Arbuscular mycorrhizas are the most common non-pathogenic symbioses in the roots of plants. It is generally assumed that this symbiosis facilitated the colonization of land by plants. In arbuscular mycorrhizas, fungal hyphae often extend between the root cells and tuft-like branched structures (arbuscules) form within the cell lumina that act as the functional interface for nutrient exchange. In the mutualistic arbuscular-mycorrhizal symbiosis the host plant derives mainly phosphorus from the fungus, which in turn benefits from plant-based glucose. The molecular basis of the establishment and functioning of the arbuscular-mycorrhizal symbiosis is largely not understood. Here we identify the phosphate transporter gene StPT3 in potato (Solanum tuberosum). Functionality of the encoded protein was confirmed by yeast complementation. RNA localization and reporter gene expression indicated expression of StPT3 in root sectors where mycorrhizal structures are formed. A sequence motif in the StPT3 promoter is similar to transposon-like elements, suggesting that the mutualistic symbiosis evolved by genetic rearrangements in the StPT3 promoter.

Membrane-spanning transport proteins are responsible for the selective passage of most mineral nutrients and metabolites across cellular and intracellular membranes. This review's focus is on summarising the current state of research covering the molecular regulation and biochemical mechanisms involved in the transport of phosphorus, an often growth-limiting nutrient, in vascular plants. Physiological data illustrating the tight control of Pi homeostasis on the cellular as well as on the organism's level are discussed together with the recent results on molecular transport mechanisms.

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An Arabidopsis genomic sequence was recently shown to share similarity with bacterial and eukaryotic phosphate (Pi) transporters. We have cloned the corresponding cDNA, which we named Pht2;1 , and subsequently performed gene expression studies and functional analysis of the protein product. The cDNA encodes a 61-kD protein with a putative topology of 12 transmembrane (TM) domains interrupted by a large hydrophilic loop between TM8 and TM9. Two boxes of eight and nine amino acids, located in the N-and C-terminal domains, respectively, are highly conserved among species across all kingdoms (eubacteria, archea, fungi, plants, and animals). The Pht2;1 gene is predominantly expressed in green tissue, the amount of transcript staying constant in leaves irrespective of the Pi status of the shoot; in roots, however, there is a marginal increase in mRNA amounts in response to Pi deprivation. Although the protein is highly similar to eukaryotic sodium-dependent Pi transporters, functional analysis of the Pht2;1 protein in mutant yeast cells indicates that it is a proton/Pi symporter dependent on the electrochemical gradient across the plasma membrane. Its fairly high apparent K m for Pi (0.4 mM) and high mRNA content in the shoot, especially in leaves, suggest a role for shoot organs in Pi loading. Pht2;1 thus differs from members of the recently described plant Pi transporter family in primary structure, affinity for Pi, and presumed function.

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SummaryA cDNA encoding Pht2;1 from potato, a new member of the plant Pht2 gene family of low-af®nity orthophosphate (Pi) transporters, was isolated. The expression pattern of the corresponding gene as well as its ortholog from Arabidopsis was analyzed and the encoded proteins were localized in the two plants. Pht2;1 expression is strongly upregulated by light in potato and Arabidopsis leaf tissue. RNA gel blot analysis, reverse transcription-polymerase chain reaction (RT-PCR), promoter/GUS, and protein/green¯uorescent protein (GFP) fusion studies, respectively, indicate that the gene is expressed in both auto-and heterotrophic tissues and its encoded protein is localized to the plastids. The similar patterns of Pht2;1 gene regulation in potato and Arabidopsis prompted us to screen publicly available gene expression data from 228 Arabidopsis oligonucleotide microarrays covering 83 different experimental conditions. Modulation of Pht2;1 transcript levels was overall moderate, except for a limited number of experimental conditions where Pht2;1 mRNA concentrations varied between 2-and 3.7-fold. Overall, these analyses suggest involvement of the Pht2;1 protein in cell wall metabolism in young, rapidly growing tissues, independent of other Pi transporters such as the high-af®nity Solanum tuberosum Pi transporter 1 (StPT1). Cluster analysis allowed identi®cation of colinear or antiparallel expression pro®les of a small set of genes involved in post-translational regulation, and photosynthetic carbon metabolism. These data give clues about the possible biological function of Pht2;1 and shed light on the complex web of interactions in which Pht2;1 could play a role.

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