•The allelochemicals of Glycyrrhiza uralensis and their allelopathic effects were evaluated.•The extracts exhibited phytotoxicity on Lactuca sativa and autotoxicity on tabacco.•Liquiritin, isoliquiritigeninand glycyrrhizic acid were the major allelochemicals.•Three allelochemicals showed observably phytotoxicity and autotoxicity.•Their concentrations in dry in soils were 2.32, 0.09 and 12.5 μg/g, respectively.Autotoxicity is considered as a major factor causing replant problems. As one of the most frequently used herbs in traditional Chinese medicine and industrial crop, Glycyrrhiza uralensis Fisch. is cultivated extensively in north of China, but it has often been hampered by replant failure. In light of the serious threat of replant failure, the allelochemicals from Glycyrrhiza uralensis Fisch. and their allelopathic effects were investigated. Extract of Glycyrrhiza uralensis Fisch. rhizosphere soil showed phytotoxic activity against Lactuca sativa and autotoxic activity on its own seedlings. Six compounds were isolated from the extract and elucidated by spectroscopic analysis. Among them, liquiritin (1), isoliquiritigenin (2) and glycyrrhizic acid (4) showed observably phytotoxicity and autotoxicity. They were further verified and quantified in the rhizosphere soils of Glycyrrhiza uralensis Fisch. by high-performance liquid chromatography (HPLC), and their concentrations in dry soils were 2.32, 0.09 and 12.5 μg/g, respectively. These allelochemicals, showed remarkably phytotoxic and autotoxic activity, may play an important role in the replant failure of Glycyrrhiza uralensis Fisch.

Astragalus hoantchy, a widely cultivated medicinal plant species in traditional Chinese and Mongolian medicine, has been often hampered by replant failure during cultivation, like many other herbs of the genus Astragalus. Root aqueous extracts of Astragalus herbs were reported to exhibit allelopathic activity against other plants and autotoxic activity on their own seedlings, but the allelochemicals released by Astragalus plants have not been specified so far. Ten compounds were isolated from the rhizosphere soil extract of cultivated A. hoantchy and elucidated by spectroscopic analysis. Compounds 1–6 observably showed allelopathic activity against Lactuca sativa seedlings and autotoxic activity against A. hoantchy seedlings. The isolated compounds were further confirmed and quantified by high-performance liquid chromatography (HPLC) in the rhizosphere soil, with a total concentration of 9.78 μg/g (dry weight). These results specify and verify the allelochemicals released by cultivated A. hoantchy into the soil environment, which may provide new insights into the allelopathic mechanisms of this medicinal plant and probably assist in clarifying the replant problems of Astragalus plants.

The presence of hydroxyl groups at the C4 and C7 positions in coumarin backbone has been proposed as a potential modification site for providing excellent bioactivity according to previous studies. A series of novel coumarin derivatives were rationally designed and synthesized by use of a complex catalytic system for a targeted modification at the above sites. These derivatives were assayed for nematicidal activity. As predicted, the derivatization enhanced the activity of the coumarins against five nematodes. Compounds 7b, 9a, 10c and 11c showed significant strong nematicidal broad spectrum activity against all tested nematodes. Compound 10c was the most effective with the lowest LC50 values against Meloidogyne incognita (5.1 μmol/L), Ditylenchus destructor (3.7 μmol/L), Bursaphelenchus mucronatus (6.4 μmol/L), Bursaphelenchus B. xylophilus (2.5 μmol/L) and Aphelenchoides besseyi (3.1 μmol/L), respectively. A brief investigation on the structure–activity relationships (SAR) revealed that the targeted modification by a C7 hydroxyl was optimum compared with that of a C4 hydroxyl and that the coupling chain length was crucial for the nematicidal activity.A series of novel coumarin analogs were effectively synthesized by a targeted modification at C4 and C7 hydroxyl groups and their nematicidal activity was investigated to discover promising compounds. As predicted, the derivatization enhanced the activity of the coumarins against five phytonematodes. The investigation on the structure–activity relationships (SAR) has provided useful guidelines for the development of coumarin based nematicides.

