Quang Le Dang

Nematicidal and antifungal activities of annonaceous acetogenins from Annona squamosa against various plant pathogens.

The methanol extract of Annona squamosa seeds was highly active against two phytoparasitic nematodes, Bursaphelenchus xylophilus and Meloidogyne incognita. It efficiently suppressed plant diseases, caused by Phytophthora infestans and Puccinia recondita. Ten annonaceous acetogenins (AAs) were isolated, and their chemical structures were identified by mass and nuclear magnetic resonance spectral data. Out of 10 substances, eight displayed strong in vitro nematicidal activity against B. xylophilus with LD(50) values ranging 0.006 to 0.048 μg/mL. Squamocin-G showed potent nematicidal activity against M. incognita. Squamocin, squamocin-G, and squamostatin-A also displayed potent in vitro and in vivo antifungal activities against P. infestans causing tomato late blight. In addition, squamostatin-A effectively controlled the development of wheat leaf rust caused by P. recondita. Our findings suggested that A. squamosa seeds and its bioactive AAs can be an alternative resource of a promising botanical nematicide and fungicide to control various plant diseases.

Antimicrobial activities of novel mannosyl lipids isolated from the biocontrol fungus Simplicillium lamellicola BCP against phytopathogenic bacteria.

The antagonistic fungus Simplicillium lamellicola BCP has been developed as a microbial biopesticide that effectively controls the development of various plant diseases caused by both pathogenic bacteria and pathogenic fungi. Antibacterial bioassay-directed fractionation was used to isolate mannosyl lipids from S. lamellicola BCP, and the structures of these compounds were elucidated using spectral analysis and chemical degradation. Three novel mannosyl lipids were characterized and identified as halymecins F and G and (3R,5R)-3-O-β-D-mannosyl-3,5-dihydrodecanoic acid. Massoia lactone and (3R, 5R)-3-hydroxydecan-5-olide were also isolated from S. lamellicola BCP. The three novel compounds inhibited the growth of the majority of phytopathogenic bacteria that were tested, and halymecin F displayed the strongest antibacterial activity. Agrobacterium tumefaciens was the most sensitive to the three novel compounds, with IC₅₀ values ranging from 1.58 to 24.8 μg/mL. The ethyl acetate extract of the fermentation broth from the antagonistic fungus effectively reduced the bacterial wilt caused by Ralstonia solanacearum on tomato seedlings. These results indicate that S. lamellicola BCP suppresses the development of plant bacterial diseases through the production of antibacterial metabolites.

Pyochelin isolated from Burkholderia arboris KRICT1 carried by pine wood nematodes exhibits phytotoxicity in pine callus

Pine wilt disease is a very complex disease known to be caused by the pine wood nematode, Bursaphelenchus xylophilus , and its accompanying bacteria. The phytotoxin-producing bacteria have been reported to be involved in the development of pine wilt disease. In this study, we attempted to characterise phytotoxins produced by Burkholderia arboris KRICT1, which was carried by the pine wood nematode. It produced phytotoxic compounds in the pine seedling assay, and the ethyl acetate (EtOAc) layer of the fermentation broth of the strain displayed phytotoxic activity on the pine callus of Pinus densiflora. One active compound was isolated from the EtOAc layer by repeated Sephadex LH-20 column chromatography and preparative TLC. Using mainly mass spectroscopy and nuclear magnetic resonance (NMR) spectroscopy, the chemical structure was determined to be that of pyochelin (a mixture of two interconvertible diastereoisomers, pyochelin I and II). Pyochelin exposure reduced the viability of pine callus. Compared with phenylacetic acid, a phytotoxin produced by Bacillus spp. that was transmitted by B. xylophilus in Japan, pyochelin showed much stronger phytotoxicity. The results suggested that pyochelin might play a role in the wilting process of pine wilt disease.

Antiviral peptide from Pseudomonas chlororaphis O6 against tobacco mosaic virus (TMV)

Although Pseudomonas chlororaphis O6 (O6) is known to be a rhizobacterium capable of inducing systemic resistance against plant virus, its antiviral products from O6 remain unknown. In the present study, an antiviral cyclic peptide was identified from the cell-free supernatant of O6. O6 cultures grown on Luria Bertani medium were centrifuged, and the resulting supernatant was extracted with organic solvent, followed by a series of column chromatography and preparative high performance liquid chromatography (HPLC). Bioassay-guided fractionations were involved in the isolation of antiviral products against tobacco mosaic virus (TMV). Time of flight mass spectrometry (TOF-MS) analysis of the isolated product detected (M+H)+ peak at m/z 887.4242 that generated m/z 756.3859, 657.3180, 556.2724, 459.2208, 345.1873, and 171.1130 as the main fragment ions. NMR analyses characterized all protons and carbons of the isolated product. Based on the data, the isolated antiviral product was determined to be a cyclic peptide with molecular formula C39H67N9O12S consisting of seven different amino acids. The antiviral peptide exhibited more than 95% disease suppression of TMV at 1,000 μg/mL. O6 may play a role in promoting plant growth.

