Stress & Dev Biol

The hrp gene clusters of plant pathogenic bacteria control pathogenicity on their host plants and ability to elicit the hypersensitive reaction in resistant plants. Some hrp gene products constitute elements of the type III secretion system, by which effector proteins are exported and delivered into plant cells. Here, we show that the hrpZ gene product from the bean halo-blight pathogen, Pseudomonas syringae pv. phaseolicola (HrpZPsph), is secreted in an hrp-dependent manner in P. syringae pv. phaseolicola and exported by the type III secretion system in the mammalian pathogen Yersinia enterocolitica. HrpZPsph was found to associate stably with liposomes and synthetic bilayer membranes. Under symmetric ionic conditions, addition of 2 nM of purified recombinant HrpZ Psph to the cis compartment of planar lipid bilayers provoked an ion current with a large unitary conductivity of 207 pS. HrpZPsphrelated proteins from P. syringae pv. tomato or syringae triggered ion currents similar to those stimulated by HrpZPsph. The HrpZPsphmediated ion-conducting pore was permeable for cations but did not mediate fluxes of Cl ؊ . Such pore-forming activity may allow nutrient release and͞or delivery of virulence factors during bacterial colonization of host plants.

Mitogen-activated protein kinase (MAPK)-mediated responses are in part regulated by the repertoire of MAPK substrates, which is still poorly elucidated in plants. Here, the in vivo enzyme-substrate interaction of the Arabidopsis thaliana MAP kinase, MPK6, with an ethylene response factor (ERF104) is shown by fluorescence resonance energy transfer. The interaction was rapidly lost in response to flagellin-derived flg22 peptide. This complex disruption requires not only MPK6 activity, which also affects ERF104 stability via phosphorylation, but also ethylene signaling. The latter points to a novel role of ethylene in substrate release, presumably allowing the liberated ERF104 to access target genes. Microarray data show enrichment of GCC motifs in the promoters of ERF104 -up-regulated genes, many of which are stress related. ERF104 is a vital regulator of basal immunity, as altered expression in both erf104 and overexpressors led to more growth inhibition by flg22 and enhanced susceptibility to a non-adapted bacterial pathogen. defense ͉ elicitor ͉ FRET ͉ signal transduction M itogen-activated protein kinase (MAPK) cascades transduce This article contains supporting information online at