COI1 is a critical component of a receptor for jasmonate and the bacterial virulence factor coronatine

Jasmonate (JA) is a lipid-derived hormone that regulates diverse aspects of plant immunity and development. An amino acid-conjugated form of JA, jasmonoyl–isoleucine (JA–Ile), stimulates binding of the F-box protein coronatine-insensitive 1 (COI1) to, and subsequent ubiquitin-dependent degradation of, jasmonate ZIM domain (JAZ) proteins that repress transcription of JA-responsive genes. The virulence factor coronatine (COR), which is produced by plant pathogenic strains of Pseudomonas syringae, suppresses host defense responses by activating JA signaling in a COI1-dependent manner. Although previous data indicate that COR acts as a molecular mimic of JA–Ile, the mechanism by which JA–Ile and COR are perceived by plant cells remains unknown. Here, we show that interaction of tomato COI1 with divergent members of the JAZ family is highly specific for JA–Ile and structurally related JA conjugates and that COR is ≈1,000-fold more active than JA–Ile in promoting this interaction in vitro. JA–Ile competes for binding of COR to COI1–JAZ complexes, demonstrating that COR and JA–Ile are recognized by the same receptor. Binding of COR to the COI1–JAZ complex requires COI1 and is severely impaired by a point mutation in the putative ligand-binding pocket of COI1. Finally, we show that the C-terminal region of JAZ3 containing the highly conserved Jas motif is necessary and sufficient for hormone-induced COI1–JAZ interaction. These findings demonstrate that COI1 is a critical component of the JA receptor and that COR exerts its virulence effects by functioning as a potent agonist of this receptor system

Plant defense in the absence of jasmonic acid: The role of cyclopentenones

The Arabidopsis opr3 mutant is defective in the isoform of 12-oxo-phytodienoate (OPDA) reductase required for jasmonic acid (JA) biosynthesis. Oxylipin signatures of wounded opr3 leaves revealed the absence of detectable 3R,7S-JA as well as altered levels of its cyclopentenone precursors OPDA and dinor OPDA. In contrast to JA-insensitive coi1 plants and to the fad3 fad7 fad8 mutant lacking the fatty acid precursors of JA synthesis, opr3 plants exhibited strong resistance to the dipteran Bradysia impatiens and the fungus Alternaria brassicicola. Analysis of transcript profiles in opr3 showed the wound induction of genes previously known to be JA-dependent, suggesting that cyclopentenones could fulfill some JA roles in vivo. Treating opr3 plants with exogenous OPDA powerfully up-regulated several genes and disclosed two distinct downstream signal pathways, one through COI1, the other via an electrophile effect of the cyclopentenones. We conclude that the jasmonate family cyclopentenone OPDA (most likely together with dinor OPDA) regulates gene expression in concert with JA to fine-tune the expression of defense genes. More generally, resistance to insect and fungal attack can be observed in the absence of JA

A Tonic Hyperpolarization Underlying Contrast Adaptation in Cat Visual Cortex

ith a significance level of 5% (SYSTAT, Systat Inc., Evanston, IL ). 34. This project was funded in part by a grant from the Swedish Natural Science Research Council. We thank H. Karlsson, A. T. Proveaux, and D. Powell for assistance with mass spectrometric analysis, J. Lockerman and S. Sharp for oral secretion collection, and M. Brennan for technical assistance. We also thank J. G. Millar, C. A. Ryan, and G. G. Still for helpful comments on the manuscript. 24 January 1997; accepted 17 March 1997 A Tonic Hyperpolarization Underlying Contrast Adaptation in Cat Visual Cortex Matteo Carandini* and David Ferster The firing rate responses of neurons in the primary visual cortex grow with stimulus contrast, the variation in the luminance of an image relative to the mean luminance. These responses, however, are reduced after a cell is exposed for prolonged periods to high-contrast visual stimuli. This phenomenon, known as contrast adaptation, occurs in the cortex and is not present at e