Metabolism of 1-naphthaleneacetic acid in explants of tobacco: Evidence for release of free hormone from conjugates

1-Naphthaleneacetic acid (1-NAA), required for in vitro flower bud formation, was taken up by pedicel explants of tobacco (Nicotiana tabacum L.) in large amounts and rapidly metabolized into various conjugates. These conjugates have been tentatively identified in four thin-layer Chromatographic systems using authentic standards as references. The major metabolite formed during the first hours of culture comigrated with 1-NAA-glucoside (1-NAGlu). From the 6th hour on, most 1-NAA had been converted into a yet unidentified metabolite. 1-NAglu was an intermediate in the formation of this metabolite. After 24 h, 1-NAA-aspartate (1-NAAsp) became the second major metabolite. The increase in 1-NAAsp formation was induced by 1-NAA. The inactive analog 2-naphthaleneacetic acid (2-NAA) was metabolized similar to 1-NAA, but was unable to increase the formation of the aspartate conjugate. When explants were fed labeled 1-NAGlu, 1-NAAsp or the major unidentified metabolite, radioactivity became associated with free 1-NAA and all major conjugates, indicating interconversion of conjugates and breakdown to free 1-NAA. A regulatory role of conjugation in maintaining a particular level of free 1-NAA in the tissue is proposed herein.