Plant roots generate electrical currents and associated electrical fields as a consequence of electrogenic ion transport at the root surface. Here we demonstrate that the attraction of swimming zoospores of oomycete plant pathogens to plant roots is mediated in part by electrotaxis in natural root-generated electric fields. The zones of accumulation of anode- or cathode-seeking zoospores adjacent to intact and wounded root surfaces correlated with their in vitro electrotactic behavior. Manipulation of the root electrical field was reflected in changes in the pattern of zoospore accumulation and imposed focal electrical fields were capable of overriding endogenous signals at the root surface. The overall pattern of zoospore accumulation around roots was not affected by the presence of amino acids at concentrations expected within the rhizosphere, although higher concentrations induced encystment and reduced root targeting. The data suggest that electrical signals can augment or override chemical ones in mediating short-range tactic responses of oomycete zoospores at root surfaces.

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Zoospores of plant pathogenic oomycetes exhibit distinct swimming speeds and patterns under natural conditions. Zoospore swimming is influenced by ion homeostasis and changes in the ionic composition of media. Therefore, we used video microscopy to investigate swimming patterns of five oomycete species in response to changes in potassium homeostasis. In general, zoospore speed tended to be negatively correlated with zoospore size. Three Phytophthora species (Phytophthora palmivora, Phytophthora megakarya, and Phytophthora infestans) swam in straight patterns with speeds ranging from 50 to 250 microm/s whereas two Pythium species (Pythium aphanidermatum and Pythium dissotocum) swam at similar speeds ranging from 180 to 225 microm/s with a pronounced helical trajectory and varying amplitudes. High external concentrations of potassium salts reduced the swimming speed of Ph. palmivora and induced encystment. This was not observed for Py. aphanidermatum. Application of the potassium ionophores gramicidin, nigericin and valinomycin resulted in reduced swimming speeds and changes in the swimming patterns of the Phytophthora species. Therefore, potassium ions play a key role in regulating zoospore behavior.

In the plant pathogen Phytophthora infestans, nuclear integration of inf1 transgenic DNA sequences results in internuclear gene silencing of inf1. Although silencing is regulated at the transcriptional level, it also affects transcription from other nuclei within heterokaryotic cells of the mycelium. Here we report experiments exploring the mechanism of internuclear gene silencing in P. infestans. The DNA methylation inhibitor 5-azacytidine induced reversion of the inf1-silenced state. Also, the histone deacetylase inhibitor trichostatin-A was able to reverse inf1 silencing. inf1-expression levels returned to the silenced state when the inhibitors were removed except in non-transgenic inf1-silenced strains that were generated via internuclear gene silencing, where inf1 expression was restored permanently. Therefore, inf1-transgenic sequences are required to maintain the silenced state. Prolonged culture of non-transgenic inf1-silenced strains resulted in gradual reactivation of inf1 gene expression. Nuclease digestion of inf1-silenced and nonsilenced nuclei showed that inf1 sequences in silenced nuclei were less rapidly degraded than non-silenced inf1 sequences. Bisulfite sequencing of the endogenous inf1 locus did not result in detection of any cytosine methylation. Our findings suggest that the inf1-silenced state is based on chromatin remodelling. INTRODUCTIONIn many organisms, including animals, plants, fungi and protists, transcriptional gene silencing (TGS) can occur as a consequence of the introduction of transgenes into the somatic genome (Selker, 1997;Henikoff, 1998;Vaucheret et al., 1998;Cogoni & Macino, 1999;van West et al., 1999). TGS is thought to have evolved as a mechanism to protect the nuclear genome against potentially harmful transforming sequences such as transposons and viruses (Kooter et al., 1999). TGS is reminiscent of paramutation found in plants, and other imprinting phenomena (Hollick et al., 1997). Paramutation is an epigenetic phenomenon in which one allele or locus spontaneously acquires a less transcriptionally active state, which is then imposed on homologous sequences in the genome (Chandler et al., 2000). In plants and animals TGS is often, but not invariably associated with altered methylation patterns and changes in chromatin structure. Organisms lacking efficient DNA methylation systems, such as Drosophila, Caenorhabditis elegans and yeast, use chromatin modification to achieve imprinting and to maintain epigenetic states. Several experiments have shown that chromatin remodelling and histone deacetylation processes influence higher-order chromatin structure and can result in transcriptional repression (Tyler & Kadonaga, 1999;Grewal & Elgin, 2002;Vermaak et al., 2003).Other homology-dependent gene-silencing phenomena are also found; these are thought to occur at a posttranscriptional level in the cytoplasm. Post-transcriptional gene silencing (PTGS) phenomena have one major feature in common, which is that target mRNAs are degraded in a sequence-specific manner. Examples inclu...

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