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Published on December 7, 2007; 10.1104/pp.107.109934

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Received September 26, 2007
Accepted November 27, 2007

An Arabidopsis Purple Acid Phosphatase with Phytase Activity Increases Foliar Ascorbate

Wenyan Zhang , Hope A. Gruszewski , Boris I. Chevone , and Craig L. Nessler *

Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

* Corresponding author; email: cnessler{at}vt.edu.

Ascorbate (AsA) is the most abundant antioxidant in plant cells and a co-factor for a large number of key enzymes. However, the mechanism of how AsA levels are regulated in plant cells remains unknown. The Arabidopsis thaliana activation tagged mutant AT23040 showed a pleiotropic phenotype including ozone resistance, rapid growth and leaves containing higher AsA than wild-type (wt) plants. The phenotype was caused by activation of a purple acid phosphatase (PAP) gene, AtPAP15 that contains a dinuclear metal center in the active site. AtPAP15 was universally expressed in all tested organs in wt plants. Overexpression of AtPAP15 with the 35S CaMV promotor produced mutants with up to 2-fold increase in foliar AsA, 20-30% decrease in foliar phytate, enhanced salt tolerance and decreased ABA sensitivity. Two independent SALK T-DNA insertion mutants in AtPAP15 had 30% less foliar AsA and 15-20% more phytate than wt plants and decreased tolerance to abiotic stresses. Enzyme activity of partially purified AtPAP15 from plant crude extract and recombinant AtPAP15 expressed in bacteria and yeast was highest when phytate was used as substrate indicating that AtPAP15 is a phytase. Recombinant AtPAP15 also showed enzyme activity on substrate myo-inositol-1-phosphate indicating that the AtPAP15 is a phytase which hydrolyzes myo-inositol hexakisphosphate to yield myo-inositol and free phosphate. myo-Inositol is a known precursor for AsA biosynthesis in plants. Thus, AtPAP15 may modulate AsA levels by controlling the input of myo-inositol into this branch of AsA biosynthesis in Arabidopsis.




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