The role of Phytoalexins in the disease resistance of V.Faba. L to infection by Botrytis

Abstract

Wyerone, wyerone acid and four other phytoalexins were isolated from broad bean tissues infected by Eotrytis. Three of the new phytoalexins were shown to be wyerone epoxide, wyerol and nedica'-y-n, the fourth vis tentatively identified as hydrohydroxy - Koto - wyerone. All six phytoalexins accumulated in pod endoearp infected with 3. cinerea end in cotyledons infected with 3. cir.cr-i or 3. fabec. In pod tissue a??e8red t0 be able to metabolize the phytoalexins and prevent their accumulation to antifungal concentrations. Wyerone was the predominant phytoalexin in cotyledons infected with both fungi, whereas wycrone acid was the major component of the phytoalexin response in nods and leaves infected with ¿^pinjgea. The other phytoalexins seem unlikely to contribute significantly to the restriction of Botrytis in invaded tissue. The role of wyerone in preventing fungal growth invivo is not clear. It nay have a direct role in providin' an antifungal environment in the lesion, by beinm deposited on cell walls adjacent to the invading f"ngal hyphae. Alternatively, it may act indirectly, serving as a precursor -or wyerone acid. Wyerone acid and wyerone epoxide had similar activities against "*rm tube ...... h of B. cinerea and B. fabaa. Wyerone, hr»-.: v< r, was less a tifungal than these two phytoalexJns but more active than wyerol, Bj^cinerea was more sensitive than Ik fabae to all of the phytoalexms. In vitro both 3. fab»e and B. cineroa were able to i.iotabol’ r.e the phytoalexins wyerone, wyerone ej >xi ie and wyerone acid, ft« 'oductB tb,J metabolism of these phytoalexins by dotrytir were identified. The metabolites p .. d by B. fabae fro i ........ ixide and wyerone acid 1 ut v’ore shown to accumulato in spreading lesions caused by 2?.-i2222. on no' * " e detection of vryerol, the metabolite produced by both fun-1 from perone injltro, in restricted B. ciprea lesions but not in spreading: lesions caused by 3-, in pods suggested that vyerol is more likely to be a nr cursor for the other furano-acetylene phytoalexins, than a fungal metabolite. The inability to detect wyerol in spreading lesions caused by B^fabae in nods also indicates that i nj±S wyerone is not metabolized by the same pathways as detected in vitro. Circumstantial evidence indicated a conversion of wyerone to reduced vyei-one acid via wyerone acid in 3. fabae lesions. A model illustrating the possible Pathways of synthesis and of metabolism of the phytoalexins is oresented. Results suggest that the differential pathogenicities o' rL.J>nae and B. cinerea towards the broad bean can not be explained in terms of differential rates of phytoalexin metabolism alone. Other factors, --uch as the rate of phytoalexin accumulation and the sensitivity of the fungi to the phytoalexins are urobably also involved. The pathogenicity of ^ * ¿ 2 2. towards the broad bean is discussed in terms of induction and metabolism of the phytoalexins and of sensitivity of the fungus to the inhibitors.