Low-affinity transporter for external inorganic phosphate (Pi) probably involved in the acquisition of phosphate released by arbuscular mycorrhizal (AM) fungi (e.g. Gigaspora gigantea, Glomus versiforme and G.intraradices) during AM symbiosis; required for propper mycorrhizal arbuscule morphology (PubMed:12368495, PubMed:25841038). Acts as a Pi-sensing machinery at the root tip level, independently of AM fungi, involved in the regulation of early root branching and lateral roots formation (PubMed:26476189). NH4(+)(in) = NH4(+)(out) Optimum pH is 3-4. Present on the periarbuscular membrane in cells containing arbuscules during arbuscular mycorrhizal (AM) symbiosis with AM fungi (e.g. Gigaspora gigantea, Glomus versiforme and G.intraradices). Mostly expressed in mycorrhizal roots (PubMed:12368495). Also observed in root tips of non-mycorrhizal roots, in a phosphate (Pi) depended-manner, highest expression levels being observed in low Pi conditions (PubMed:26476189). Expressed in cortical cells containing arbuscules of mycorrhizal roots upon arbuscular mycorrhizal (AM) symbiosis with AM fungi (e.g. Gigaspora gigantea, Glomus versiforme and G.intraradices). Accumulates in roots, in a RAM1-dependent manner, during colonization by arbuscular mycorrhizal (AM) fungi (e.g. Gigaspora gigantea, Glomus versiforme and G.intraradices) (PubMed:26511916, PubMed:20687807, PubMed:25841038). Induced in root tips by low phosphate (Pi) levels (PubMed:26476189). Impaired phosphate (Pi) starvation induction of lateral roots formation (PubMed:26476189). Plants missing PT4 or both PT4 and PT8 fail to establish arbuscular mycorrhizal (AM) symbiosis with AM fungi in high nitrogen conditions, leading to premature arbuscule degeneration (PAD); these phenotypes are suppressed in nitrogen-deprived conditions in an AMT2-3-dependent manner (PubMed:25841038). Although related to the sugar transporter family, it does not transport sugars. Belongs to the major facilitator superfamily. phosphate:H(+) symporter (TC 2.A.1.9) family.