Phytophthora cinnamomi is a destructive oomycete plant pathogen in both natural and agricultural systems worldwide. Natural plant communities all around Australia are greatly affected by the disease caused by this pathogen. Only a very limited number of plants have been described as field resistant and that survive within areas that are affected by P. cinnamomi. Lomandra longifolia was assessed for its resistance under controlled laboratory conditions and was found to be highly resistant. We then explored potential resistance mechanisms in roots following zoospore inoculation. Microscopic analysis and quantitative studies of lignin, callose and hydrogen peroxide (H2O2) showed their induction in inoculated roots and genes related to their biosynthesis (PAL, Callose synthase 5 and 7, GST and Chalcone synthase) were upregulated following infection. The transcriptome of L. longifolia inoculated with P. cinnamomi was then analysed to provide an overview of defence-related gene induction. More specifically, the genes encoding multiple defence signalling proteins and pathogenesis-related proteins, phytohormone biosynthetic enzymes, phytoalexin biosynthetic enzymes and known defence-related transcription factors were highly induced in roots in response to infection.
Elicitins are 10 kDa proteins secreted by almost all Phytophthora species. We isolated and purified the β-cinnamomin elicitin from P. cinnamomi with the aim to develop an immunoaffinity purified antibody as a tool to examine virulence. The specificity of β-cinnamomin antiserum was confirmed through western-blot and isoelectric focusing analysis. The β-cinnamomin production was shown in different P. cinnamomi life stages and in inoculated plant roots by using immuno detection and confocal microscopy. Pre-treatment of zoospores with the β-cinnamomin antiserum and then inoculation of susceptible Lupinus angustifolius roots revealed a partial loss of virulence suggesting the intrinsic role of the elicitin. Exploration of resistance-related mechanisms in plants and pathogenicity factors in P. cinnamomi facilitate the possibility of modifying susceptible plant responses and identifying targets to prevent infection.