Free indole-3-acetic acid levels were measured by gas chromatography-mass spectrometry in three ultra-tall ;slender' Pisum sativum L. lines differing in gibberellin content. Measurements were made for apices and stem elongation zones of light-grown plants and values were compared with wild-type, dwarf, and nana phenotypes in which internode length is genetically regulated, purportedly via the gibberellin level. Indole-3-acetic acid levels of growing stems paralleled growth rates in all lines, and were high in all three slender genotypes. Growth was inhibited by p-chlorophenoxyisobutyric acid, demonstrating the requirement of auxin activity for stem elongation, and also by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. It is concluded that the slender phenotype may arise from constant activation of a gibberellin receptor or transduction chain event leading directly or indirectly to elevated levels of indole-3-acetic acid, and that increased indole-3-acetic acid levels are a significant factor in the promotion of stem elongation.
The gibberellin biosynthesis inhibitor uniconazol reduces both the elongation and indole‐3‐acetic acid content of growing Pisum sativum cv. Alaska intemodes. Both internode growth and indole‐3‐acetic acid content in uniconazol‐treated plants can be elevated by gibberellin A3 treatment. The lengths of the growing intemodes are directly related to the indole‐3‐acetic acid contents.
Stem segments excised from light‐grown Pisum sativum L. (cv. Little Marvel) plants elongated in the presence of indole‐3‐acetic acid and its precursors, except for L‐tryptophan, which required the addition of gibberellin A, for induction of growth. Segment elongation was promoted by D‐tryptophan without a requirement for gibberellin, and growth in the presence of both D‐tryptophan and L‐tryptophan with gibberellin A3, was inhibited by the D‐aminotransferase inhibitor D‐cycloserine. Tryp‐tophan racemase activity was detected in apices and promoted conversion of L‐tryptophan to the D isomer; this activity was enhanced by gibberellin A3. When applied to apices of intact untreated plants, radiolabeled D‐tryptophan was converted to indole‐3‐acetic acid and indoleacetylaspartic acid much more readily than L‐tryptophan. Treatment of plants with gibberellin A3, 3 days prior to application of labeled tryptophan increased conversion of L‐tryptophan to the free auxin and its conjugate by more than 3‐fold, and led to labeling of N‐malonyl‐D‐tryptophan. It is proposed that gibberellin increases the biosynthesis of indole‐3‐acetic acid by regulating the conversion of L‐tryptophan to D‐tryptophan, which is then converted to the auxin.
Two-week-old dwarf peas (Pisum sativum cv Little Marvel) were sprayed with gibberellic acid (GA3), and after 3 or 4 days the upper stem and young leaf samples were analyzed for indole-3-acetic acid (IAA) and indole-3-acetyl aspartic acid by an isotope dilution high performance liquid chromatography method. GA3 increased IAA levels as much as 8-fold and decreased indole-3-acetyl aspartic acid levels.
Exogenously applied indole-3-acetic acid (IAA) strongly promoted stem elongation over the long term in intact light-grown seedlings of both dwarf (cv Progress No. 9) and tall (cv Alaska) peas (Pisum sativum L.), with the relative promotion being far greater in dwarf plants. In dwarf seedlings, solutions of IAA (between 10-4 and 10-3 M), when continuously applied to the uppermost two internodes via a cotton wick, increased whole-stem growth by at least 6-fold over the first 24 h. The magnitude of growth promotion correlated with the applied IAA concentration from 10-6 to 10-3 M, particularly over the first 6 h of application. IAA applied only to the apical bud or the uppermost internode of the seedling stimulated a biphasic growth response in the uppermost internode and the immediately lower internode, with the response in the latter being greatly delayed. This demonstrates that exogenous IAA effectively promotes growth as it is transported through intact stems. IAA withdrawal and reapplication at various times enabled the separation of the initial growth response (IGR) and prolonged growth response (PGR) induced by auxin. The IGR was inducible by at least 1 order of magnitude lower IAA concentrations than the PGR, suggesting that the process underlying the IGR is more sensitive to auxin induction. In contrast to the magnitude of the IAA effect in dwarf seedlings, applied IAA only doubled the growth in tall seedlings. These results suggest that endogenous IAA is more growth limiting in dwarf plants than in tall plants, and that auxin promotes stem elongation in the intact plant probably by the same mechanism of action as in isolated stem segments. However, since dwarf plants to which IAA was applied failed to reach the growth rate of tall plants, auxin cannot be the only limiting factor for stem growth in peas.
Endogenous indole-3-acetic acid (IAA) was found in axenically cultured gametophytes of the leafy liverwort, Plagiochila arctica Bryhn and Kaal., by high-performance liquid chromatography with electrochemical detection. Identification of the methylated auxin was confirmed by gas chromatography-mass spectrometry. Addition of 57 micromolar IAA to cultures increased relative production of ethylene. This is the first definitive (gas chromatography-mass spectrometry) demonstration of the natural occurrence of IAA in a bryophyte.
