Marshall E. Mace

Fungal wilt diseases of plants.

Fungal wilt diseases of plants , Fungal wilt diseases of plants , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Contributions of tyloses and terpenoid aldehyde phytoalexins to Verticillium wilt resistance in cotton

Abstract Verticillium wilt-resistant cv. Seabrook Sea Island and -susceptible cv. Rowden cottons were inoculated in the hypocotyl at the ten leaf stage with conidia of Verticillium dahliae . Stem xylem was observed from 10 to 96 h after inoculation for Verticillium hyphal growth and sporulation, vascular occlusion and terpenoid aldehyde synthesis. Rapid occlusion of xylem vessels by tyloses followed by synthesis of fungitoxic terpenoid aldehydes is apparently required for the resistance of Seabrook Sea Island to Verticillium wilt. Occlusion of infected vessels throughout the stem of Seabrook Sea Island by 48 h after inoculation appeared to prevent the systemic distribution of most, if not all, secondary conidia released at the primary infection sites between 24 and 48 h after inoculation. The fungitoxic terpenoid aldehydes, predominantly hemigossypol and methoxyhemigossypol, then accumulated within the localized primary infection sites. Once V. dahliae was effectively localized, formation of new, non-infected xylem tissue apparently enabled Seabrook Sea Island to compensate for vessels occluded by responses to the primary infections. Occlusions of infected vessels in the stem of Rowden were delayed. This delay allowed secondary conidia to escape containment at primary infection sites and thereby escape exposure to the localized accumulation of terpenoid aldehydes. Because of the extensive systemic distribution of secondary conidia in Rowden, infection of newly formed xylem vessels generally kept pace with their formation, and severe wilt developed.

Antimicrobial terpenoids of Gossypium: 6-methoxygossypol and 6,6′-dimethoxygossypol☆

Abstract The triterpenoid aldehydes, gossypol ( 1 ), 6-methoxygossypol ( 2 ) and 6,6′-dimethoxygossypol ( 3 ); and the sesquiterpenoid aldehydes, hemigossypol ( 4 ) and methoxyhemigossypol ( 5 ), were isolated from 1-week-old roots of Gossypium hirsutum and G. barbadense and identified. This is the first report of 2 and 3 in nature and of 4 and 5 from healthy roots. Compounds 2 and 3 also constituted 30% of the total terpenoid aldehydes in the seeds of 1 cultivar of G. barbadense , but occurred only in trace quantities in those of G. hirsutum . Spectral data (UV, IR, NMR, MS) and proof of structure for 2 and 3 are presented.

Toxicity and role of terpenoid phytoalexins in verticillium wilt resistance in cotton

Abstract Hemigossypol (HG), methoxyhemigossypol (MHG), desoxyhemigossypol (dHG) and desoxymethoxyhemigossypol (dMHG), the four major terpenoids formed in the stem stele of Verticillium dahliae -infected, wilt-resistant Seabrook Sea Island (SBSI) cotton, were tested at pH 6·3–7·5 in liquid nutrient media for toxicity to V. dahliae . The terpenoids dHG, HG, dMHG, and MHG at 25 °C killed all conidia after 18–40 h at 10, 45, 25 and 60 μg ml −1 , respectively; and all mycelia after 48 h at 15, 32, 25 and 45 μg ml −1 , respectively. Inhibition of conidia germination also occurred at concentrations well below the fungicidal concentrations. Dimethylsulfoxide at 2 or 5% was required to solubilize HG, MHG and dMHG at fungicidal concentrations. Only dHG had the water solubility apparently required to reach fingicidal concentration in the aqueous medium of infected xylem vessels and thus account for the death of V. dahliae conidia and mycelia in most infected vessels in the stem stele of SBSI cotton 10 days after inoculation. The dHG in the stem stele at 10 days after inoculation was in excess of fungicidal concentration.

Histochemistry and identification of flavanols in Verticillium wilt-resistant and -susceptible cottons

Abstract The flavanols catechin (3,5,7,3′,4′-pentahydroxyflavan) and gallocatechin (3,5,7,3′,4′,5′-hexahydroxyflavan) were the major precursors of condensed proanthocyanidins in the stem steles of healthy and V. dahliae -inoculated Verticillium wilt-resistant Seabrook Sea Island (SBSI) and -susceptible Rowden cottons. Histochemical studies of the flavanols in noninfected and infected plants were conducted at daily intervals for 4 days after stem-inoculation of 10-leaf-stage plants. The dimethoxybenzaldehyde (DMB) histochemical reagent revealed the presence of the flavanols (catechin, gallocatechin, and their condensed proanthocyanidins) as red-colored products. In non-infected stem steles of both cultivars the flavanols were sparsely distributed in solitary ray, axial and pith parenchyma cells. Greater frequencies of flavanol-containing cells in diseased than healthy stems were first noted in ray parenchyma at 2 and 3 days after inoculation of SBSI and Rowden, respectively. This apparently infection-induced synthesis, much more intense in SBSI than in Rowden, intensified progressively at 3 to 4 days after inoculation, and became evident in the pith adjacent to infected xylem vessels and in a diffuse band in ray and axial parenchyma between the cambium and newly formed xylem vessels. Fungitoxic flavanols in both SBSI and Rowden may create a toxic environment throughout the stem stele which confines V. dahliae to the vessel lumens.

Relation between sensitivity to terpenoid phytoalexins and virulence to cotton of Verticillium dahliae strains

Abstract The terpenoid phytoalexins hemigossypol (HG), methoxyhemigossypol (MHG), desoxyhemigossypol (dHG), and desoxymethoxyhemigossypol (dMHG) from Verticillium dahliae -infected, wilt-resistant Seabrook Sea Island cotton were tested at pH 6.3–7.5 in liquid nutrient media for toxicity to two defoliating and two nondefoliating isolates of V. dahliae . Terpenoid concentrations of 20, 40, 50, and 90 μg/ml of dHG, HG, dMHG, and MHG, respectively, were required to kill uniformly mycelia of all four isolates of V. dahliae . Using this bioassay, differential sensitivity to terpenoid phytoalexins does not appear to be a factor in the differences in virulence of the defoliating and nondefoliating isolates of V. dahliae on cotton.

The role of free radicals in the decomposition of the phytoalexin desoxyhemigossypol

The cotton phytoalexin desoxyhemigossypol (dHG) decomposed rapidly in solution to give hemigossypol (HG). The rate of decomposition was retarded by the reducing agents ascorbic acid, reduced glutathione and cysteine, by the metal chelator diethylenetriaminepentaacetic acid, and by the enzyme catalase. However, the chelator, ethylenediaminetetraacetic acid did not reduce the rate of decomposition and the enzyme superoxide dismutase increased the rate of decomposition. Solutions of the phytoalexin desoxyhemigossypol-6-methyl ether were significantly more stable than were those of dHG. Oxygen-18 from water but not from oxygen gas was incorporated into HG during this decomposition. A hydroperoxynaphthalenone which loses hydrogen peroxide is proposed as an intermediate to explain this observation. The formation of hydrogen peroxide may be involved in the toxicity of this phytoalexin to plant pathogens such as Verticillium dahliae.