Natriuretic peptides and cGMP modulate K+, Na+, and H+ fluxes in Zea mays roots.

Recent evidence suggests that in plants, as in vertebrates, natriuretic peptides (NPs) regulate homeostasis. In this study noninvasive ion-selective vibrating microelectrodes were used to measure net fluxes of K+, Na+, and H+ in Zea mays root conductive tissue. Immunoreactant plant natriuretic peptides (irPNP) cause immediate net H+ influx and delayed net K+ and Na+ uptake. Delayed net K+ influx was also observed in response to 8-Br-cGMP, however, not accompanied by significant changes in net H+ fluxes. Furthermore, 8-Br-cGMP does not stimulate the plasma membrane H+-ATPase implying that cGMP directly affects cation channels. The data are consistent with NP and cGMP-dependent stimulation of nonselective cation channels with P(K) > P(Na) and point to a complex role for NPs in plant homeostasis.

Osmotic sensitivity of Ca2+ and H+ transporters in corn roots: effect on fluxes and their oscillations in the elongation region.

Seedling roots of corn were treated with different concentrations of mannitol-containing solution for 1 to 1.5 hr, and net fluxes of Ca2+ and H+ were measured in the elongation region. H+ fluxes were much more sensitive to osmotic pressure than were Ca2+ fluxes. Oscillations of 7-min period in H+ flux, normally observed in the control, were almost fully suppressed at high osmotic concentrations. Net H+ flux was shifted from average efflux of 25 +/- 3 nmol m-2 sec-1 to average influx of 10 +/- 5 nmol m-2 sec-1 after the incubation in 100 mm mannitol. The larger the osmotic concentration, the larger was the H+ influx. This flux caused the unbuffered solution of pH 4.85 to change to pH 5.3 after mannitol application. It appears that the osmoticum suppresses oscillatory H+ extrusion at the plasma membrane. Discrete Fourier Transforms of the H+ flux data showed that, apart from suppression of the 7-min oscillations in H+ flux, mannitol also promoted the appearance of faster 2-min oscillations. Ca2+ influx slightly increased after mannitol treatment. In addition the 7-min oscillatory component of Ca2+ flux remained apparent thereby showing independence of H+ flux.

Proton and calcium flux oscillations in the elongation region correlate with root nutation.

The involvement of Ca2+ and H+ flux oscillations in root nutation was studied for decapped roots of corn (Zea mays L. cv. Aussie Gold) placed horizontally. Net ion fluxes were measured around the elongation and meristematic regions using a microelectrode ion flux measuring system. High correlation between H+ flux oscillations and root nutations was found in the elongation region. Two oscillatory components of H+ flux, with periods of about 90 min and 7 min, correlated with root circumnutations and micronutations, respectively. The periods of H+ flux oscillations and rhythmical root movements in this region could be modified similarly by external factors including pH. In the meristematic region no association between ion flux behaviour and nutation was apparent. Ion flux oscillations and nutations both decreased in amplitude as the growth rate at the measured location decreased. Possible involvement of ion flux oscillations in root circumnutation is discussed. It is concluded that a model involving an internal oscillator must be developed to explain the H+ flux involvement in root nutations.

Oscillations in H+ and Ca2+ Ion Fluxes around the Elongation Region of Corn Roots and Effects of External pH.

Net fluxes of H+ and Ca2+ around the elongation region of low-salt corn (Zea mays L.) roots were measured using the microelectrode ion flux estimation (MIFE) technique. At pH 5.2 two oscillatory components were found. Fast, 7-min oscillations in H+ flux were superimposed on slow oscillations of about 1.5 h. Fast oscillations in Ca2+ flux showed a strong dependence on the H+ oscillations and were normally leading in phase by about 1 to 1.5 min. Both oscillatory components were strongly affected by external pH values. Preincubation for 20 h in buffered pH 4.0 solution suppressed the slow oscillatory component and caused huge H+ influxes in the elongation region. The fast oscillations were 8 times larger in amplitude and were slightly lengthened. Preincubation at pH 6.0 did not suppress the rhythmic character of the ion fluxes but it shifted the average H+ flux to greater efflux. The fast and slow oscillatory components of H+ flux seem to relate to biophysical and biochemical mechanisms of intracellular pH homeostasis, respectively. The origin of the Ca2+ flux oscillations is discussed in terms of the "weak acid Donnan Manning" model of cell wall ion exchanges.