Calcium Chelator and Channel Blockers Suppress the IAA-Induced Membrane Hyperpolarization without Inhibiting the Following Growth Promotion in Hypocotyl Sections of Vigna unguiculata under Xylem Perfusion

The effects of the Ca2+ chelator EGTA and the Ca2+ channel blockers La3+, Gd3+, nifedipine, and verapamil on the IAA-induced early responses of hypocotyl sections of Vigna unguiculata were studied to assess the involvement of Ca2+ in the IAA-induced hyperpolarization followed by growth promotion using the xylem perfusion method. The IAA-induced hyperpolarization was distinctly inhibited by pretreatment with EGTA, La3+, and/or Gd3+ (5 mM each). EGTA inhibited by about 80–90% while La3+ and Gd3+ inhibited by about 60–75%. The sustained hyperpolarization after IAA application was also inhibited by these reagents. Nifedipine and verapamil (50 μM each) had little effect on the IAA-induced hyperpolarization and growth promotion. The inhibitory effects of EGTA, La3+ and Gd3+ on the membrane potentials were not due to changes in a passive component, but mainly to decreases in an elec-trogenic component. The effect of EGTA was reversible, while those of La3+ and Gd3+ were not. Present results suggest that the influx of Ca2+ through the plasma membrane might be required not only for the initiation of hyperpolarization by IAA but also for the following sustained hyperpolarization and may therefore play an essential role in the IAA-induced activation of the electrogenic proton pumps at the plasma membrane in Vigna hypocotyls. Inhibition of the hyperpolarization by EGTA, La3+, and Gd3+, however, did not involve inhibition of the promotion of growth by IAA. It seems that activation of the electrogenic proton pumps at the plasma membrane is not always the indispensable primary step of the IAA-induced promotion of elongation growth. Some alternative pathway of proton extrusion might take part.