A rapid and sensitive auxin binding system for detecting N6-substituted adenines, and some urea and thiourea derivatives, that show cytokinin activity in cell division tests.

A selective, sensitive and rapid (2 min or less) method for detecting compounds with potential for cytokinin activity is described. The method does not measure cytokinesis; instead, it determines the ability of cytokinin-active agents to (i) activate the intake of either L-tryptophan or indoleacetic acid by germinated spores of the water-mould Achlya, while inhibiting the energy-dependent transport of all L-amino acids usually found in proteins; (ii) inhibit the energy-dependent transport of nucleosides and sugars by the same organism. The compounds with cytokinin activity generally activate auxin (tryptophan) intake at 10(-8) M or greater and inhibit at 10(-6) M or greater. The most effective activating compounds were N6-(delta2-isopentenyl)adenine, N6-benzyladenine. N6-furfuryladenine, and N6-(trans-hydroxy-3-methyl-but-2-enyl)adenine. These compounds are classed generally as cytokinins in plant growth studies. A cell membrane - localized glycopeptide of molecular weight 6000 was isolated from this organism and shown to be the site at which cytokinins, auxin, and tryptophan bind. An earlier study had also established that calcium ions bind to this entity as well. Tryptophan binding to the glycopeptide was enhanced by cytokinins, suggesting that this may be the way in which whole cells display enhanced tryptophan binding in the bioassay. On the other hand, calcium binding was antagonized by cytokinin. The results suggest that this may be an important experimental system for use in studying one possible way in which cytokinin may regulate plant growth.