Control of the plant pathogen, Phytophthora


A project undertaken at the School of Life and Environmental Sciences, Deakin University, and supervised by Dr Peter Beech

Plant diseases caused by species of Phytophthora have had, and continue to have, significant and often devastating impacts on the environment and agriculture. This genus of pathogens causes billions of dollars of crop losses per annum, extensive environmental degradation and, most notably, was the cause of the Irish potato famine of the 1840’s that lead to an estimated one million deaths.


Currently, there are only two chemical inhibitors available for the control of Phytophthora—Metalaxyl and phosphite—neither of which is species-specific and both are plagued by issues of emerging resistance. Containment of the pathogen though quarantine protocols has also had limited success, mainly due to the production of swimming zoospores that migrate to new infection sites during rain events and an inability of regulatory authorities to limit anthropogenic transport. Given these factors, there is an urgent need to develop targeted antibiotics to control this pathogen.


The release of three Phytophthora genomes in 2006 resulted in many findings of interest to antibiotic developers. One such finding was that Phytophthora does not contain a gene for phospholipase C (PLC). PLC is an essential signalling enzyme that, among other things, triggers cytokinesis, activates protein kinases, activates phospholipase D and regulates lipid rafts that are critical for numerous cellular functions. PLC does all this by hydrolysing the phospholipid phosphatidylinositol (4,5) bisphosphate into the secondary signalling molecules inositol (1,4,5) triphosphate and diacylglycerol. Inositol triphosphate is probably the most important secondary signalling molecule generated by PLC, as this molecule initiates the calcium flux required for cytokinesis. The PLC enzyme is found in all eukaryotes and almost all bacteria, which raised the question: How does Phytophthora survive without a PLC?


While it might have seemed that the PLC pathway had therefore become redundant in Phytophthora, other work had shown that the downstream biochemical pathway for PLC (the production of inositol triphosphate and calcium flux) was still functional in Phytophthora. In addition, all of the other enzyme components of the PLC pathway appeared to exist in the three sequenced Phytophthora genomes. From these findings, we postulated, that Phytophthora had evolved a novel phospholipase C that we have named AltPLC, for alternate PLC.


Because this protein is presumably essential for and unique to Phytophthora, it is an ideal target for Phytophthora-specific antibiotics. This project aims to identify and characterise AltPLC and to initiate the development of such antibiotics.

 

Figure 1. Xanthorrhoea plants dying following infection with the Dieback pathogen, Phytophthora cinnamomi at Wilsons Promontory, Victoria.

Figure 2. Hyphae of Phytophthora visualised under the flourescence microscope.