Richard N. Strange

Phytotoxins produced by microbial plant pathogens

Phytotoxic compounds produced by plant pathogens are often crucial determinants of plant disease. Knowledge of them provides insights into disease syndromes and may be exploited by conventional breeding and biotechnology to obtain resistant crops.

Chickpea blight: Production of the phytotoxins solanapyrones A and C by Ascochyta rabiei

Abstract Filtrates from 12-day-old stationary cultures of Ascochyta rabiei grown on Czapek-Dox medium, supplemented with an extract of chickpea seed, killed the cells in cell suspensions obtained by enzymic digestion of chickpea leaflets. Two toxins were isolated by solvent partitioning with ethyl acetate and flash chromatography of the organic fraction on silica. Mass spectrometry, UV, 13 C/ 1 H NMR and X-ray analysis showed that the two compounds were identical to the phytotoxins solanapyrones A and C isolated previously from culture filtrates of the fungus Alternaria solani .

A new QTL for Ascochyta blight resistance in an RIL population derived from an interspecific cross in chickpea

SummaryA linkage map in a population of recombinant inbred lines (RILs) derived from an interspecific cross between Cicer arietinum (ILC72) × Cicer reticulatum (Cr5-10), resistant and susceptible to blight, caused by Ascochyta rabiei, respectively, was obtained using RAPD, ISSR, STMS, isozyme (Pdf6) and flower colour (pink/white) markers. The map comprised ten linkage groups and covered a distance of 601.2 cM. When the population was evaluated for reaction to Ascochyta blight under field conditions by determining the Area Under the Disease Progress Curve (AUDPC), the distribution of frequencies was bimodal: most of the lines had an intermediate reaction, fewer were nearly as susceptible as the susceptible parent and none had values close to the resistant parent. A QTL explaining 28% of the variation in resistance was located in linkage group 2 (LG2). Five RAPD markers on this linkage group showed significant association with resistance (OPX04372, UBC881621, OPAI09746, OPAI09352 and OPAC12700) and the major QTL peak lay midway between OPAI09746 and UBC881621 which are 14.1 cM apart. Contrary to other studies, no association of linkage group 4 with resistance was found. The QTL for resistance to Ascochyta blight in this study is therefore different from QTLs for this character reported in other interspecific crosses and may be the same as that reported in linkage group 2 in intraspecific crosses where genes for resistance to races of Fusarium oxysporum f. sp. ciceri, causing wilt, are also located.

Two novel stilbene phytoalexins from Arachis hypogaea

Abstract Three phytoalexins were isolated from groundnut seeds which had been sliced and incubated for 48 hr at 25†. Two were novel isoprenylated stilbene der

Two novel stilbene-2-carboxylic acid phytoalexins from Cajanus cajan

Abstract Three phytoalexins were isolated from leaves of pigeon pea which had been challenged with Botrytis cinerea . One was identified as pinostrobin chalc

Involvement of the hydroxyl radical in the abiotic elicitation of phytoalexins in legumes

Benzoate, dimethylsulphoxide, mannitol and methionine, which are scavengers of the hydroxyl radical (OH·), inhibited the accumulation of the glyceollin phytoalexins in soybean cotyledons elicited by silver nitrate. Benzoate also inhibited phytoalexin accumulation in groundnut cotyledons elicited by slicing and in pea cotyledons elicited by slicing or silver nitrate. Superoxide, generated by the xanthine oxidase/acetaldehyde reaction, did not cause glyceollin accumulation in soybean cotyledons and superoxide dismutase did not prevent glyceollin accumulation by silver nitrate treatment. These results are discussed in terms of a direct role for OH· but not superoxide in the process of abiotic elicitation of phytoalexins in legume cotyledons.

Circumstantial evidence for phytoalexin involvement in the resistance of peanuts to Aspergillus flavus.

Three stilbene phytoalexins, elicited by slicing and incubating imbibed peanut kernels under aerobic conditions, inhibited spore germination and hyphal extension of Aspergillus flavus with ED50 values in the range 4.9-12.8 micrograms ml-1. Phytoalexin yield was dependent on cultivar, conditions and duration of incubation after slicing, and crop history. The yield of phytoalexin from ten cultivars studied, after slicing and incubating at 25 degrees C for 24 h, ranged from 28 to 935 micrograms per g fresh weight and was negatively correlated with dry kernel colonization by A. flavus [r = -0.868 when plotted as 1n (phytoalexin concn) against 1n (percentage peanut colonization)]. When the incubation period was extended to 96 h there was no such correlation. Reduced phytoalexin yields were obtained when sliced kernels of one cultivar studied were incubated in water or at 37 degrees C, and no phytoalexin was obtained when the slices were incubated under nitrogen gas or frozen before aerobic incubation. Drought stress during pod development in four cultivars studied reduced phytoalexin yields of sliced kernels incubated at 25 degrees C for 24 h by 17-65% compared with non-stressed controls.

Two isoprenylated isoflavone phytoalexins from Cajanus cajan

Abstract Four phytoalexins were isolated from sliced seeds of pigeonpea which had been incubated with its native microflora. Two were identified as the known pigeonpea phytoalexins, cajanin and cajanol, and the other two were characterized as new isoprenylated isoflavones.

A dienyl stilbene phytoalexin from arachis hypogaea

Abstract The novel stilbene, 3-isopentadienyl-4,3′,5′-trihydroxystilbene, was identified as the major antifungal component elicited by slicing imbibed kernels of 11 genotypes of groundnut. Incubation for 24 hr after slicing yielded values which varied between 38.5 and 105.8 μ of the compound/g of unimbibed kernels according to cultivar. After incubation for 48 hr, yields rose to between 89.5 and 189.7 μ/g. The compound was inhibitory to both spore germination and hyphal extension of the fungus, Aspergillus flavus, at 14.0 and 11.3 μ/ml, respectively.

Chlamydosporol, a new metabolite from Fusarium chlamydosporum

Extracts of rice on which an isolate of Fusarium chlamydosporum had been cultured were toxic to brine shrimps. The toxic fraction was purified by flash chromatography to give two compounds which were identified by UV, IR, NMR and mass spectroscopy at the 6α and 6β isomers of 5-hydroxy-4-methoxy-6, 8a-dimethyl-6,7-dihydro-2H,8aH-pyrano[2,3-b]pyran-2-one. These lactones for which the name chlamydosporol is proposed have not been reported previously. When tested in brine shrimp and HeLa cell assays, the LC50 concentration for a mixture of the isomers was approximately 400 μg/ml in both systems.