AN AUXIN-RESISTANT MUTANT OF NICOTIANA-PLUMBAGINIFOLIA VIV IS IMPAIRED IN 1-NAPHTHALENEACETIC ACID-INDUCED HYPERPOLARIZATION OF HYPOCOTYL CELL-MEMBRANES IN INTACT SEEDLINGS

The electrical response to the synthetic auxin 1-naphthaleneacetic aci d (1-NAA) of Nicotiana plumbaginifolia wild type and the monogenic, do minant auxin-resistant mutant R25 was studied. Membrane potentials wer e continuously recorded in hypocotyl cells of light-grown, intact seed lings, and the time course of the response to 1-NAA addition was follo wed. Wild-type cells responded to greater than or equal to 10(-5) M1-N AA with a delayed, transient hyperpolarization. The R25 cells hyperpol arized significantly only in response to 1-NAA at 10(-3) M, and with m aximal amplitudes lower than those recorded with the wild type. In con trast, the two genotypes reacted similarly in terms of kinetics and am plitude to 10(-)5 M fusicoccin, which rapidly and strongly hyperpolari zed the cells, and to 10(-3) M benzoic acid, which induced rapid and w eak hyperpolarization. The resting membrane potentials of the wild typ e and R25 were also not significantly different. Unlike wild-type hypo cotyls, those of R25 ceased elongating before the time chosen for the electrophysiological measurements, but control experiments performed a t a time when the elongation of both genotypes had terminated indicate d that the difference in electrical response to auxin is independent o f hypocotyl growth. The inefficiency of 1-NAA in inducing hyperpolariz ation of R25 hypocotyl cells suggests a defect at an early step in aux in action.