Hamid, Khalid; (1999) Separation and phytotoxicity of solanapyrone compounds produced by Ascochyta rabiei (Pass.) Labr. and their metabolism by chickpea (Cicer arietinum L.). Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An isolate of Ascochyta rabiei secreted the phytotoxins, solanapyrones A, B and C when grown on Czapek Dox nutrients supplemented with five cations. The toxins were identified and quantified by high performance liquid chromatography with diode array detection and isolated from culture filtrates by partitioning into ethyl acetate and flash chromatography on silica gel. Cells isolated from leaflets of 12 chickpea cultivars differed by up to five fold in their sensitivity to solanapyrone A and this compound was 2.6-12.6 times more toxic than solanapyrone B, depending on cultivar. When chickpea shoots were placed in solanapyrone A, the compound could not be recovered from the plant and symptoms developed consisting of turgor loss of stems and flame-shaped, chlorotic zones in the leaflets. In similar experiments with solanapyrone B, only 9.4% of the compound taken up was recovered and stems remained turgid but their leaflets became twisted and chlorotic. Glutathione reacted with solanapyrone A, rapidly reducing the amount of free toxin and forming a Sol.A-glutathione conjugate as well as reducing its activity when incorporated in the cell assay. Measurement of reduced glutathione concentration and GST activity among cultivars showed that the differences of their means were highly significant and both were negatively correlated with their relative sensitivity to solanapyrone A. Treatment of shoots with solanapyrone A enhanced total, reduced and oxidized glutathione content as well as GST activity 1.26, 1.23, 1.50 and 1.94 fold, respectively. Similarly, treatment of shoots with the safener, dichlormid, also raised total, oxidized and reduced glutathione levels and GST activity. Cells isolated from shoots treated with dichlormid at 150μg/shoot and 300μg/shoot were 2.45 times and 2.66 times less sensitive to solanapyrone A with LD50 values of 71.5 μg/ml and 77.8 μg/ml, respectively as compared to 29.2 μg/ml for controls. hi preliminary experiments designed to identify microbial genes capable of detoxifying the solanapyrones a basal mineral salts medium caused demethylation of solanapyrone A. Demethylated solanapyrone A was 16.4 fold less toxic than solanapyrone A in the cell assay, requiring 514.0 μg/ml to kill 50% of the cells compared with 31.3 μg/ml for Sol.A.

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