Nematicidal activities of 4-quinolone alkaloids isolated from the aerial part of Triumfetta grandidens against Meloidogyne incognita.

The methanol extract of the aerial part of Triumfetta grandidens (Tiliaceae) was highly active against Meloidogyne incognita, with second-stage juveniles (J2s) mortality of 100% at 500 μg/mL at 48 h post-exposure. Two 4-quinolone alkaloids, waltherione E (1), a new alkaloid, and waltherione A (2), were isolated and identified as nematicidal compounds through bioassay-guided fractionation and instrumental analysis. The nematicidal activities of the isolated compounds against M. incognita were evaluated on the basis of mortality and effect on egg hatching. Compounds 1 and 2 exhibited high mortalities against J2s of M. incognita, with EC50 values of 0.09 and 0.27 μg/mL at 48 h, respectively. Compounds 1 and 2 also exhibited a considerable inhibitory effect on egg hatching, which inhibited 91.9 and 87.4% of egg hatching, respectively, after 7 days of exposure at a concentration of 1.25 μg/mL. The biological activities of the two 4-quinolone alkaloids were comparable to those of abamectin. In addition, pot experiments using the crude extract of the aerial part of T. grandidens showed that it completely suppressed the formation of gall on roots of plants at a concentration of 1000 μg/mL. These results suggest that T. grandidens and its bioactive 4-quinolone alkaloids can be used as a potent botanical nematicide in organic agriculture.

Antifungal activity of sterols and dipsacus saponins isolated from Dipsacus asper roots against phytopathogenic fungi

The in vivo antifungal activity of crude extracts of Dipsacus asper roots was evaluated against the phytopathogenic fungi Botrytis cinerea, Colletotrichum coccodes, Blumeria graminis f. sp. hordei, Magnaporthe grisea, Phytophthora infestans, Puccinia recondita and Rhizoctonia solani using a whole-plant assay method. Ethyl acetate and acetone extracts, at 1000μg/mL, suppressed the development of tomato gray mold (TGM) and tomato late blight (TLB) by 90%. Through bioassay-guided isolation, five antifungal substances were isolated from the D. asper roots and identified as β-sitosterol (1), campesterol (2), stigmasterol (3), cauloside A (4) and a novel dipsacus saponin, named colchiside (3-O-β-d-xylopyranosyl-23-O-β-d-glucopyranosyl-28-O-β-d-(6-O-acetyl)-glucopyranosyl hederagenin) (5). Of those, cauloside A (4) displayed the greatest antifungal efficacy against rice blast, TGM and TLB. Colchiside (5) moderately suppressed the development of TLB, but exhibited little effect against the other diseases. The synergistic effects of the isolated compounds against TLB were also assessed. Synergistic and additive interactions were observed between several of the sterol compounds. This study indicated that the crude extracts of, and bioactive substances from, the roots of D. asper suppress TGM and TLB. In addition, cauloside A (4) and colchiside (5) could be used as antifungal lead compounds.

Nematicidal activity against Bursaphelenchus xylophilus of isoobtusilactone A isolated from Persea americana

Methanol extracts of 50 plant species were assessed for nematicidal activity against the pine wood nematode (PWN), Bursaphelenchus xylophilus . Out of these, five extracts: Justicia diclipteroides subsp. usambarica ; Nerium oleander ; Persea americana ; Scadoxus multiflorus subsp. multiflorus ; and Thespesia populnea , caused 96-100% mortalities of pine wood nematode at 1000 μ g ml –1 for 48 h. Methanol extract of the stem bark of P. americana (avocado) and the n -hexane layer obtained from the methanol extract by solvent partitioning caused 100% mortality of PWN at low concentrations of 125 and 63 μ g ml –1 , respectively. One potent nematicidal compound was isolated from the stem bark of P. americana and its chemical structure was determined to be isoobtusilactone A ( 1 ) by mass and NMR spectral analyses. The chemical showed a very strong nematicidal activity against PWN; it caused mortalities of over 97% at concentrations higher than 50 μ g ml –1 . Compared with a commercial synthetic nematicide, morantel tartrate, compound 1 showed a little more potent activity. The results suggested that compound 1 may have the potential to be explored as a natural nematicide or be useful as a lead molecule for development of new nematicidal agents for controlling the pine wilt disease caused by B. xylophilus .

