An investigation of the fate of Exogenous Auxin in the pea, with special reference to the formation and role of Auxin Conjugates

Abstract

The physiological significance of indole-acetyl aspartic acid (lAAsp) in auxin metabolism was examined with special reference to its role as part of a homeostatic mechanism to control hormone levels in pea seedlings (Pisum sativum L.)• A technique is described for the chemical synthesis of lAAsp. This was successfully used to obtain samples of the authentic compound. Techniques for the isolation, purification and concentration of lAAsp from plant tissues were examined in detail and a technique giving improved recoveries of the compound is described. Several techniques were used in an attempt to obtain more reliable identification of the presumed lAAsp in plant extracts. These included paper and thin layer chromatography in several solvent systems; mass spectroscopy (MS); gas chromatography/mass spectroscopy (GC-MS) and acid and alkali hydrolysis of the isolated compound. Although the presumed lAAsp consistently co-chromatographed with samples of the authentic compound, technical problems during isolation and derivatization prevented its successful identification by MS and GC-MS. When the presumed 14C-labelled lAAsp was reapplied to intact pea plants, 14C was found with indoleacetic acid (lAA) indicating that the conjugate could be hydrolysed at the presumed peptide bond to release the free hormone. The formation of lAAsp by plants may therefore represent a means of storing excess lAA in a protected and inactive form from which it can be released if tissue concentrations fall' The time course of uptake, conjugation and export of 14C-IAA following its application to the apfcgs of intact pea seedlings was examined in relation to photomorphogenesis and dwarfism.

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