Regulation of Arbuscular Mycorrhizal Phosphate Transporter Genes in Acidic Forest Soils.

Many temperate hardwood forests of the Northeastern U.S. experience chronic acid deposition, which can influence ecosystem processes and productivity. Soils with a low pH (<5) are known to experience phosphorus (P) limitation; however previous studies have been unable to document widespread P limitation in these temperate forest trees. We hypothesize that forest trees do not experience P limitation due to the activity of soil microorganisms, particularly arbuscular mycorrhizal fungi (AMF). AMF possess high affinity inorganic phosphate transporters (PTs), and under greenhouse and laboratory conditions, P concentrations have been shown to regulate AMF-specific PT gene expression. This suggests that acidity may impact PT activity by indirectly affecting P availability. However, because PTs are proton-coupled, soil pH may also directly affect their expression. Despite the potential importance of AMF-specific PTs in the acquisition of P, we know little about the interactive effects of soil pH and P availability on their functioning, especially under field conditions. We investigated AMF-specific PTs in 12 established temperate forest plots where we experimentally altered soil pH and P availability. Using previously published primers that are specific to certain species in the genus Glomus, two regions of PT genes were successfully PCR-amplified, cloned, and sequenced from Acer root DNA extracts. Blastx searches to the non-redundant protein sequence database at the NCBI site found that the best matches for all of our cloned sequences were to the three known PT proteins from Glomus. Though our amplified PT regions were restricted to Glomus spp., sequencing of the 18S AMF gene from root samples in our plots found only Glomus species present. Our findings indicate that AMF-specific PT genes can be successfully detected in environmental samples. Additionally, quantification of the expression of these genes can elucidate the importance of AMF for maintaining tree P nutrition under acidic conditions.