UPTAKE, ACCUMULATION AND METABOLISM OF AUXINS IN TOBACCO LEAF PROTOPLASTS

Uptake and metabolism of exogenous naphthalene-l-acetic acid (NAA) and indole-3-acetic acid (IAA) have been studied in tobacco (Nicotiana ta bacum L. cv. Xanthi) mesophyll protoplasts. Both auxins entered protop lasts by diffusion under the action of the transmembrane pH gradient w ithout any detectable participation of an influx carrier. Molecules we re accumulated by an anion-trapping mechanism and most of them were me tabolized within hours, essentially as glucose-ester and amino-acid co njugates. Protoplasts were equipped with a functional auxin-efflux car rier as evidenced by the inhibitory effect of naphthylphtalamic acid o n IAA efflux. Basically, similar mechanisms of NAA and IAA uptake occu rred in protoplasts. However, the two auxins differed in their levels of accumulation, due to different membrane-transport characteristics, and the nature of the metabolites produced. This shows the need to est imate the accumulation and the metabolism of auxins when analyzing the ir effects in a given cell system. The internal auxin concentration co uld be modulated by changing the transmembrane pH gradient, giving an interesting perspective for discriminating between the effects of intr a- and extracellular auxin on physiological processes.