Use of plasma membrane vesicles for examination of phosphorous deficiency-induced root excretion of citrate in cluster roots of withe lupin (Lupinus albus L.)
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In cluster roots of P-deficient white lupin, intense exudation of citrate, and the contemporaneous acidification of the rhizosphere, are an adaptation to mobilise sparingly soluble soil P sources. Transport processes involved in citrate exudation were investigated through a membrane physiological approach, using isolated root plasma membrane (PM) vesicles. Exogenous application of citrate [100–250 μM] to root PM vesicle preparations stimulated H+-ATPase, activity. However, a strong inhibition occurred at concentrations >2 mM, independently of Mg supply. No such effects were observed when malate was applied. Accordingly, application of 5 mM citrate to inside-out PM vesicles decreased ATP-dependent intravesicular H+ accumulation, which was increased after the application of 5 mM malate. In contrast, lower citrate concentrations [2 mM] stimulated vesicle acidification even in the absence of Mg-ATP. Acidification was further enhanced by the addition of Mg-ATP, and was most evident in vesicles isolated from cluster roots of P-deficient plants. Moreover, the uptake of 14C citrate in the presence of ATP and PM H+-ATPase activity were particularly enhanced in PM vesicles obtained from cluster roots. The results provide evidence for a P deficiency-induced transport mechanism for citrate anions at the PM of cluster root cells, coupled with the activity of the PM H+-ATPase, On the other hand, PM H+-ATPase, activity may be modulated by concentration levels of citrate characteristic for the cytosol.