Plasma membrane H⁺-ATPase-dependent citrate exudation from cluster roots of phosphate-deficient white lupin

ABSTRACT

White lupin (Lupinus albus L.) is able to grow on soils with

sparingly available phosphate (P) by producing specialized

structures called cluster roots.To mobilize sparingly soluble

P forms in soils, cluster roots release substantial amounts of

carboxylates and concomitantly acidify the rhizosphere.The

relationship between acidification and carboxylate exudation

is still largely unknown. In the present work,we studied

the linkage between organic acids (malate and citrate) and

proton exudations in cluster roots of P-deficient white lupin.

After the illumination started, citrate exudation increased

transiently and reached a maximum after 5 h. This effect

was accompanied by a strong acidification of the external

medium and alkalinization of the cytosol, as evidenced by in

vivo nuclear magnetic resonance (NMR) analysis. Fusicoccin,

nuclear magnetic resonance (NMR) analysis. Fusicoccin,an activator of the plasmamembrane (PM)H+-ATPase,

stimulated citrate exudation, whereas vanadate, an inhibitor

of the H+-ATPase, reduced citrate exudation. The burst of

citrate exudation was associated with an increase in expression

of theLHA1PMH+-ATPase gene,an increased amount

of H+-ATPase protein, a shift in pH optimum of the enzymeand post-translational modification of an H

++-ATPase

protein involving binding of activating 14-3-3 protein.Taken

together, our results indicate a close link in cluster roots of

P-deficient white lupin between the burst of citrate exudation

and PM H+-ATPase-catalysed proton efflux.