The regulatory pigment phytochrome induces rapid and opposite growth changes in different regions of etiolated maize seedlings: it stimulates the elongation rate of coleoptiles and inhibits that of mesocotyls. As measured by a quantitative immunoassay, phytochrome also promotes rapid and opposite changes in the extractable content of a Mr 98,000 anionic isoperoxidase in the cell walls of these same organs: it induces a decrease of this peroxidase in coleoptiles and an increase in mesocotyls. The peroxidase changes precede the growth changes. As measured by video stereomicroscopy or a position transducer, red light (R), which photoactivates phytochrome, stimulates coleoptile elongation with a lag of about 15-20 min and suppresses mesocotyl growth with a lag of 45-50 min. R also induces a 50% reduction in the extractable level of the anionic peroxidase in coleoptile walls in less than 10 min and a 40% increase in the level of this peroxidase in mesocotyl walls within 30 min. Ascorbic acid, an inhibitor of peroxidase activity, blocks the effects of R on mesocotyl section growth.These results are relevant to hypotheses that postulate that certain wall peroxidases can participate in light-induced changes in growth rate by their effects on wall extensibility.The regulatory pigment phytochrome can initiate rapid growth changes in plants. In etiolated seedlings of oats, maize, and other grasses, the photoactivation of phytochrome by red light (R) stimulates two distinct growth responses: it promotes coleoptile elongation and inhibits mesocotyl elongation (1).Changes in the extensibility of cell walls can help to mediate certain environmentally stimulated growth changes in plants (2), including those induced by phytochrome (3). Cell wall extensibility is at least partially controlled in many plants by one or more wall-localized enzymes that catalyze the formation or breakage of cell wall bonds (4). Consistent with this finding, several authors have noted that there is an inverse correlation between wall peroxidase activity and the growth of cell walls (5); e.g., when cucumber hypocotyl elongation is inhibited by blue light, there is an increase in wall peroxidase activity (6). The inverse correlation is consistent with the function of wall peroxidases in cross-linking wall macromolecules (7). To the extent peroxidases can decrease wall extensibility, they may help to mediate the inhibitory effects Ca2+ can have on wall extensibility (8), for Ca2+ can stimulate both the activity and secretion of wall peroxidases (9).There are numerous wall isoperoxidases, and there has been little or no previously published information correlating induced growth changes with changes in the content of any specific wall peroxidase isozyme. We recently characterized a monoclonal antibody, mWP3, which specifically crossreacts with a Mr 98,000 anionic peroxidase, which represents 15% of the total peroxidases present in soluble extracts of wall proteins from maize seedlings (10). Analysis by immunogold localization methods revealed t...

A library of 22 hybridomas, which make antibodies to soluble wall antigens from the coleoptiles and primary leaves of etiolated corn (Zea mays L.) seedlings, was raised and cloned three times by limit dilution to assure monoclonal growth and stability. Two of these hybridomas made immunoglobulin G antibodies, designated mWP3 and mWP19, which both effectively immunoprecipitated peroxidase activity from crude and partially purified preparations of wall peroxidases. Direct peroxidase-binding assays revealed that both antibodies bound enzymes with peroxidase activity. As judged by immunoblot analyses, mWP3 recognized a Mr 98,000 wall peroxidase with an isoelectric point near 4.2, and mWP19 recognized a Mr 58,000 wall peroxidase. Immunogold localization studies showed both peroxidases are predominately in cell walls.