A Phosphate Transporter from <i>Medicago truncatula</i> Involved in the Acquisition of Phosphate Released by Arbuscular Mycorrhizal Fungi

TLDR

Plants acquire phosphate through symbiosis with arbuscular mycorrhizal fungi, which release phosphate from arbuscules into the periarbuscular membrane, but in Medicago truncatula the root phosphate transporters do not mediate this symbiotic uptake. Using EST database searches for conserved phosphate transporter motifs, the authors identified a novel Medicago truncatula transporter, MtPT4. MtPT4 differs from known root transporters, functions as a low‑affinity phosphate transporter in yeast, is expressed exclusively in mycorrhizal roots and arbuscule‑containing cells, localizes to the periarbuscular membrane, and its properties support a role in fungal phosphate acquisition.

Abstract

Many plants have the capacity to obtain phosphate via a symbiotic association with arbuscular mycorrhizal (AM) fungi. In AM associations, the fungi release phosphate from differentiated hyphae called arbuscules, that develop within the cortical cells, and the plant transports the phosphate across a symbiotic membrane, called the periarbuscular membrane, into the cortical cell. In Medicago truncatula, a model legume used widely for studies of root symbioses, it is apparent that the phosphate transporters known to operate at the root–soil interface do not participate in symbiotic phosphate transport. EST database searches with short sequence motifs shared by known phosphate transporters enabled the identification of a novel phosphate transporter from M. truncatula, MtPT4. MtPT4 is significantly different from the plant root phosphate transporters cloned to date. Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K m suggest a relatively low affinity for phosphate. MtPT4 is expressed only in mycorrhizal roots, and the MtPT4 promoter directs expression exclusively in cells containing arbuscules. MtPT4 is located in the membrane fraction of mycorrhizal roots, and immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane. The transport properties and spatial expression patterns of MtPT4 are consistent with a role in the acquisition of phosphate released by the fungus in the AM symbiosis.