K+ channels of Cf-9 transgenic tobacco guard cells as targets for Cladosporium fulvum Avr9 elicitor-dependent signal transduction.
The Cf-9 gene encodes an extracytosolic leucine-rich repeat (LRR) protein that is membrane anchored near its C-terminus. The protein confers resistance in tomato to races of the fungus Cladosporium fulvum expressing the corresponding avirulence gene Avr9. In Nicotiana tabacum the Cf-9 transgene confers sensitivity to the Avr9 elicitor, and leads on elicitation to a subset of defence responses qualitatively similar to those normally seen in the tomato host. One of the earliest responses, both in the native and transgenic hosts, results in K+ salt loss from the infected tissues. However, the mechanism(s) underlying this solute flux and its control is poorly understood. We have explored the actions of Avr9 on Cf-9 transgenic Nicotiana using guard cells as a model. Much detail of guard cell ion channels and their regulation is already known. Measurements were carried out on intact guard cells in epidermal peels, and the currents carried by inward- (IK,in) and outward-rectifying (IK,out) K+ channels were characterized under voltage clamp. Exposures to Avr9-containing extracts resulted in a 2.5- to 3-fold stimulation of IK,out and almost complete suppression of IK,in within 3-5 min. The K+ channel responses were irreversible. They were specific for the Avr9 elicitor, were not observed in guard cells of Nicotiana lacking the Cf-9 transgene and, from kinetic analyses, could be ascribed to changes in channel gating. Both K+ channel responses were found to be saturable functions of Avr9 concentration and were completely blocked in the presence of 0.5 microM staurosporine and 100 microM H7, both broad-range protein kinase antagonists. These results demonstrate the ability of the Cf-9 transgene to couple Avr9 elicitation specifically to a concerted action on two discrete K+ channels and they indicate a role for protein phosphorylation in Avr9/Cf-9 signal transduction leading to transport control.
Blatt
M R
MR
Laboratory of Plant Physiology and Biophysics, Wye College, University of London, Wye, Kent, UK. mblatt@wye.ac.uk
Grabov
A
A
Brearley
J
J
Hammond-Kosack
K
K
Jones
J D
JD
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine / Q000494:pharmacology; D000818:Animals; D002963:Cladosporium / Q000472:pathogenicity / Q000502:physiology*; D004553:Electric Conductivity; D005656:Fungal Proteins / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000235:genetics / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D010766:Phosphorylation / Q000187:drug effects; D010935:Plant Diseases / Q000382:microbiology; D010940:Plant Proteins / Q000235:genetics / Q000502:physiology*; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism*; D024661:Potassium Channels, Inwardly Rectifying; D024683:Potassium Channels, Tandem Pore Domain; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D019311:Staurosporine / Q000494:pharmacology; D014026:Nicotiana / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism / Q000382:microbiology
D005656:Fungal Proteins; D008562:Membrane Glycoproteins; D010940:Plant Proteins; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D024683:Potassium Channels, Tandem Pore Domain; C073605:AVR9 protein, Cladosporium fulvum; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D017346:Protein Serine-Threonine Kinases; D019311:Staurosporine; D011188:Potassium