The Uptake of Growth Substances: IX. FURTHER STUDIES OF THE MECHANISM OF UPTAKE OF 2,3,6-TRICHLOROBENZOIC ACID BY AVENA SEGMENTS

In a previous paper it was established that during the courseof uptake of radioactive 2,3,6-trichlorobenzoic acid (2,3,6-TCBA)by mesocotyl segments of Avena, the rate, initially positive,became negative within six hours. This phase of net loss isprevented by streptomycin and by synthalin, while an enhancementof accumulation is brought about by cetyl trimethylammoniumbromide (CTAB). It was postulated that the initial accumulationis governed by an unstable accumulatory process (Type 1) whichinvolves adsorption by some cell membrane system through aninteraction between the carboxyl anion of the growth-regulatormolecule and the quaternary ammonium group of the choline moietyof -lecithin. Hydrolysis of lecithin by phospholipase-D destroysthis Type 1 binding, while cationic nitrogen compounds maintainpositive uptake by competing with the choline quaternary ammoniumgroup of -lecithin for the anionic site of phospholipase-D. The effects of pretreatment at a low temperature on the subsequentuptake of 2,3,6-TCBA and the influence of pH on the course ofuptake, as well as studies of the egress of choline, providesome support for the role of phospholipase-D in determiningthe instability of the accumulatory system. Synthalin and CTAB inhibit the activity of phospholipase-D invitro. However, other investigations with this enzyme alsoemphasize that such inhibition can only partially account forthe great enhancement of the uptake of TCBA produced by CTAB.Related experiments on the uptake of alkyl pyridinium compoundsby Avena segments and on their adsorption to lecithin in vitrofavour a suggestion that quaternary ammonium compounds, suchas CTAB, act largely by providing aftificial Type 1 sites. The mechanism and significance of Type 1 accumulation are discussedand compared with similar postulates for the binding of auxinsand salts.