Acid phosphatase activity in phosphorus‐deficient white lupin roots

Abstract

White lupin (Lupinus albus L.) develops proteoid roots when grown in phosphorus (P)‐deficient conditions. These short, lateral, densely clustered roots are adapted to increase P availability. Previous studies from our laboratory have shown proteoid roots have higher rates of non‐photosynthetic carbon fixation than normal roots and altered metabolism to support organic acid exudation, which serves to solubilize P in the rhizosphere. The present work indicates that proteoid roots possess additional adaptations for increasing P availability and possibly for conserving P in the plant. Roots from P‐deficient (–P) plants had significantly greater acid phosphatase activity in both root extracts and root exudates than comparable samples from P‐sufficient (+P) plants beginning 10 d after emergence. The increase in activity in –P plants was most pronounced in the proteoid regions. In contrast, no induction 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 phosphorus supplied, whether organic or inorganic forms. Adding molybdate to the roots increased the number of proteoid roots in plants supplied with organic P, but not inorganic P. Increased acid phosphatase activity was detected in root exudates in the presence of organic P sources. Native‐polyacrylamide gel electrophoresis demonstrated that under P‐deficient conditions, a unique isoform of acid phosphatase was induced between 10 and 12 d after emergence. This isoform was found not only within the root, but it comprised the major form exuded from proteoid roots of –P plants. The fact that exudation of proteoid‐root‐specific acid phosphatase coincides with proteoid root development and increased exudation of organic acids indicates that white lupin has several coordinated adaptive strategies to P‐deficient conditions.