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An in vitro investigation of the reduction in lesion development caused by infection with Phytophthora cinnamomi zoospores in young axenic Pisolithus tinctorius/Eucalyptus sieberi ectomycorrhizae

Howard, Kay (1995) An in vitro investigation of the reduction in lesion development caused by infection with Phytophthora cinnamomi zoospores in young axenic Pisolithus tinctorius/Eucalyptus sieberi ectomycorrhizae. Honours thesis, Murdoch University.

Abstract

It has been suggested that ectomycorrhizae protect roots from invasion by soilborne plant pathogens. The present study tested this hypothesis with in vitro studies using axenic conditions. When Eucalyptus sieberi . roots were in association with Pisolithus tinctorius, a degree of protection was afforded, whereby lesion lengths caused by Phytophthora cinnamomi in primary roots are reduced. Exudates from P. tinctorius resulted in a significant increase in the number of lateral roots. Lateral root apices were observed as a popul,ar site for P. cinnamomi infection in control roots. By 48 h, lesion development in ectomycorrhizae was 1.96 - 4.58 and 0.26 - 4.75 mm.d-1 (P. tinctorius isolates H445 and M56, respectively), whereas the control lesion extenstion rate was 11.2 mm.d-1.

In non-mycorrhizal plants, P. cinnamomi caused a brown lesion, in the stele of the root, extending from the root tip within 12 h. At 5 mm beyond the visible lesion front, the amount and position of hyphae observed varied from root to root. Staining of embedded sections with methylene blue and azure II in borax detected lignin in the xylem and in large amounts in the metaxylem, of all inoculated roots. Lignin was only observed in the xylem of the non-inoculated control roots. Sudan black B detected suberin only within sections of visible lesions. Whole mounts stained with Trypan blue showed most hyphae were arranged randomly along the root surface, with many orientated along cell wall junctions. Hyphae were concentrated at the apex of newly formed lateral roots, and were detected growing intercellulary throughout the cortex and the phloem of primary roots.

The influence of the P. tinctorius hyphae on• zoospore germination and mycelial growth, on solid media, varied between the two P. cinnamomi isolates used in this study. The interaction between P. cinnamomi isolate MP112 and P. tinctorius isolate M56 resulted in the greatest inhibition of the pathogen's mycelium. In this reaction, P. cinnamomi hyphae tended to aggregate, but did not stop growing. Zoospore germination of P. cinnamomi isolate MP94048 were suppressed at 0.5 h only, while germination of zoospores of isolate MP112 were significantly reduced by both P. tinctorius isolates for the entire trial.

Staining of embedded sections revealed the ectomycorrhizae (P. tinctorius isolate M56) produced more elongated epidermal cells with approximately 30% more primary root tips covered by hyphae, than isolate H445. However, both isolates were observed to have a loose mantle. Isolate M56 produced some defence reactions, detected by aniline blue fluorescence in the cortex, and both isolates resulted in greater lignification in the metaxylem in comparison to the control roots.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
Supervisor: Hardy, Giles and Dell, Bernard
URI: http://researchrepository.murdoch.edu.au/id/eprint/32816
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