Elicitor treatment or mechanical damage to Scutellaria baicalensis Georgi (skullcap plants) callus causes an immediate insolubilization of a 36-kDa protein into cell walls. The 36-kDa protein was identified as peroxidase 1 by analysis of its internal amino acid sequence and by immunoblotting using affinity-purified anti-peroxidase 1. Insolubilized peroxidase 1 is cross-linked to lignin through covalent bonds, and the cross-linking is catalyzed in the presence of H(2)O(2) by peroxidase 1 itself. The properties of insolubilized peroxidase 1 resemble those of defense-related structural proteins (extensins and proline-rich proteins) cross-linked to cell wall. Although the isozymes peroxidases 2 and 3 have enzyme activities similar to peroxidase 1, they are not insolubilized by stress treatment. Molecular characterization established that peroxidase 1 contains regions characteristic of structural proteins, but peroxidases 2 and 3 do not have such regions. These results suggest that among the three isozymes, only peroxidase 1 has a structural protein-like function as well as an enzymatic function.