2008
Threat abatement project
Enhancing the efficacy of phosphite with the addition/supplementation of other chemicals such as those known to be involved in resistance
Prepared by the Centre for Phytophthora Science and Management for the
Australian Government Department of the Environment, Water, Heritage and the Arts, May 2008
About the report
Chemically–induced resistance to Phytophthora cinnamomi
To test the efficacy of treatments against a pathogen it is necessary to have a standard pathosystem. We assessed the suitability of intact Arabidopsis plants from 20 ecotypes, root inoculated with 29 P. cinnamomi isolates, as Arabidopsis offers so many advantages for physiological and molecular work and allows fast throughput of trials. It was shown that intact Arabidopsis plants, grown hydroponically and root inoculated with P. cinnamomi zoospores are not a suitable pathosystem, as although root mass is reduced, shoots are qualitatively and quantitatively unaffected and plants remain healthy. An ideal pathosystem is one where susceptible Arabidopsis ecotypes die and resistant Arabidopsis ecotypes survive.
Detached leaves of 4-week old Arabidopsis (ecotype Landsberg erecta) plants inoculated with zoospores or mycelium of P. cinnamomi provided a good model system for some of the analysis of the effects of phosphite as infection could be assessed through lesion size and abundance of callose papillae. Leaves treated with phosphite showed reduced lesion size and increased numbers of callose papillae. Using qPCR an increase in the level of expression of the defense gene PRI was quantified. Although not an ideal pathosystem, Arabidopsis can be used to examine very early defense responses (in the first few days following inoculation) after treatment with phosphite.
A rapid assay was developed to compare the effect of phosphite and metabolic inhibitors on pathogenicity of P. cinnamomi. Filter paper discs overgrown with P. cinnamomi were treated with 20 ìL drops of phosphite or inhibitors, then tested for pathogenicity (ability to colonise lupin roots), or growth on NARPH plates. It was shown that c-AMP is likely to be involved in the reduction by phosphite of P. cinnamomi pathogenicity. The technique provides a means of screening compounds that might enhance phosphite efficacy, and to explain the mode of action of phosphite.
Quantification of phosphite uptake and movement in the plant is hampered by the lack of a quick, cheap method of measuring accurately the concentration of phosphite in different plant tissues. Costs of the existing HPLC method (~$30 per sample) prevent such studies, and analyses in the region of $1 - $5 per sample are required. We examined two methods (a silver nitrate assay, and a phosphite dehydrogenase assay) as potential methods of accurately and cost effectively measuring phosphite in plant tissues. Both methods proved promising and represent exciting advances in phosphite analysis. Some additional fine tuning is required to ensure that the methods are reliable across a range of plant species from different families. A cheap, accurate and robust analytical method will allow many important questions about phosphite uptake and movement to be investigated.
Research into natural and induced resistance in Australian native vegetation of Phytophthora cinnamomi and innovative methods to contain and/or eradicate within localised incursions in areas of high biodiversity in Australia.