Molecular studies of the oomycete biocontrol agent Pythium oligandrum

• Main Author: Horner, Neil R.
• Published: University of Aberdeen 2007
• Subjects: 579.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485627

The oomycete Pythium oligandrnm is used as a biocontrol agent because ofthe symbioses it forms with plants, fungi, and other oomycetes. It can parasitize phytopathogenic fungi and oomycetes providing protection to crop plants. P. oligandrnm grows as an endophyte within roots ofplants without causing disease symptoms or defense responses associated with pathogen attack. Growth of P. oligandrnm within plant tissues induces systemic resistance to pathogens and also stimulates plant growth. Little is known ofthe molecular processes that are involved in these proc7sses. Limited molecular tools were currently available to study this organism. The work set out to develop basic tools, and to acquire the fIrst signifIcant DNA sequencing data for this orgalllsm. P. oligandrnm expressed sequence tags derived from vegetative mycelia and a P. oligandrnm-P. infestans interaction were analyzed in an attempt to fmd sequences that may be involved in its biotic interactions. Many sequences with similarity to previously described effectors from fungi, oomycetes, and bacteria were revealed. ' A transformation protocol was adapted for use in P. oligandrnm, which was used to heterologously express green flourescent protein (gfp), producing florescent hyphae that could be used to follow the interaction with a plant and oomycete host. Several genes were identifIed from the EST libraries that were similar to nematode eggshell protein-encoding genes. Using the transformation protocol, it was possible to silence the expression ofthese genes by homology-dependent gene silencing. Oospores .from silenced strains displayed major ultrastructural abnormalities and were sensitive to degradative enzyme treatment. Using immunocytochemistry, these proteins were localized to the oogonial and oospore wall. It was therefore suggested that these proteins are integral components of the oospore/oogonial wall. From the EST libraries three sequences were identifIed that were similar to sea slug pheromones. Similar sequences were found to form a la:.ge gene family in the genomes ofPhytopthora infestans, P. sojae and P. ramorum. One of these genes in P. infestans, was shown to be up-regulated in the motile zoospore stage, leading to the hypothesis that this gene may be acting as a pheromone in zoospore autoaggregation. An initial characterization found no evidence to support this hypothesis.