Abstract
Certain kinds of plants, such as lupin and Proteaceae, form a unique root structure so-called 'cluster roots'. Cluster root is a bottle-brush like structure with dense rootlets at a certain part of secondary roots and increases the area of root surface and the amount of exudates. P deficiency significantly stimulates cluster root formation, which contributes P accumulation from unavailable forms in the soil. Mature cluster roots of lupin secrete huge amount of citrate and mobilize sparingly soluble phosphate by chelating function. It was suggested that acidification and secretion of antifungal compounds protects citrate from consumption by microbes in the rhizosphere. Secretion of acid phosphatase (APase) is also stimulated at cluster roots. It was indicated that the activity of APase in the rhizosphere of cluster roots was almost root-secreted APase instead of microbial activities. The optimal pH of the APase is 4.3. It is consistent with the citrate protection by acidification in the rhizosphere of cluster roots. Parallel increase of APase and citrate exudation could be important for organic P mobilization, because organic P frequently exists as insoluble forms in the soil.
