Acid phosphatase (APase) produced by the cluster roots of white lupin (Lupinus albus L.) plays an important role in inorganic phosphate (Pi) acquisition. Although the importance of cluster roots in Pi acquisition is well known, information on the distribution of APase within tissues of normal and cluster roots is lacking. Isoelectric focusing of APase isoforms as well as histochemical localization and visualization of APase were used to clarify the importance of secretory APase for P nutrition of white lupin grown under P deficiency. Isoelectric focusing revealed that both the secretory type and other major APase isoforms probably involved in P translocation were inducible. The major activity in the rhizosphere soil of cluster roots and roots grown under hydroponic conditions corresponded to LASAP2, a previously purified APase secreted from white lupin roots. Histochemical localization using enzyme-labeled fluorescence (ELF)-97 phosphate as a substrate was applied to rhizosphere samples. This substrate provides fluorescent precipitates after hydrolysis by phosphatase. Strong APase activity in the epidermal tissues of normal roots and cluster rootlets and in root hairs of cluster rootlets under P deficiency was detected. These results support the hypothesis that APase activities in the rhizosphere liberate Pi and supply it to white lupin plants grown under P-deficient conditions.
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