Gibberellin‐enhanced indole‐3‐acetic acid biosynthesis: D‐Tryptophan as the precursor of indole‐3‐acetic acid
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Abstract:
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.Keywords:
Indole-3-acetic acid
Gibberellic acid
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Gibberellic acid promotes doubling, early flowering, and fruiting directly in Impatiens. It also lengthens the stem, probably through a neutralization of auxin inhibitors. Such a mechanism would leave the growth-promoting auxins unchecked to produce the increased elongation characteristic of gibberellin application. It seems that application of additional auxin produces a rapid build-up of the auxin inhibitor(s) in the plant.
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Grape (Vitis vinifera L.) is one in every of the most precious fruit plants of temperate regions, but it is successfully grown in tropical and sub-tropical agro-climatic situations. This review is undertaken to evaluate the effects of gibberellic acid on the Quality and yield of grapes. The plant hormones are extraordinarily essential agents inside the integration of developmental activities. GA3 is also called gibberellic acid, but the term gibberellic is often used in describing all gibberellins. Active gibberellins show many physiological effects, each depending on the type of gibberellin present in the grape plant. This review Results suggest foliar sprays of only gibberellic acid (combine with different chemicals) with various concentrations at the different developmental stages, increases both quality and yield in grapes.
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Spraying celery plants with gibberellic acid (GA) resulted in differing growth responses to the different treatments, such as to suggest that the response exhibited was controlled more by the age of the plant at the time of spraying than by the dosage of GA applied.
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The highly active, polar gibberellin‐like substance found in the apical region of shoots of tall (genotype Le ) peas ( Pisum sativum L.) is shown by combined gas chromatography‐mass spectrometry (GC/MS) to be GA 1 . This substance is either absent or present at only low levels in dwarf ( le ) plants. Multiple ion monitoring (MIM) tentatively suggests that GA 8 may also be present in shoot tissue of tall peas. Gibberellin A 1 is the first 3 β‐hydroxylated gibberellin positively identified in peas, and its presence in shoot tissue demonstrates the organ specificity of gibberellin production since GA 1 has not been detected in developing seeds. Application of GA 1 can mask the Le/le gene difference. However, whilst Le plants respond equally to GA 20 and GA 1 , le plants respond only weakly to GA 20 , the major biologically active gibberellin found in dwarf peas. These results suggest that the Le gene controls the production of a 3 β‐hydroxylase capable of converting GA 20 to GA 1 . Further support for this view comes from feeds of [ 3 H] GA 20 to Le and le plants. Plants with Le metabolise [ 3 H] GA 20 to three major products whilst le plants produce only one major product after the same time. The metabolite common to Le and le plants co‐chromatographs with GA 29 . The additional two metabolites in Le peas co‐chromatograph with GA 1 and GA 8 .
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Auxin promotes GA biosynthesis in the aboveground parts of plants. However, it has not been demonstrated previously that this interaction occurs in roots. To understand the interactions between auxin and GAs in these organs, we treated wild-type pea (Pisum sativum L.) roots with the inhibitors of auxin action, p-chlorophenoxyisobutyric acid (PCIB) and yokonolide B (YkB), and with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). These compounds generally downregulated GA synthesis genes and upregulated GA deactivation genes, and reduced the level of the bioactive GA1. These effects indicate that in pea roots, auxin at normal endogenous levels stimulates GA biosynthesis. We show also that supra-optimal levels of exogenous auxin reduce the endogenous level of bioactive GA in roots, although the effect appears too small to account for the strong growth-inhibitory effect of high auxin levels.
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In shoots of the garden pea ( Pisum sativum L.), the main bioactive gibberellin (GA) is GA 1 , which is synthesised from GA 20 by 3 β ‐hydroxylation. Gibberellin A 20 is produced from GA 19 , as part of the process known as GA 20‐oxidation. Because these steps are thought to be negatively regulated by GA 1 , we compared the metabolism of labelled GA 19 and GA 20 in mutants deficient in GA 1 , with that observed in isogenic wild‐type (WT) plants. There was a large and specific increase in the 3 β ‐hydroxylation of labelled GA 20 in the GA 1 ‐deficient (dwarf) mutants, compared with the WT. Metabolism experiments did not provide convincing evidence for feedback regulation of 20‐oxidation, possibly because GA 19 akppears to be metabolised rapidly, even in WT pea shoots. Both 3 β ‐hydroxylase and 20‐oxidase transcript levels were markedly higher in the mutants than in isogenic WT lines. The results sukpport previous suggestions that both biosynthetic steps are feedback‐regulated by GA 1 in pea.
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Gibberellic acid promotes doubling, early flowering, and fruiting directly in Impatiens. It also lengthens the stem, probably through a neutralization of auxin inhibitors. Such a mechanism would leave the growth-promoting auxins unchecked to produce the increased elongation characteristic of gibberellin application. It seems that application of additional auxin produces a rapid build-up of the auxin inhibitor(s) in the plant.
Gibberellic acid
Elongation
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