Feeding Hungry Plants: The Secreted Purple Acid Phosphatase Isozymes AtPAP12 and AtPAP26 Play a Pivotal Role in Extracellular Phosphate Scavenging in Arabidopsis Thaliana
Robinson, Whitney Drummond
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Orthophosphate (Pi) is a limiting macronutrient in most soils and is essential for plant metabolism. Massive amounts of Pi-fertilizers are applied to agricultural fields to compensate for this limitation. However, Pi-fertilizers are made from non-renewable rock Pi-sources and their application is environmentally destructive. Plants have evolved numerous ways to survive in Pi-deficient (-Pi) soils, including the upregulation and secretion of acid phosphatases (APases). APases catalyze the hydrolysis of phosphate (Pi) from Pi-esters in an acidic environment. The major group of plant secreted APases, purple acid phosphatases (PAPs), have been hypothesized to scavenge Pi from organic-Pi (Po) sources that can compose up to 80% of the total P-content of some soils. Previous biochemical and proteomic studies indicate that AtPAP26 and AtPAP12 are the predominant secretory PAP isozymes upregulated by –Pi Arabidopsis thaliana cell cultures and seedlings. This thesis examines the influence of different Po supplements on the growth, Pi content, secretory APase activity, and secreted AtPAP12 and AtPAP26 polypeptides of wildtype (Col-0) Arabidopsis seedlings. Additionally, this thesis assesses the potential role that AtPAP12 and AtPAP26 play in scavenging Pi from extracellular Po sources by utilizing a homozygous atpap12/atpap26 double knockout mutant. Loss of AtPAP26 and AtPAP12 expression resulted in a 64% decrease in root secreted APase activity of –Pi seedlings. These results corroborate previous findings implying that: (i) Arabidopsis are able to grow on a variety of extracellular Po sources as their sole source of P-nutrition, and (ii) AtPAP12 and AtPAP26 are the principal contributors to secreted APase activity of –Pi Arabidopsis. Total shoot Pi levels, and growth of atpap12/atpap26 Arabidopsis seedlings cultivated in -Pi/+Po media were significantly lower relative to Col-0 controls, but unaffected under Pi sufficient conditions. The atpap12/atpap26 seedlings were unable to grow in a –Pi/+Po soil, whereas the Col-0 seedlings were able to develop. Additionally, both PAPs were strongly upregulated on root surfaces and in shoot cell wall extracts of –Pi seedlings. Taken together, these results strongly suggest that AtPAP12 and AtPAP26 play an important role in the hydrolysis of Pi from extracellular Po and make a large contribution to Pi-recycling and scavenging in –Pi Arabidopsis.