Functional Genomics Indicates a Minor Role for the Purple Acid Phosphatase Isozyme AtPAP17 in Arabidopsis Thaliana Phosphorus Metabolism

Abstract

Phosphorus (P) is a crucial plant macronutrient, as it is a constituent of essential biomolecules involved in nearly all major metabolic processes. However, soluble orthophosphate (Pi), the only form of P that roots directly assimilate, is often limiting in most soils. This has prompted the widespread use of unsustainable and polluting Pi-containing fertilizers in agriculture. Plants have evolved a complex variety of biochemical adaptations that facilitate Pi acquisition, utilization, and recycling during nutritional Pi-deficiency. This includes the up-regulation of vacuolar and secreted purple acid phosphatases (PAPs) which catalyze Pi hydrolysis from Pi-monoesters at acidic pH. Of the 29 predicted PAP isozymes encoded by the genome of the model plant Arabidopsis thaliana, several high molecular weight isozymes, particularly AtPAP26, have been shown to play a key role in scavenging and recycling Pi during nutritional Pi-deprivation or leaf senescence. It has been hypothesized that AtPAP17, a low molecular weight PAP isozyme, also has a crucial function in Arabidopsis Pi metabolism as its transcripts show a marked induction during Pi-starvation or leaf senescence. This thesis uses a reverse genetic approach to examine AtPAP17’s function in Arabidopsis Pi acquisition and use efficiency by taking advantage of the publicly available T-DNA tagged insertional mutagenized populations of Arabidopsis. An anti-AtPAP17 peptide antibody was also raised in rabbits for detecting 35 kDa AtPAP17 polypeptides on immunoblots of Arabidopsis extracts. A null atpap17 allele was identified and characterized that abrogated AtPAP17 expression at both the transcript and protein level. However, loss of AtPAP17 expression did not influence the progression of Arabidopsis leaf senescence nor plant growth during Pi-deprivation relative to wild type (Col-0) controls. A sizable (~40%) decrease in extractable APase activity was only observed when the atpap17-2 mutant was subjected to Pi-deprivation. These results suggest that, contrary to inferences drawn from transcript profiling, AtPAP17 does not play a direct role in Pi scavenging during leaf senescence, and only plays a minor role during Pi-starvation. As AtPAP17 also exhibits alkaline peroxidase activity, future studies are needed to establish its potential involvement in reactive oxygen species metabolism during Pi stress or leaf senescence.