Electrophysiological responses of maize roots to low water potentials: relationship to growth and ABA accumulation

The maintenance of root elongation is an important adaptive response to low water potentials (psi(w)), but little is known about its regulation. An important component may be changes in root cell electrophysiology, which both signal and maintain growth maintenance processes. As a first test of this hypothesis, membrane potentials (E(m)) were measured within the cell elongation zone of maize (Zea mays L.) primary roots. Seedlings were grown in oxygenated solution culture, and low psi(w) was imposed by the gradual addition of polyethylene glycol. Cells hyperpolarized approximately 25 mV in response to low psi(w), and after 48 h resting potentials remained significantly hyperpolarized at psi(w) lower than -0.3 MPa compared with roots at high psi(w). Inhibitor experiments showed that the hyperpolarization was dependent on plasma membrane H(+)-ATPase activity. Previous work showed that accumulation of abscisic acid (ABA) is required for the maintenance of maize primary root elongation at low psi(w). To determine if the mechanism of action of ABA involves changes in root electrophysiology, E(m) measurements were made during long-term exposure to low psi(w). Steady-state resting E(m) were measured in regions in which maintenance of cell elongation was dependent on ABA accumulation (2-3 mm from the apex), or in which elongation was inhibited regardless of ABA status (6-8 mm from the apex). E(m) was substantially more negative in ABA-deficient roots specifically in the 2-3 mm region. The results suggest that set-points for ion homeostasis shifted in association with the maintenance of root cell elongation at low psi(w), and that ABA accumulation plays a role in regulating the ion transport processes involved in this response.