Stellera chamaejasme, a perennial weed which is an ecological threat, is widely distributed in some grasslands of Central and Eastern Asia. Our previous studies have identified several allelochemicals including two coumarins (umbelliferone and daphnoretin), from S. chamaejasme, and confirmed that allelopathy contributed to the competitive behavior of this weed. In this study, the inhibitory effects of umbelliferone and daphnoretin on lettuce seedlings and the mechanisms of their phytotoxicity were investigated. Results showed that shoot and root elongation and fresh weight of lettuce seedlings were effectively inhibited by umbelliferone in a concentration-dependent manner. Daphnoretin showed a weaker phytotoxicity. Both of the coumarins arrested the mitosis process in lettuce root tips and induced proline overproduction. Additionally, loss of cell viability and overproduction of reactive oxygen species in lettuce root cells were found after treatments with umbelliferone. Moreover, umbelliferone caused lipid peroxidation. These results suggested that umbelliferone displayed stronger phytotoxicity than daphnoretin on lettuce growth, and that the two coumarins had different mechanisms of phytotoxicity. That of daphnoretin was mainly dependent on its inhibitory effects on mitosis. Umbelliferone caused membrane lipid peroxide formation and cell death by inducing ROS overproduction, and impacted cell division, which resulted in growth inhibition of the receptor plant.

Autotoxicity, defined as a deleterious allelopathic effect among individuals of the same plant species, is considered as one of the factors that contributes to replant failure. Tobacco, as an important cultured and economic crop over the world, has been often hampered by replant failure. In view of the seriousness of this problem, the allelochemicals of flue-cured tobacco and their allelopathic effects were investigated. The extracts of rhizosphere soil exhibited phytotoxic activities against Lactuca sativa and autotoxic activities against tobacco itself. Bioassay-guided fractionation of the extract led to the isolation of six compounds, the structures of which were elucidated by spectroscopic analysis. Among them, β-cembrenediol (1), di-n-hexyl phthalate (2), and bis(2-propylheptyl) phthalate (3) showed observably phytotoxic activities against L. sativa seedlings and autotoxic activities on tobacco. The allelochemicals were then verified in the root zone soils of flue-cured tobacco by high-performance liquid chromatography (HPLC). These results provide new insights into the allelopathic mechanisms involved in the replant failure of flue-cured tobacco.

Gravitropism is affected by many exogenous factors such as environmental stresses. In the present study, the effect of ellagic acid (EA) on the gravitropic curvature of Arabidopsis thaliana (L.) Heynh. roots was characterized. Exogenous application of EA influenced root gravitropic responses in a concentration-dependent fashion. KI/I staining research revealed that the starch grain in the root cap was reduced by EA treatments. Simultaneously, expression of auxin-responsive reporter gene DR5::GFP showed that the auxin redistribution in roots treated with EA was less sensitive to that of control groups under gravity stimulus. Moreover, the expression of one of the auxin flux-facilitators—PIN2, was also suppressed by EA. Taken together, our results indicated that the effects of EA on root gravitropism were largely dependent on the reduction of starch grain and the disorder of lateral auxin redistribution.

Populations of Stellera chamaejasme L. have been increasing constantly in recent years in some areas of the grassland in north China but why this toxic weed has become highly competitive is not clear. In order to determine if any potential allelochemicals are released into the soil environment by S. chamaejasme, we investigated the chemical composition of a water-washed solution of the living roots with rhizosphere soil. This led to the isolation and identification of seven compounds: umbelliferone (1), daphnoretin (2), chamaechromone (3), 7-methoxyneochamaejasmine A (4), mesoneochamaejasmin A (5), neochamaejasmin B (6), dihydrodaphnodorin B (7). All are secondary metabolites of S. chamaejasme. Bioassay showed that 1, 5 and 6 had a strong inhibitory effect on Festuca rubra L. and Medicago sativa seedlings. These compounds were quantified by high performance liquid chromatography in 25 root zone soil samples of S. chamaejasme collected at altitudes between 165 and 4741 m from the northeast to the Tibetan Plateau of China. All samples contained at least one of the phytotoxic compounds. Their content did not correlate with the altitude of the growing site. However, the level of chamaechromone negatively correlated with the soil pH. Principle components analysis indicated that the flavonoids might come from the same source. These potential allelochemicals from root release into the soil might play an important role in the highly competitive nature and broad ecological adaptability of S. chamaejasme in the wild.

