Young stem sections of dwarf peas (Progress No. 9) grown in light contain at least seven peroxidase isozymes separable by electrophoresis on starch gel. An eighth isozyme appears as the tissue elongates and ages, on or off the plant. The appearance of this isozyme in excised sections is repressed by application of the plant growth hormone, indole-3-acetic acid.

Summlary. Joint application of gibberellic acid and indole-3-acetic acid to excised stem sections, terminal cuttings, and decapitated plants of a green dwarf pea results in a markedly synergistic growth response to these hormones. Synergism in green tall pea stem sections is comparatively small, although growth is kinetically indistinguishable from similarly treated dwarf sections.Gibberellin-induced growth does not appear to be mediated throtugh its effect on auxin synthesis, since gibberellin pretreatment of dwarf cuttings fails to elicit an enhanced tryptophan-induced growth response of sections, whereas auxin-induced growth is strongly enhanced. Also, tryptophan-gibberellin synergism is not significant in sections and cuttings of green dwarf peas, while auxin-gibberellin synergism is.Administration of gibberellic acid prior to indole-3-acetic acid results in greatly increased growth. In reversed order, the application fails to produce any synergistic interaction. This indicates that gibberellin action must precede aulxin action in growth regulation.Both gibberellins and auxins contribute to growth in length of excised etiolated Alaska pea epicotyl sections (4, 7, 16) and green pea stem sections (1,3) although gibberellin is less effective in green sections. \Vhen gibberellin and auxin are applied together to etiolated Alaska sections, growth is either equal to or less than the sum of the separate responses (17). This has led to the conclusion that gibberellin and auxin make independent contributions to stem growth throtugh different biochemical pathways. In contrast to these results, a small degree of atuxin-gibberellin synergism has been demonstrated in sections from Progress No. 9 pea seedlings grown in weak red light (21).Joint application of the 2 hormones to green pea tissuies, such as Alaska stem sections (3) and Meteor stem sections (1) results in a marked synergistic interaction between auxin and gibberellin, being more pronounced in the latter (dwarf) cultivar. These results, together with those derived from the petiole of single-node cuttings of sweet potato (11) indicate that gibberellin-induced growth is dependent on auxin. It thus appears that auxingibberellin synergism is a phenomenon characteristic of sections from dwarf peas, and is more

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Peroxidase activity in excised stem segments of Pisum sativum L. cv. Progress No. 9 increases linearly during incubation in buffered medium. Gibberellic acid (GA) causes a slight reduction in activity whereas indoleacetic acid (IAA) treatment completely prevents this rise. Excision produces two new cathodic isoperoxidases near the cut ends. Their appearance is prevented by cycloheximide, actinomycin D, and IAA; G A enhances this IAA-induced repression. GA alone stimulates one of the isozymes but does not affect the other one. Peroxidase leakage is stimulated by GA and inhibited by IAA treatment. The activity is entirely confined to the isoperoxidases produced in response to injury.IAA oxidase activity in incubated segments was slightly elevated over that of freshly cut ones. However, differences in activity among hormone treatments were small. IAA oxidase was also demonstrated in growth media and only IAA treatment reduced activity. Both peroxidases in the medium were isolated by column chromatography. Surprisingly, the purified isozymes appear to be essentially devoid of IAA oxidase activity.

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