Indirect evidence suggests that vascular coatings formed by plants in response to stress consist of suberin-like substances containing lipid and phenolic compounds. To provide more direct chemical evidence that coatings are suberin, we used a natural pathogen, Verticillium albo-atrum, or a stress-responsive hormone, abscisic acid, to induce coating in two isolines of tomato (Lycopersicon esculentum L. cultivar Craigella) that are resistant or susceptible to the pathogen. Using treated petioles that had been monitored cytologically, chemical depolymerization followed by combined gas-liquid chromatography-mass spectrometry analysis of alkane-a,w-diol levels confirmed the presence of suberin after induction of coating and showed quantitative differences between the isolines that correlated with cytological measurements of the coating response. Northem analysis of suberization-associated anionic peroxidase mRNA showed corresponding increases, and tissue blot analysis further indicated that induction of the mRNA was localized in the responding vascular bundles, as determined by suberin histochemistry. Taken together, these results provide chemical evidence that the coatings are mainly suberin. Stress, whether biotic or abiotic, frequently induces the vascular coating response in plants (for review see ref. 19). A recent study (25) ofearly interactions between the wilt fungus, Verticillium albo-atrum, and its tomato host indicated that the coating of xylem vessel walls involves the secretion and deposition of normal or modified cell wall components. The nature of the polymeric materials deposited on the cell wall remains unknown. Although pathogen-induced wall coatings have been variously designated as lignin, induced lignin, lignin-like, and suberin, and although most studies have not satisfactorily distinguished between lignification and suberization, in most cases the deposits probably result from suberization (9, 15). The same process also is known to be injuryinduced in all plant organs (5).According to the current model, suberin consists of an aromatic domain and an aliphatic domain that is probably attached to the aromatic domain (10). The phenolic monomers derived from the phenylpropanoid pathway are polym-
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