Upon binding to a high‐affinity plasma membrane (PM) protein (a member of the 14‐3‐3 family of regulatory proteins), the fungal phytotoxin fusicoccin (FC) activates the H+‐ ATPase by hindering the inhibitory interaction of the enzyme’s C‐terminus with its catalytic site. Protease protection experiments carried out with sealed PM vesicles of different orientation proved that the FC‐binding site faces the cytoplasmic surface of the membrane. The in vivo induced activation of the H+‐ATPase by FC was retained during solubilization of PM proteins. Two‐dimensional gel systems combining a native separation of membrane protein complexes with a denaturing dimension as well as high‐performance anion‐exchange chromatography proved the existence of a labile ATPase:14‐3‐3 complex in plasma membranes. Stabilization of this complex could be achieved by FC treatment in vivo or in vitro. Mild proteolytic removal of the C‐terminal auto‐inhibitory domain of the H+ATPase liberated apparent hydrophobic 14‐3‐3 isoforms from the membrane in soluble form. During size exclusion chromatography of the proteolytically released proteins, co‐elution of 14‐3‐3 dimers, protein‐bound FC and the C‐terminus of the H+ATPase was observed. Moreover, the data suggest that 14‐3‐3 dimers themselves are not able to bind FC. Based on these results, it is proposed that the ‘FC receptor’ is represented by a labile complex between a 14‐3‐3 dimer and the H+‐ATPase whose formation is part of a mechanism regulating ATPase‐activity under physiological conditions. In our working model, binding of FC stabilizes this labile complex, thus leading to a strong and persistent activation of the H+‐ATPase in vivo. The possibility that the C‐terminus of the enzyme represents the binding domain for 14‐3‐3 homologs is discussed.

The chemical structures and accumulation kinetics of several major soluble as well as wall-bound, alkali-hydrolyzable compounds induced upon infection of Arabidopsis thaliana leaves with Pseudomonas syringae pathovar tomato were established. All identified accumulating products were structurally related to tryptophan. Most prominent among the soluble substances were tryptophan, ␤-D-glucopyranosyl indole-3-carboxylic acid, 6-hydroxyindole-3-carboxylic acid 6-O-␤-D-glucopyranoside, and the indolic phytoalexin camalexin. The single major accumulating wall component detectable under these conditions was indole-3-carboxylic acid. All of these compounds increased more rapidly, and camalexin as well as indole-3-carboxylic acid reached much higher levels, in the incompatible than in the compatible P. syringae͞A. thaliana interaction. The only three prominent phenylpropanoid derivatives present in the soluble extract behaved differently. Two kaempferol glycosides remained largely unaffected, and sinapoyl malate decreased strongly upon bacterial infection with a time course inversely correlated with that of the accumulating tryptophan-related products. The accumulation patterns of both soluble and wallbound compounds, as well as the disease resistance phenotypes, were essentially the same for infected wild-type and tt4 (no kaempferol glycosides) or fah1 (no sinapoyl malate) mutant plants. Largely different product combinations accumulated in wounded or senescing A. thaliana leaves. It seems unlikely that any one of the infection-induced compounds identified so far has a decisive role in the resistance response to P. syringae.

The chemical structures and accumulation kinetics of several major soluble as well as wall-bound, alkali-hydrolyzable compounds induced upon infection of Arabidopsis thaliana leaves with Pseudomonas syringae pathovar tomato were established. All identified accumulating products were structurally related to tryptophan. Most prominent among the soluble substances were tryptophan, ␤-D-glucopyranosyl indole-3-carboxylic acid, 6-hydroxyindole-3-carboxylic acid 6-O-␤-D-glucopyranoside, and the indolic phytoalexin camalexin. The single major accumulating wall component detectable under these conditions was indole-3-carboxylic acid. All of these compounds increased more rapidly, and camalexin as well as indole-3-carboxylic acid reached much higher levels, in the incompatible than in the compatible P. syringae͞A. thaliana interaction. The only three prominent phenylpropanoid derivatives present in the soluble extract behaved differently. Two kaempferol glycosides remained largely unaffected, and sinapoyl malate decreased strongly upon bacterial infection with a time course inversely correlated with that of the accumulating tryptophan-related products. The accumulation patterns of both soluble and wallbound compounds, as well as the disease resistance phenotypes, were essentially the same for infected wild-type and tt4 (no kaempferol glycosides) or fah1 (no sinapoyl malate) mutant plants. Largely different product combinations accumulated in wounded or senescing A. thaliana leaves. It seems unlikely that any one of the infection-induced compounds identified so far has a decisive role in the resistance response to P. syringae.

No abstract