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...

SUMMARYThe majority of land plants live in symbiosis with arbuscular mycorrhizal fungi from the phylum Glomeromycota. This symbiosis improves acquisition of phosphorus (P) by the host plant in exchange for carbohydrates, especially under low-P availability. The symbiosome, constituted by root cortex cells accommodating arbuscular mycorrhizal fungal hyphae, is the site at which bi-directional exchange of nutrients and metabolites takes place. Uptake of orthophosphate (Pi) in the symbiosome is facilitated by mycorrhizaspecific plant Pi transporters. Modifications of the potato Pi transporter 3 (StPT3) promoter were analysed in transgenic mycorrhizal roots, and it was found that the CTTC cis-regulatory element is necessary and sufficient for a transcriptional response to fungal colonization under low-Pi conditions. Phylogenetic footprinting also revealed binary combination of the CTTC element with the Pi starvation response-associated PHR1-binding site (P1BS) in the promoters of several mycorrhiza-specific Pi transporter genes. Scanning of the Lotus japonicus genome for gene promoters containing both cis-regulatory elements revealed a strong over-representation of genes involved in transport processes. One of these, LjVTI12, encoding a member of the SNARE family of proteins involved in membrane transport, exhibited enhanced transcript levels in Lotus roots colonized with the arbuscular mycorrhizal fungus Glomus intraradices. Down-regulation of LjVTI12 by RNA interference resulted in a mycorrhiza-specific phenotype characterized by distorted arbuscule morphology. The results highlight cooperative cis-regulation which integrates mycorrhiza and Pi starvation signaling with vesicle trafficking in symbiosome development.

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SummaryThe active endogenous dTph1 system of the Petunia hybrida mutator line W138 has been used in several forward-genetic mutant screens that were based on visible phenotypes such as flower morphology and color. In contrast, defective symbiotic phosphate (P i ) transport in mycorrhizal roots of Petunia is a hidden molecular phenotype as the symbiosis between plant roots and fungi takes place below ground, and, while fungal colonization can be visualized histochemically, P i transport and the activity of P i transporter proteins cannot be assessed visually. Here, we report on a molecular approach in which expression of a mycorrhiza-inducible bi-functional reporter transgene and insertional mutagenesis in Petunia are combined. Bi-directionalization of a mycorrhizal P i transporter promoter controlling the expression of two reporter genes encoding firefly luciferase and GUS allows visualization of mycorrhiza-specific P i transporter expression. A population of selectable transposon insertion mutants was established by crossing the transgenic reporter line with the mutator W138, from which the P i transporter downregulated (ptd1) mutant was identified, which exhibits strongly reduced expression of mycorrhiza-inducible P i transporters in mycorrhizal roots.