Tetsuo Kokubun

Plant–fungal interactions: the search for phytoalexins and other antifungal compounds from higher plants

A brief review is given of some biological, chemical and chemotaxonomic aspects of phytoalexin research. Emphasis is placed on the search for antifungal compounds in the plant families Leguminosae and Rosaceae, and in rice, Oryza sativa. The possible role of phytoalexins in the resistance of rice plants against the fungus Pyricularia oryzae (= Magnaporthe grisea) is discussed, and the future prospects of phytoalexin research are outlined.

Dihydroisocoumarins and a tetralone from Cytospora eucalypticola

Two dihydroisocoumarins, 3,5-dimethyl-8-hydroxy-7-methoxy-3,4-dihydroisocoumarin and 3,5-dimethyl-8-methoxy-3,4-dihydroisocoumarin were isolated from a culture filtrate of Cytospora eucalypticola, together with three known dihydroisocoumarins and a tetralone derivative. Their structures were determined by spectroscopic methods. These isocoumarins are mildly antifungal, and antibacterial towards gram positive bacteria. A known compound, 5-hydroxymethylmellein, showed mild antifeedant activity towards Spodoptera littoralis.

The chemotaxonomic significance of two bioactive caffeic acid esters, nepetoidins A and B, in the Lamiaceae.

A survey of leaf surface constituents in the family Lamiaceae using HPLC with diode array detection revealed the presence of two characteristic phenolic compounds in many species. The distribution of these phenolics in the Lamiaceae was found to be of taxonomic significance, as they were present in the great majority of species investigated for the subfamily Nepetoideae, including representatives of the well-known genera of culinary herbs, mint, rosemary, sage, thyme and basil. In contrast, they were absent from species of the other subfamilies of Lamiaceae studied and from the related families Verbenaceae, Scrophulariaceae, Acanthaceae and Buddlejaceae. The compounds were isolated from Plectranthus crassus and identified by NMR spectroscopy as the known caffeic acid esters (Z,E)-[2-(3,5-dihydroxyphenyl)ethenyl] 3-(3,4-dihydroxyphenyl)-2-propenoate and (Z,E)-[2-(3,4-dihydroxyphenyl)ethenyl] 3-(3,4-dihydroxyphenyl)-2-propenoate, for which the trivial names nepetoidins A and B are proposed. The presence of this pair of caffeic acid esters adds another character to the chemical, palynological and embryological features distinguishing the Nepetoideae from the other subfamilies of Lamiaceae and related families, and supports the view that the Nepetoideae are a specialised and monophyletic group within the family. Nepetoidin B was shown to have a greater antioxidant activity than gallic, rosmarinic and caffeic acids, and showed activity as an insect phagostimulant. Both compounds were antifungal.

Chemical constituents and antimicrobial activity of medicinal plants from Ghana: Cassia sieberiana, Haematostaphis barteri, Mitragyna inermis and Pseudocedrela kotschyi

The antimicrobial activity of the sequential n‐hexane, acetone and 50% aqueous methanol extracts of leaves, stem bark and roots of four species of medicinal plants, Cassia sieberiana DC. (Leguminosae), Haematostaphis barteri Hook. f. (Anacardiaceae), Mitragyna inermis (Willd.) O. Kuntze (Rubiaceae) and Pseudocedrela kotschyi (Schweinf.) Harms (Meliaceae), from Ghana were tested against Bacillus subtilis, Pseudomonas syringae and Cladosporium herbarum using TLC direct‐autobiographic methods. Extracts from leaves, stem bark and roots of the four species gave a positive result against at least one test organism. Twelve of the 36 extracts were active against B. subtilis, four extracts were active against P. syringae and six were active against C. herbarum. Preliminary chemical analysis revealed the presence of flavonoids, stilbenes and alkaloids. This is the first report of a stilbene from the Anacardiaceae. Copyright

Unexpected Secoiridoid Glucosides from Manulea corymbosa

From an extract of Manulea corymbosa were isolated four known secoiridoid glucosides (1-4), 10 new monoterpenoid esters of secologanol, namely, manuleosides A-I (5-11, 13, and 14) and dimethyl rhodanthoside A (12), and four new phenylpropanoid esters of carbocyclic iridoid glucosides, manucorymbosides I-IV (15-18). Also, the caffeoyl phenylethanoid glycoside verbascoside was isolated. The presence of secoiridoids apparently derived from loganic acid in the family Scrophulariaceae is unprecedented and greatly unexpected.

Iridoid glucosides in the genus Veronica (Plantaginaceae) from New Zealand

Four simple iridoid glucosides, three known esters of catalpol, seven esters of aucubin, and two phenylethanoids were isolated from Veronica hookeri (syn. Hebe ciliolata; Plantaginaceae). Of these, none of four aromatic (p-methoxybenzoyl, isovanilloyl, veratroyl, caffeoyl) 6-O-esters of aucubin and 6″-O-benzoyl mussaenosidic acid, had been reported from nature before. Similarly, three simple iridoid glucosides, two esters of 6-O-rhamnopyranosylcatapol, and two phenylethanoid glucosides, as well as 1-O-benzoyl-3-α-glucuronosylglycerol, and 1-O-β-benzoyl rutinoside were isolated from Veronica pinguifolia (syn. Hebe pinguifolia). The compound 3″-O-benzoyl-2″-O-caffeoyl 6-O-rhamnopyranosylcatalpol had not been reported previously. The pattern of the structural features of the iridoid glucosides is overlaid onto the latest molecular phylogenetic framework of Veronica sects. Hebe and Labiatoides, and discussed in the context of evolutionary trends.

Protoilludane, Illudane, Illudalane, and Norilludane Sesquiterpenoids from Granulobasidium vellereum

Two new and seven known sesquiterpene compounds were isolated from an agar plate culture of Granulobasidium vellereum, isolated from a log of Ulmus sp. The two new structures were elucidated with spectroscopic methods as an illudalane derivative, granulolactone (1), and a 15-norilludane, granulodione (9). The acaricidal and insecticidal activities of the isolated compounds were examined in vitro against two major horticultural pests, the two-spotted spider mite Tetranychus urticae and the glasshouse thrips Heliothrips haemorrhoidalis, respectively.

Further iridoid glucosides in the genus Manulea (Scrophulariaceae)

From Manulea altissima (Scrophulariaceae) were isolated five known secoiridoid glucosides sweroside, eustomoside, eustoside, secoxyloganin and secologanoside as well as the 4″-O-rhamnopyranosyl-feruloyl ester of adoxosidic acid, named altissimoside. Also, the caffeoyl phenylethanoid glycoside verbascoside was isolated. In addition two previously unknown terpenoid esters of 6β-hydroxy 8-epi-boschnaloside, named manucoside A and B were isolated from a formerly obtained fraction from the work-up of Manulea corymbosa. The distribution of iridoid glucosides in the Scrophulariaceae is discussed.

Iridoid glucosides in the endemic Picconia azorica (Oleaceae)

In our continued investigation of plants from the family Oleaceae we have now investigated Picconia azorica endemic to the Azores. Like most species within the family it contains the oleoside-based secoiridoid glucosides ligstroside and oleuropein as the main compounds and in addition verbascoside and echinacoside. As with the previously investigated Picconia excelsa, it also contained the carbocyclic iridoid glucosides involved in the biosynthetic pathway to the oleoside derivatives. However, while P. excelsa contained loganin esterified with some monoterpenoid acids, P. azorica contains similar esters of 7-epi-loganic acid named Picconioside A and B. In addition were found the two 7-O-E/Z-cinnamoyl esters of 7-epi-loganic acid named Picconioside C and D.