrass coexistence system followed by correlation analysis to access rice allelochemicals. Both rice and barnyardgrass biosynthesized JA and SA, but their contents varied greatly with species, tissues and coexistence. There was a positive correlation in contents between rice allelochemicals and JA in roots or SA in shoots. Endogenous JA was exuded from barnyardgrass roots eliciting the production of rice allelochemicals. SA was not detected in growth media as an exogenous signaling compound in a rice-barnyardgrass coexistence system, but SA content in rice shoots was an indicator for distinguishing the allelopathic rice traits from the non-allelopathic ones.Highlights► JA and SA are quantified in a rice-barnyardgrass coexistence system. ► There is a positive correlation in level between rice allelochemicals and JA or SA. ► Elicitation of rice allelochemicals production by root-exuding and applying JA. ► Endogenous SA in rice shoots linking to allelopathic traits in rice varieties.

Despite increasing knowledge of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and 6-methoxy-benzoxazolin-2-one (MBOA) as allelochemicals involved in the defense of wheat against pests, relatively little is known about their levels in the rhizosphere and interactions with the soil microbial community. This study quantified DIMBOA and MBOA in the wheat rhizosphere and analyzed the soil microbial community structure. MBOA rather than DIMBAO was found in the wheat rhizosphere, and its concentration varied with cultivars, plant densities, and growth conditions. Wheat could detect the presence of competing weeds and respond by increased MBOA in the rhizosphere. There was a linear positive relationship between the MBOA level in the wheat rhizosphere and soil fungi/bacteria. When DIMBOA was applied to soil, yielding MBOA increased soil fungi. There were different phospholipid fatty acid (PLFA) patterns in soil incubated with DIMBOA and MBOA. These results suggested that DIMBOA and MBOA could affect the soil microbial community structure to their advantage through the change in fungi populations.

The establishment and productivity of a Manchurian walnut (Juglans mandshurica Maxim.) plantation can be improved by inter-planting with larch (Larix gmelini Rupr.) in Northeast China, but the potential mechanism remains obscure. We carried out a series of experiments in a 20-year-old mixed-species plantation, as well as in Manchurian walnut and larch plantations. Manchurian walnut seedlings had difficulty surviving in the Manchurian walnut plantation because their growth was inhibited by their own soil and root exudates. In sharp contrast, Manchurian walnut seedlings grew well in larch and mixed-species plantations. Larch soil and root exudates greatly stimulated the growth of Manchurian walnut seedlings in controlled conditions. In particular, larch root exudates can increase the soil microbial populations, including bacteria, actinomycetes, azotobacter and cellulose-decomposing microorganisms; larch root exudates can also increase the enzyme activities of saccharase, urease, proteinase and polyphenol oxidase. Significant results led to a rapid degradation of the root-exuded phytotoxic juglone from Manchurian walnut. Manchurian walnut root exudates contained juglone at a high concentration of 121.3 ± 6.6 mg g−1, while juglone concentrations in the soil beneath Manchurian walnut trees ranged from 2.9–6.2 µg g−1 soil. It appears from the results that juglone may be released from Manchurian walnut roots into the soil in a sufficient quantity but rapidly degrades due to interactions with soil factors. Furthermore, juglone was more resistant toward degradation in the Manchurian walnut soil (t1/2 = 7.36 ± 0.63 h) when compared to the larch soil (t1/2 = 4.66 ± 0.82 h). The results suggest that larch may improve the establishment and productivity of Manchurian walnut in a mixed-species plantation through the release of root exudates.

Despite an increase in the understanding of the soybean isoflavones involved in root-colonizing symbioses, relatively little is known about their levels in the rhizosphere and their interactions with the soil microbial community. Based on a 13-year experiment of continuous soybean monocultures, in the present study we quantified isoflavones in the soybean rhizosphere and analyzed the soil microbial community structure by examining its phospholipid fatty acid (PLFA) profile. Two isoflavones, daidzein (7, 4′-dihydroxyisoflavone) and genistein (5,7,4′- trihydroxyisoflavone), were detected in the rhizosphere soil of soybean plants, with the concentrations in the field varying with duration of mono-cropping. Genistein concentrations ranged from 0.4 to 1.2 μg g−1 dry soil over different years, while daidzein concentrations rarely exceeded 0.6 μg g−1 dry soil. PLFA profiling showed that the signature lipid biomarkers of bacteria and fungi varied throughout the years of the study, particularly in mono-cropping year 2, and mono-cropping years 6–8. Principal component analysis clearly identified differences in the composition of PLFA during different years under mono-cropping. There was a positive correlation between the daidzein concentrations and soil fungi, whereas the genistein concentration showed a correlation with the total PLFA, fungi, bacteria, Gram (+) bacteria and aerobic bacteria in the soil microbial community. Both isoflavones were easily degraded in soil, resulting in short half-lives. Concentrations as small as 1 μg g−1 dry soil were sufficient to elicit changes in microbial community structure. A discriminant analysis of PLFA patterns showed that changes in microbial community structures were induced by both the addition of daidzein or genistein and incubation time. We conclude that daidzein and genistein released into the soybean rhizosphere may act as allelochemicals in the interactions between root and soil microbial community in a long-term mono-cropped soybean field.Highlights► Isoflavones (daidzein and genistein) are quantified in the soybean rhizosphere. ► Rhizosphere isoflavones and soil microbes vary with period of soybean mono-cropping. ► There is a positive correlation between isoflavones levels and soil fungi. ► Isoflavones participate in the interactions between soybean root and soil microbes.