In vivo assessment of plant extracts for control of plant diseases: A sesquiterpene ketolactone isolated from Curcuma zedoaria suppresses wheat leaf rust

ABSTRACT As an alternative to synthetic pesticides, natural materials such as plant extracts and microbes have been considered to control plant diseases. In this study, methanol extracts of 120 plants were explored for in vivo antifungal activity against Rhizoctonia solani, Botrytis cinerea, Phytophthora infestans, Puccinia triticina, and Blumeria graminis f. sp. hordei. Of the 120 plant extracts, eight plant extracts exhibited a disease control efficacy of more than 90% against at least one of five plant diseases. In particular, a methanol extract of Curcuma zedoaria rhizomes exhibited strong activity against wheat leaf rust caused by P. triticina. When the C. zedoaria methanol extracts were partitioned with various solvents, the layers of n-hexane, methylene chloride, and ethyl acetate showed disease control values of 100, 80, and 43%, respectively, against wheat leaf rust. From the C. zedoaria rhizome extracts, an antifungal substance was isolated and identified as a sesquiterpene ketolactone based on the mass and nuclear magnetic resonance spectral data. The active compound controlled the development of rice sheath blight, wheat leaf rust, and tomato late blight. Considering the in vivo antifungal activities of the sesquiterpene ketolactone and the C. zedoaria extracts, these results suggest that C. zedoaria can be used as a potent fungicide in organic agriculture.

Current Status of Botanical Pesticides for Crop Protection

The problems caused by synthetic pesticides have led the need for effective biodegradable pesticides with greater selectivity. Botanical pesticides are generally recognized as safe in agriculture systems. Thus, they have been regarded as attractive alternatives to synthetic chemical pesticides for the pest management. Both lower efficacy and higher costs of production make botanicals more expensive to use than conventional pesticides. Moreover, only a small portion of plant-derived metabolites among a number of bioactive metabolites are in use because commercialization of botanicals is inhibited by several problems such as toxicity, or high production cost. However, with the growing acceptance of botanical pesticides as an efficient crop protection alternative resulting in increasing demand, plant-based pesticides will play a significant role in achieving sustainable agriculture in future.

Identification of novel compounds, oleanane- and ursane-type triterpene glycosides, from Trevesia palmata: their biocontrol activity against phytopathogenic fungi

Plants contain a number of bioactive compounds that exhibit antimicrobial activity, which can be recognized as an important source of agrochemicals for plant disease control. As part of our search for new antimicrobial agents from natural sources, we found that a crude methanol extract of Trevesia palmata exhibited a promising antifungal activity against phytopathogenic fungi, such as Magnaporthe oryzae and Botrytis cinerea. Furthermore, based on activity-guided fractionation, we isolated five antifungal compounds from the methanol extract of T. palmata: two new triterpene glycosides (TPGs), TPG1 (hederagenin-3-O-β-D-glucopyranosyl-(1  → 3)-α-L-rhamnopyranosyl-(1 → 2)-α-L-rhamnopyranosyl-(1 → 2)-α-L-arabinopyranoside) and TPG5 (3-O-α-L-rhamnopyranosyl asiatic acid), along with three known TPGs (TPG2 [macranthoside A], TPG3 [α-hederin], and TPG4 [ilekudinoside D]). The chemical structures of the TPGs were determined by spectroscopic analyses and by comparison with literature data. An in vitro antifungal bioassay revealed that except for TPG4 (ilekudinoside D; IC50 >256 μg/ml), the other TPGs exhibited strong antifungal activities against the rice blast pathogen M. oryzae with IC50 values ranging from 2–5 μg/ml. In particular, when the plants were treated with compound TPG1 (500 μg/ml), disease control values against rice blast, tomato grey mold, tomato late blight, and wheat leaf rust were 84, 82, 88, and 70%, respectively, compared to the non-treatment control. Considering the in vitro and in vivo antifungal activities of the TPGs and the T. palmata methanol extracts, our results suggest that T. palmata can be a useful source to develop new natural fungicides.