The Dynamics of Auxin Transport in Tobacco Cells

The polar transport of auxins plays a key role in the regulation of growth and development in plants. The experimental evidence indicates that the polarity of auxin transport through cells and tissues probably results from the asymmetrical distribution of auxin efflux carriers in the plasma membrane. A substantial amount of molecular and cytological data about the polar auxin transport machinery comes from studies at the tissue level on Arabidopsis thaliana and its mutants. However, there is a need for some alternative experimental system that would enable us to study auxin transport from biochemical and cytological points of view in more detailed way. Cell suspension cultures provide good model systems in which to study biochemical and cytological aspects of auxin transport and to investigate the responses of transport to various inhibitors at the level of a single cell and its compartments. This mini-review summarises the present state-of-the-art in the investigation of the auxin transport machinery in tobacco cell cultures. The specificity of auxin efflux carriers is described, together with the effects on auxin transport and on the arrangement of the cytoskeleton and of the endoplasmic reticulum (ER), of the polar auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA) and the vesicle-trafficking inhibitor brefeldin A (BFA). The data emphasise the importance of actin filaments and possibly also of the ER in vesicle-mediated trafficking of proteins and in the cycling of the auxin efflux catalyst. The data also imply that different cellular mechanisms are involved in the actions of NPA and BFA. Contrary to the recent suggestion that auxin transport inhibitors influence auxin efflux by generally interfering with membrane trafficking processes, in tobacco cells NPA seems to act specifically