Protein synthesis and Golgi-mediated vesicle traffic is not required to maintain a polarised membrane voltage in the presence of auxin

The contribution of protein synthesis and secretion to indol acetic acid (IAA) induced polarisation of the plasma membrane voltage (V(M)) was investigated. The V(M) of coleoptiles from Zea mays was measured in the presence of known inhibitors of protein- and RNA synthesis, as well as those of Golgi-mediated vesicle secretion. Inhibitors were applied under conditions at which they are known to abolish IAA stimulated H+ secretion and cell elongation effectively. Cycloheximide (CHI), an inhibitor of protein synthesis, caused depolarisation of V(M) with a half maximal concentration of approximately 20 μM. At 100 μM CHI. V(M) depolarised to a new stable voltage with a half time of 9.8 ± 0.6 min. The temporal similarity of CHI-induced depolarisation and cessation of coleoptile elongation suggests that the induced change in V(M) underlies inhibition of elongation. CHI evoked membrane depolarisation to a final voltage of about -100 mV irrespective of the presence or absence of auxin in the external medium. Thus, CHI probably affected constitutive membrane transport properties independently of IAA-induced modulation of transport proteins. Cordycepin (COR), an inhibitor of RNA synthesis, had no significant effect at 400 μM on V(M) of IAA-treated cells, suggesting that gene transcription for transport- or regulatory protein synthesis was not essential for IAA-generated polarisation of V(M). Brefeldin-A (BFA), an inhibitor of Golgi-mediated vesicle secretion in maize coleoptiles, had no perceivable effect at 20 mg/l on V(M) of IAA-treated coleoptile cells, demonstrating that constitutive or IAA-stimulated protein secretion was not essential for the mechanism underlying IAA-evoked V(M) polarisation. Hence, IAA-stimulated and COR/BFA-depressed H+ extrusion in elongating coleoptiles may not be entirely mediated by auxin-enhanced ATPase activity.