Abstract
Peroxidase localization is cytoplasmic in juvenile onion root tissues and associated with the walls in mature cells. The enzyme reaction is strongest in tissues which do not lignify, but is wholly absent in the lignifying xylem throughout all stages of development. One wall surface—the cortical-endodermal junction—exhibits a thermostable, cyanide-insensitive peroxidase reaction. This "pseudoperoxidase" is located at a site which stains most heavily for pectic material. Phenolic deposition occurs at the opposite wall surface in the endodermal cell. The reciprocal interaction observed between peroxidase enzyme and the plant growth hormone, indoleacetic acid, is not compatible with the concept that plant peroxidase is identical with indoleacetic acid-oxidase. Although auxin treatment changes the pattern of peroxidase localization in cell walls, the accumulation of hormone is greatest in hypodermal cells which are strongly positive for peroxidase. A mutual exclusion phenomenon observed for peroxidase and sulfhydryl proteins localized in the cells of young root tissues does not persist into the mature tissues where these two reactions can occur at the same wall sites. The data do not support the theory that plant peroxidase is involved in cell wall synthesis. Its occurrence in conjunction with cell wall fractions may be incidental and only indirectly related to the natural redox function of the enzyme.