Cloning and characterization of two secreted acid phosphatases from rice calli Plant Nutrition

We have studied the mechanism of expression and secretion of secreted acid phosphatase (S-APase) in lupin roots. Since S-APase activity of rice is lower than in lupin, we are interested in improving S-APase secretion ability in rice. We identified two S-APase cDNAs (OsSAP1 and OsSAP2) from calli that had been induced from root segments of rice. The deduced amino acid sequence of OsSAP1 (OsSAP1) shared 54% identity with that of one lupin S-APase (LaSAP2) , and 48% identity with that of another lupin S-APase (LaSAP1) . The predicted structure of OsSAP1 was similar to Fe(III)-Zn(II) purple APase (KBPAP) of Phaseolus vulgaris . The PSORT computer program indicated a secretion probability of this enzyme of 0.82. We have also partially purified the 51 kDa S-APase protein (OsSAP2) secreted from rice calli. The first 15 amino acids of the N-terminal sequence were identical to an EST (Accession No. AU065845). However, the rest of the sequence was less homologous to that of OsSAP1 cDNA. The full-length of OsSAP2 cDNA was identified by sequencing the EST clone. Probability of secretion was 0.58. The predicted proteins corresponding to the two S-APase cDNAs showed high homology with S-APases and acid phosphatases (APase) from other plants. Therefore, it is assumed that the difference in secretion between lupin and rice might be due to differences at the transcriptional level or in the secretory processes.