Seed auxin precursor

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the amounts and rate of metabolic turnover of the indolylic components of the kernels of lea mays (cf. 27, 28 Nowacki, unpublished Epstein, unpublished).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. 

Thus, the manner in which auxins were discovered led to confusion as to the site and mechanism of auxin synthesis. Was the hormone synthesized in the tip by some completely de novo route from simple non-aromatic precursors, or was it stored in the tip in an aromatic form such as tryptophan, or a related indolic compound that could readily be converted to lAA Alternatively, was an aromatic precursor transported from the seed to the tip of the seedling as it grew This latter possibility was suggested by Skoog [12], who called the upward transported form the seed auxin precursor , which he believed to be tryptamine. 

Radiolabeled lAA applied to the endosperm of germinating maize seedlings is transported to the shoot [113]. About 1% of the radioactivity in the shoot remained as free lAA, 2% was esterified, and 97% was metabolized to compounds not hydrolyzable to IA A. [ C]-labeled lAInos [131] was used to demonstrate that labeled lAInos applied to the endosperm could also be transported to the shoot and then hydrolyzed to yield free lAA [98,114]. lAInos-galactose, however, did not move out of the kernel and into the shoot until the galactose moiety was removed by hydrolysis. The resultant lAInos was then transported into the shoot [132]. From these results it was concluded that lAInos is a specific seed auxin precursor and that lAInos is the primary form in which LAA is transported from kernel to shoot. 

It has not previously been possible to determine the relationship between the rate of growth and the amount of the growth hormone, lA A. Early experiments involved exodiffusion of coleoptile tips, followed by a bioassay of the amount of lAA in the agar blocks that were used to collect the diffusing growth substance. Such experiments measured the capacity of the tips to supply lA A over a sustained time period and undoubtedly involved conversion of a seed auxin precursor [2, 23]. We now believe that the seed auxin precursor is indole-3-acetyl-wj< -inositol (lAInos), which upon hydrolysis yields free lAA [8,9,15,22]. Also, it is now possible to extract the tissue rapidly with aqueous acetone and to measure the free lAA by GC-MS. 

Nowacki J, Bandurski RS (1980) Myo-inositol esters of indole-3-acetic acid as seed auxin precursors of Zea mays L. Plant Physiol 65 422-427 Ohwaki Y, Tsurumi S, Nagao M (1974) Auxin transport in Vicia roots. In Plant growth substances 1973. Hirokawa, Tokyo, pp 1071-1078 Osborne DJ (1968) A theoretical model for polar auxin transport. In Vardar Y (ed) The transport of plant hormones. North-Holland, Amsterdam, pp 97-107 Osborne DJ (1974) Hormones and the shedding of leaves and bolls. Cotton Grow Rev 51 256-365... 

---