IAA myo-inositol

The seeds of oats [Avena sativa) have been studied by Dr. Percival and shown to have 85% of their IAA esterified to a glucoprotein (28). The glucan is of the lichenan type having both 61-9-3 and 6l- -4 linkages. Recently, Ms. P. Hall in our laboratory (personal communication) has isolated IAA-myo-inositol from rice [Oryza sativa) thus showing the compound originated early in cereal evolution. 

Ms. Pat Hall and Drs. J. Nowacki and E. Epstein have provided our knowledge of the amounts and rate of metabolic turnover of the indolylic components of the kernels of lea mays (cf. 27, 28 Nowacki, unpublished Epstein, unpublished). This knowledge Fas enabled us, 1) to identify the "seed auxin precursor"—that is the compound which is transported from the seed to the growing shoot (39J, and 2) has provided a portion of the proof that IAA-myo-inositol and IAA are in reversible equilibrium in the shoot tissue. Proving that IAA and IAA esters are in reversible equilibrium in the tissue is essential if we wish to postulate hormonal homeostasis. 

Table II. They showed that tryptophan, IAA and IAA-myo-inositol are turning over—that is made and then destroyed, or used, at rates such that tJj was 5, 3.2 and 12 hrs, respectively. Such data permitted several important conclusions concerning the metabolism of these compounds, for example, that it is the IAA esters, and not tryptophan, which serve as a source of IAA for the germinating seed and secondly that the IAA-inositols are turning over at such a rapid rate that they must be in equilibrium with the IAA-myo-inositol glycoside pool perhaps acting as glycosylation reagents (Epstein, unpublished).

For these experiments, minute amounts of labeled IAA, tryptophan, or IAA-myo-inositol were applied to an incision in the semi-liquid endosperm of 4 day germinated Zea seedlings. After 8 hrs of incubation, the shoots were harvested and the IAA, tryptophan or IAA-7 n/o-inositol isolated using rigorous purification techniques. Now, knowing the specific activity of the applied compound at the mid-point of the experiment we could calculate the... 

This compound was synthesized by Dr. Nowacki by reacting 11+C-IAA-imidazole with myo-inositol (40). Application of these labeled compounds to corn kernels, followed immediately by homogenization of the tissue in acetone permitted us to determine the amounts of each constiuent in the kernel by the isotope dilution method of Rittenberg and Foster (41). An extension of this method, whereby the kernels are incubated for varying periods of time after application of the isotopically labeled compound permits determination of the "turnover" of the pool. Such data are shown in... 

Dr. Hamilton earlier observed that ether extraction of tissue induced autolysis, liberating active esterases and glycosi-dases, and thus leading to more free IAA than extraction of tissue by polar, and thus, enzyme-denaturing solvents (35). Thus, we knew then that there were enzymes in the tissue capaBTe of hydrolyzing IAA esters. Much later, Kopcewicz demonstrated the presence of an enzyme system which could synthesize lAA-myo-inositol from IAA, ATP, Mg++ and CoASH (43). More recently, Mr. Lech Michalczuk (unpublished) has shown that IAA-CoA will acylate inositol only in the presence of other nucleotides. Thus, the reaction is complex, but there is no doubt that enzymes to make and hydrolyze the IAA esters are present in corn. 

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