Localization and Capacity of Proton Pumps in Roots of Intact Sunflower Plants

Abstract

Proton extrusion by roots of intact sunflower plants (Helianthus annuas L.) was studied in nutrient solutions or in agar media with a pH indicator. Proton extrusion was enhanced by either iron deficiency, addition of fusicoccin, or single salt solutions of ammonium or potassium salts. The three types of proton extrusion differ in both localization along the roots and capacity. From their sensitivity to ATPase inhibitors it seems justified to characterize them as proton pumps driven by plasma membrane APTases.Enhanced proton extrusion induced by preferential cation uptake from (NH.)2SO4 or K2SO4 was uniformly distributed over the whole root system. In contrast, the enhancement effect of fusicoccin was confied to the basal root zones and that of iron deficiency to the apical root zones. Also the rates of proton extrusion per unit of root fresh weight differed remarkably and increased in the order. Fusicoccin << K2SO4< (NH4hSO4 < iron deficiency.Under iron deficiency the average values of proton extrusion for the whole root system are 5.6 micromoles H' per gram fresh weight per hour, however, for the apical root zones values of about 28 micromoles H can be calculated. This high capacity is most probably related to the iron deficiency-induced formation of rhizodermal transfer cells in the apical root zones. It can be assumed that the various types of root-induced acidification of the rhizosphere are of considerable ecological importance for the plant-soil relationships in general and for mobilization of mineral nutrients from sparingly soluble sources in particular.In higher plants, electrogenic proton pumps are localized in various membranes, such as the plasma membranes, tonoplast or mitochondrial membranes (1-4, 7). It importance. Acidification ofthe soil/root interface (rhizosphere) not only mobilizes mineral nutrients (e.g. P, Fe, Zn; [5,20]) but also affects the activity of microorganisms such as Rhizobium (16) or pathogenic fungi (23,27). This acidification of the rhizosphere can be easily demonstrated by agar techniques and depends on plant species, plant age, form and level of nutrient supply, and buffer capacity of the soil (13). Furthermore, deficiency of phosphorous (8) In this paper we report on results with intact sunflower plants on both the rate of proton extrusion and localization of proton pumps along roots as induced by iron deficiency, FC2 and different nutrient supply. In a following paper the properties of the iron deficiency-induced proton pumps will be described in more detail. MATERIALS AND METHODSGrowth of plants. Seeds of sunflower (Helianthus annuus L. cv Sobrid) were germinated for 3 d in quartz sand moistened with saturated CaS04 solution and subsequently transferred to a continuously aerated nutrient solution (25 plants per 1.5-L plas-

The global regulator GacA of Pseudomonas fluorescens CHA0 is required for suppression of root diseases in dicotyledons but not in Gramineae

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Iron homeostasis related genes in rice

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