Role of apoplast acidification by the H(+) pump : Effect on the sensitivity to pH and CO2 of iron reduction by roots of Brassica napus L.

We have studied the mechanism of the response to iron deficiency in rape (Brassica napus L.), taking into account our previous results: net H(+) extrusion maintains a pH shift between the root apoplast and the solution, and the magnitude of the pH shift decreases as the buffering power in the solution increases. The ferric stress increased the ability of roots to reduce Fe[III]EDTA. Buffering the bulk solution (without change in pH) inhibited Fe[III]EDTA reduction. At constant bulk pH, the inhibition (ratio of the Fe[III]EDTA-reduction rates measured in the presence and in the absence of buffer) increased with the rate of H(+) extrusion (modulated by the length of a pretreatment in 0.2 mM CaSO4). These results support the hypothesis that the apoplastic pH shift caused by H(+) excretion stimulated Fe[III] reduction. The shape of the curves describing the pH-dependency of Fe[III]EDTA reduction in the presence and in the absence of a buffer fitted this hypothesis. When compared to the titration curves of Fe[III]citrate and of Fe[III]EDTA, the curves describing the dependency of the reduction rate of these chelates on pH indicated that the stimulation of Fe[III] reduction by the apoplastic pH shift due to H(+) excretion could result from changes in electrostatic interactions between the chelates and the fixed chargers of the cell wall and-or plasmalemma. Blocking H(+) excretion by vanadate resulted in complete inhibiton of Fe[III] reduction, even in an acidic medium in which there was neither a pH shift nor an inhibitory effect of a buffer. This indicates that the apoplastic pH shift resulting from H(+) pumping is not the only mechanism which is involved in the coupling of Fe[III] reduction to H(+) transport. Our results shed light on the way by which the strong buffering effect of HCO 3 (-) in some soils may be involved in iron deficiency encountered by some of the plants which grow in them.

Effect of HCO - (3) concentration in the absorption solution on the energetic coupling of H(+)-cotransports in roots of Zea mays L.

The effect of HCO 3 (-) on ion absorption by young corn roots was studied in conditions allowing the independent control of both the pH of uptake solution and the CO2 partial pressure in air bubbled through the solution. The surface pH shift in the vicinity of the outer surface of the plasmalemma induced by active H(+) excretion was estimated using the initial uptake rate of acetic acid as a pH probe (Sentenac and Grignon (1987) Plant Physiol. 84, 1367). Acetic acid and orthophosphate uptake rates and NO 3 (-) accumulation were slowed down, while (86)Rb(+) uptake and K(+) accumulation rates were increased by HCO 3 (-) . These effects were similar to those induced by 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid/2-amino-2-(hydroxymethyl)-1,3-propanediol (Hepes-Tris). They were more pronounced when the H(+) excretion was strong, were rapidly reversible and were not additive to those of Hepes-Tris. The hypothesis is advanced that the buffering system CO2/H2CO3/HCO 3 (-) accelerated the diffusion of equivalent H(+) inside the cell wall towards the medium. This attenuated the surface pH shift in the vicinity the plasma membrane and affected the coupling between the proton pump and cotransport systems.