The fungal endophytes associated with medicinal plants have been demonstrated as a reservoir with novel natural products useful in medicine and agriculture. It is desirable to explore the species composition, diversity and tissue specificity of endophytic fungi that inhabit in different tissues of medicinal plants. In this study, a culture-independent survey of fungal diversity in the rhizosphere, leaves, stems and roots of a toxic medicinal plant, Stellera chamaejasme L., was conducted by sequence analysis of clone libraries of the partial internal transcribed spacer region. Altogether, 145 fungal OTUs (operational taxonomic units), represented by 464 sequences, were found in four samples, of these 109 OTUs (75.2 %) belonging to Ascomycota, 20 (13.8 %) to Basidiomycota, 14 (9.7 %) to Zygomycota, 1 (0.7 %) to Chytridiomycota, and 1 (0.7 %) to Glomeromycota. The richness and diversity of fungal communities were strongly influenced by plant tissue environments, and the roots are associated with a surprisingly rich endophyte community. The endophyte assemblages associated with S. chamaejasme were strongly shaped by plant tissue environments, and exhibited a certain degree of tissue specificity. Our results suggested that a wide variety of fungal assemblages inhabit in S. chamaejasme, and plant tissue environments conspicuously influence endophyte community structure.

Weed infestation has been known to cause considerable reductions in crop yields, thereby hindering sustainable agriculture. Many plants in genus Euphorbia affect neighboring plants and other organisms by releasing chemicals into the environment. In view of the serious threat of weeds to agriculture, the allelochemicals of Euphorbia himalayensis and their allelopathic effects were investigated. The extract of root exudates from rhizosphere soil exhibited allelopathic activities against crops (wheat, rape, and lettuce) and grasses (Poa annua, Festuca rubra, and red clover). Bioassay-guided fractionation and isolation from the root extract of E. himalayensis led to the characterization of two ellagic acid derivatives and a jatrophane diterpene, which observably showed phytotoxic activities against lettuce, Festuca arundinacea, and F. rubra. They were further confirmed by ultra-performance liquid chromatography–tandem mass spectrometry to have concentrations of 3.6, 3.8, and 8.99 nmol/g in the rhizospere soil, respectively. Bioassay indicated that the combination of the allelochemicals could be selective plant growth regulator in agriculture.

•Eight flavonoids were isolated from roots of Stellera chamaejasme.•Six flavonoids showed strong phytotoxicity on Arabidopsis thaliana.•Influence of endogenous auxin distribution by the phytotoxins in A. thaliana was analyzed.•Four flavonoids were found in surrounding soils of roots of S. chamaejasme.Allelopathy, the negative effect on plants of chemicals released to the surroundings by a neighboring plant, is an important factor which contributes to the spread of some weeds in plant communities. In this field, Stellera chamaejasme L. (Thymelaeaceae) is one of the most toxic and ecologically-threatening weeds in some of the grasslands of north and west China. Bioassay-guided fractionation of root extracts of this plant led to the isolation of eight flavonoids 1–8, whose structures were elucidated by spectroscopic analysis. All compounds obtained, except 7-methoxylneochaejasmin A (4) and (+)-epiafzelechin (5), showed strong phytotoxic activity against Arabidopsis thaliana seedlings. Seedling growth was reduced by neochamaejasmin B (1), mesoneochamaejasmin A (2), chamaejasmenin C (3), genkwanol A (6), daphnodorin B (7) and dihydrodaphnodorin B (8) with IC50 values of 6.9, 12.1, 43.2, 74.8, 7.1 and 27.3 μg/mL, respectively, and all of these compounds disrupted root development. Endogenous auxin levels at the root tips of the A. thaliana DR5::GUS transgenic line were largely reduced by compounds 1, 2 and 6–8, and were increased by compound 4. Moreover, the inhibition rate of A. thaliana auxin transport mutants pin2 and aux1–7 by compounds 1–8 were all lower than the wild type (Col-0). The influence of these compounds on endogenous auxin distribution is thus proposed as a critical factor for the phytotoxic effect. Compounds 1, 2, 4 and 8 were found in soils associated with S. chamaejasme, and these flavonoids also showed phytotoxicity to Clinelymus nutans L., an associated weed of S. chamaejasme. These results indicated that some phytotoxic compounds from roots of S. chamaejasme may be involved in the potential allelopathic behavior of this widespread weed.Isolation and structural identification of eight flavonoids from roots of S. chamaejasme and their phytotoxic activities and influences of auxin on Arabidopsis thaliana seedlings are described. Some flavonoids may act as allelochemicals because they were found in the surrounding soils of S. chamaejasme.

