Metabolism of indole-3-acetic acid in seedlings of Zea mays L

Nonhebel, Heather Margaret (1982) Metabolism of indole-3-acetic acid in seedlings of Zea mays L. PhD thesis, University of Glasgow.

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The metabolism of exogenous indole-3-acetic acid (IAA) was examined in root and coleoptile segments taken from dark-grown seedlings of Zea mays L. cv. Fronica. The rate of disappearance of IAA, and the chemical nature of the total spectrum of products in methanolic extracts of plant material which had been incubated with IAA for 2h were investigated. The difference in IAA metabolism in separated cortex plus epidermis, and stelar tissues of the root was monitored. Two methods of supplying IAA to the segments were employed, and the effect of the mode of application on the metabolism pattern noted. The metabolism of IAA, transported through coleoptile and root segments was also studied. In each experiment, the uptake of exogenous IAA was noted and compared with published measurements of endogenous IAA levels. Finally, experiments were carried out to investigate the effect of IAA on coleoptile and root elongation. Segments were usually supplied with IAA by floating them in aqueous solutions of the radioactively-labelled compound. Alternatively, agar blocks containing 14C-IAA were placed at the apical ends of coleoptile segments, or on protruding portions of stele at the basal ends of root segments. All incubations were carried out in darkness. Crude methanolic extracts of plant material were analysed using a gradient-elution, reverse-phase high-performance liquid chromatograph with an on-stream homogeneous radioactivity monitor. Sample purification was kept to a minimum to avoid selective loss of metabolites. Control experiments showed that degradation of IAA did not occur during sample preparation or analysis. Information on the chemical nature of metabolites was obtained from high-performance liquid chromatograph retention times, use of IAA labelled with 14C at different positions on the molecule, examination of the products of base-catalysed hydrolysis and methylation with diazomethane, UV spectrophotometry, and co-chromatography with standards of IAA derivatives. The identity of the IAA remaining at the end of experiments was confirmed by combined gas chromatography-mass spectrometry. IAA was metabolised rapidly by both roots and coleoptiles to a large number of products; at least 11 in roots. All metabolites, in methanolic extracts of tissue incubated for 2h, were more polar than IAA and did not involve decarboxylation. The main product in both roots and coleoptiles appeared to be oxindole-3-acetic acid. Other tentative identifications included 5-hydroxyindole-3-acetic acid, indole-3-acetyl glycine and an ester of IAA possibly with a sugar or myo-inositol. The appearance of several products, in similar quantities, after very short incubation times (10 min), indicated that metabolism probably did not take a single, linear pathway. Experiments using sterile tissues confirmed that metabolism was taking place within the plant cells. At the lowest external concentrations lised, the amounts of 14C-IAA taken into the plant represented only a small fraction of endogenous levels (measurements obtained from published data). Increasing the concentration of IAA over two orders of magnitude did not alter the pattern of metabolism. The products observed were thus unlikely to represent results of a detoxification process. Comparison of IAA metabolism by cortical and stelar tissues of the root indicated that the majority of metabolism took place in the cortex. IAA present in the stele, the site of the majority of endogenous IAA in the root, appeared to be protected from metabolism. The mode in which IAA was supplied to roots had a substantial effect on the metabolism pattern. IAA applied in agar blocks to the stele at the basal end of segments was metabolised more slowly than that taken up from solution. The relative proportions of products also differed. Although substantial metabolism took place in all experiments, radioactivity transported through both root and coleoptile segments, and collected in agar blocks, remained exclusively associated with the IAA molecule. This provided further evidence for a transport system specific for IAA. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: J R Hillman
Keywords: Plant sciences
Date of Award: 1982
Depositing User: Enlighten Team
Unique ID: glathesis:1982-72749
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Jun 2019 11:06
Last Modified: 11 Jun 2019 11:06

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