Properties of secretory acid phosphatase from lupin roots under phosphorus-deficient conditions

Abstract

Phosphorus deficiency induces the synthesis of acid phosphatases in roots of lupin and other plant species. In this study we examined the induction of secretory acid phosphatase (S-APase) at both the molecular and cellular levels. Lupin plants had increased levels of total acid phosphatase activity within two to five days after P was withered and levels approximately doubled by 15 days. Lateral roots and not main tap roots were responsible for this increase in acid phosphatase activity. Immunoblot analysis using antibodies raised against a purified S-APase showed that the synthesis of this protein was induced as early as 2 days in the P deficient treatment and that levels dramatically increased by 15 days. In contrast, no immunoreactive polypeptide was evident from crude extracts prepared from root tissues of - P treated plants. Immunocytochemical analysis revealed that the protein was located on the surface of epidermal cells of main tap roots and in the cell walls and intercellular spaces of lateral roots and lateral roots may actively secrete S-APase as soon as it is synthesized. A cDNA clone encoding the S-APase was isolated from a cDNA library constructed from lupin roots grown without P. The clone was 2,187 bp in length and had a single open reading frame of 637 amino acid residues. The deduced amino acid sequence was identical to the N-terminal region and peptide sequences of S-APase purified from lupin roots. A hydrophobic signal peptide region consisted of 31 amino acids. The primary structure was highly homologous to iron-zinc purple acid phosphatase from Phaseolus vulgaris (76%), secretory purple acid phosphatase from Arabidopsis thaliana (71%), and two Aspergillus phosphate repressible acid phosphatases (59% and 58%).