Summary Solanaceous species are among the >200 000 plant species worldwide forming a mycorrhiza, that is, a root living in symbiosis with soil‐borne arbuscular‐mycorrhizal (AM) fungi. An important parameter of this symbiosis, which is vital for ecosystem productivity, agriculture, and horticulture, is the transfer of phosphate (Pi) from the AM fungus to the plant, facilitated by plasma membrane‐spanning Pi transporter proteins. The first mycorrhiza‐specific plant Pi transporter to be identified, was StPT3 from potato [Nature414 (2004) 462]. Here, we describe novel Pi transporters from the solanaceous species tomato, LePT4, and its orthologue StPT4 from potato, both being members of the Pht1 family of plant Pi transporters. Phylogenetic tree analysis demonstrates clustering of both LePT4 and StPT4 with the mycorrhiza‐specific Pi transporter from Medicago truncatula [Plant Cell, 14 (2002) 2413] and rice [Proc. Natl Acad. Sci. USA, 99 (2002) 13324], respectively, but not with StPT3, indicating that two non‐orthologous mycorrhiza‐responsive genes encoding Pi transporters are co‐expressed in the Solanaceae. The cloned promoter regions from both genes, LePT4 and StPT4, exhibit a high degree of sequence identity and were shown to direct expression exclusively in colonized cells when fused to the GUS reporter gene, in accordance with the abundance of LePT4 and StPT4 transcripts in mycorrhized roots. Furthermore, extensive sequencing of StPT4‐like clones and subsequent expression analysis in potato and tomato revealed the presence of a close paralogue of StPT4 and LePT4, named StPT5 and LePT5, respectively, representing a third Pi transport system in solanaceous species which is upregulated upon AM fungal colonization of roots. Knock out of LePT4 in the tomato cv. MicroTom indicated considerable redundancy between LePT4 and other Pi transporters in tomato.

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Arbuscular mycorrhizae are ancient symbioses that are thought to have originated >400 million years ago in the roots of plants, pioneering the colonization of terrestrial habitats. In these associations, a key process is the transfer of phosphorus as inorganic phosphate to the host plant across the fungus-plant interface. Mycorrhiza-specific phosphate transporter genes and their regulation are conserved in phylogenetically distant plant species, and they are activated selectively by fungal species from the phylum Glomeromycota. The potato phosphate transporter gene StPT3 is expressed in a temporally defined manner in root cells harboring various mycorrhizal structures, including thick-coiled hyphae. The results highlight the role of different symbiotic structures in phosphorus transfer, and they indicate that cell-cell contact between the symbiotic partners is required to induce phosphate transport.A rbuscular mycorrhizal fungi (AMF) are important biotrophic organisms, which live in symbiosis with Ϸ80% of land plants, forming a mycorrhiza (i.e., a root colonized by a symbiotic fungus). AMF affect plant biodiversity, as well as the variability and productivity of ecosystems (1, 2). As few as Ϸ150 fungal species are known to form arbuscular mycorrhizae (AM) in the roots of a vast number of plant species. No evidence for recombination has been found in the fungi, suggesting that they reproduce clonally and have been asexual for the entire period of their association with plants (3). Recently, AMF were placed into a new monophyletic group, the phylum Glomeromycota, which probably originated from the same ancestral group as the Ascomycota and Basidiomycota (4) Ϸ1,400-1,200 million years ago and is much older than the earliest land plants, which appeared Ϸ800 million years ago and whose primitive root systems were associated with ancestral AMF. AMF may, thus, have played a crucial role in facilitating the colonization of land by plants (5-8). It is also assumed that the ancient signaling pathways evolved in AM symbiosis were recruited subsequently for the establishment of the evolutionary younger legumeRhizobia nodulation symbiosis (9). Today, despite the large number of plant species forming AM associations worldwide, only two major morphological types have been defined: the Arum and Paris types, respectively. In Arum-type mycorrhizae, fungal hyphae spread between cortical cells and form short-lived heavily branched symbiotic structures (arbuscules) within cells. In Paris-type mycorrhizae, cortical cells are colonized by intracellularly growing thick coiled hyphae, which occasionally form fine arbuscule-like ramifications (10). Materials and MethodsPlant and Fungal Material. The plant material used was Daucus carota (carrot), Lotus japonicus cv. Gifu, Medicago truncatula cv. Jemalong, Petunia ϫ hybrida (petunia), Plantago lanceolata (plantain), and Solanum tuberosum cv. Désirée (potato). AMF isolates used were Acaulospora delicata (JJ1094), Archaeospora nicolsonii (W4147), Gigaspora margarita (BEG34), Glomus caledoni...