Acid phosphatase activity in phosphorus-deficient white lupin roots

White lupin (Lupinus albus L.) develops proteoid roots when grown in phosph orus (P)-deficient conditions. These short, lateral, densely clustered root s are adapted to increase P availability. Previous studies from our laborat ory have shown proteoid roots have higher rates of non-photosynthetic carbo n fixation than normal roots and altered metabolism to support organic acid exudation, which serves to solubilize Pin the rhizosphere, The present wor k indicates that proteoid roots possess additional adaptations for increasi ng P availability and possibly for conserving P in the plant. Roots from P- deficient (-P) plants had significantly greater acid phosphatase activity i n both root extracts and root exudates than comparable samples from P-suffi cient (+P) plants beginning 10 d after emergence. The increase in activity in -P plants was most pronounced in the proteoid regions. In contrast, no i nduction of phytase activity was found in -P plants compared to +P plants. The number of proteoid roots present was not affected by the source of phos phorus supplied, whether organic or inorganic forms. Adding molybdate to th e roots increased the number of proteoid roots in plants supplied with orga nic P, but not inorganic P Increased acid phosphatase activity was detected in root exudates in the presence of organic P sources. Native-polyacrylami de gel electrophoresis demonstrated that under P-deficient conditions, a un ique isoform of acid phosphatase was induced between 10 and 12 d after emer gence. This isoform was found not only within the root, but it comprised th e major form exuded from proteoid roots of -P plants. The fact that exudati on of proteoid-root-specific acid phosphatase coincides with proteoid root development and increased exudation of organic acids indicates that white l upin has several coordinated adaptive strategies to P-deficient conditions.