Trifolium pratense, a widespread legume forage plant, is reported to exhibit phytotoxic activity on other plants, but the active metabolites have not been clarified so far. A bioassay-guided fractionation of the root extracts led to the isolation of five isoflavonoids, which were elucidated by spectroscopic analysis. All of the purified compounds observably showed phytotoxic activities against Arabidopsis thaliana. Moreover, the inhibitory effects were concentration-dependent. The furan ring linked at C-4 and C-2′ positions by an oxygen atom and a 1,3-dioxolane at C-4′ and C-5′ positions are considered to be critical factors for the phytotoxic activity. The concentrations of (6aR,11aR)-maackiain and (6aR,11aR)-trifolirhizin, concluded to be allelochemicals from soil around plants of T. pratense, were determined by HPLC and LC-MS to be 4.12 and 2.37 μg/g, respectively. These allelochemicals, which showed remarkable activities against the weed Poa annua may play an important role in assisting the widespread occurrence of T. pratense in nature.

This study was conducted to explore fungal endophyte communities inhabiting a toxic weed (Stellera chamaejasme L.) from meadows of northwestern China. The effects of plant tissue and growth stage on endophyte assemblages were characterized. Endophytes were recovered from 50 % of the samples, with a total of 714 isolates. 41 operational taxonomical units (OTUs) were identified, consisting of 40 OTUs belonging primarily to Ascomycota and 1 OTU belonging to Basidiomycota. Pleosporales and Hypocreales were the orders contributing the most species to the endophytic assemblages. The total colonization frequency and species richness of endophytic fungi were higher in roots than in leaves and stems. In addition, for the plant tissues, the structure of fungal communities differed significantly by growth stages of leaf emergence and dormancy; for the plant growth stages, the structure of fungal communities differed significantly by plant tissues. This study demonstrates that S. chamaejasme serves as a reservoir for a wide variety of fungal endophytes that can be isolated from various plant tissues.

Four new flavonoids, astragaisoflavans A–D (1–4), along with thirteen known ones (5–17) were isolated from the aerial parts of Astragalus hoantchy. Their structures were established by the extensive spectroscopic analyses including one- and two-dimensional NMR, HRESIMS, IR, and UV results. The absolute configurations of 1–4 were deduced based on circular dichroism. Compounds 1–3 all possessed a rare modified A-ring and compound 4 was a dimeric isoflavan. Compound 5 was first identified from genus Astragalus and 6–17 were found from A. hoantchy for the first time. Compound 1 and 4 were evaluated for their antifungal activity against Alternaria solani (a phytopathogenic fungus), with the result that 1 failed to exhibit significant antifungal activity at the testing concentration of 100 μg/mL and the IC50 value of 4 was 173.3 μg/mL.Download high-res image (422KB)Download full-size image

•Natural compound derived fungicides discovery.•Targeted modification and synthesis of umbelliferone.•Enhanced antifungal activity with low phytotoxicity.•Providing meaningful tactics for the further development of umbelliferone-related fungicides.Umbelliferone was an important allelochemical with a wide spectrum bioactivity. In our previous study, C7 hydroxy in the backbone of umbelliferone was identified to be responsible for its phytotoxicity and the targeted modification of the above site could lead to the phytotoxicity loss. In view of this, a series of hydroxycoumarins and C7 O-substituted umbelliferone derivatives were efficiently synthesized to evaluate their antifungal activity against four phytopathogenic fungi. Most of them, as we predicted, exhibited improved fungicidal activity. The phytotoxicity of effective compounds was also assayed by Lactuca sativa to investigate their side effects on plant growth. Compounds 9 and 17 were identified to show strong antifungal activity with low phytotoxicity. A brief investigation on structure-activity relationships revealed that the modification at the C7 hydroxy of umbelliferone could be a promising way to enhance the antifungal activity with decreasing the phytotoxicity.Download high-res image (200KB)Download full-size image