A Lycopersicon esculentum phosphate transporter (LePT1) involved in phosphorus uptake from a vesicular–arbuscular mycorrhizal fungus

Abstract

In vesicular-arbuscular mycorrhizal symbioses, specialized fungal structures (the arbuscules) are formed which are in intimate contact with plant root cortical cells. It is assumed that these arbuscules are the major sites of solute transfer between the plant and fungus, but there have been no studies that definitively show the extent or types of transfer processes that occur in this structure. Phosphate is one of the major nutrients that is acquired by mycorrhizal fungi and transferred to plants. In this study a single Lycopersicon esculentum cDNA was cloned and shown to be identical to LePT1, a previously cloned inorganic-phosphate transporter. Expression studies revealed that LePT1 transcript levels remained constant in mycorrhizal plants, but increased in phosphate-starved, nonmycorrhizal plants. Localization of the LePT1 transcript by in situ hybridization showed that this gene is highly expressed in arbuscule-containing cortical cells in mycorrhizal plants. In non-mycorrhizal plants LePT1 expression was localized to the stele and cortex. The expression studies suggest that this transporter is involved in phosphate nutrition of L. esculentum and its localization in cells that contain arbuscules indicate that it may be the mechanism used by the plant to take up phosphate that is effluxed across the fungal plasma membrane of the arbuscule. Based on our findings and those of others, an integrated model of inorganic phosphate uptake and transfer in mycorrhizal and non-mycorrhizal plants is presented.Key words : mycorrhiza, transport, phosphate, tomato, in situ RNA localization, arbuscule. Inorganic phosphate (Pi) is required in relatively large amounts and is often a limiting nutrient for plant growth. The low concentration of Pi in the soil solution (1-10 µM) and its relative immobility leads to the formation of zones depleted in Pi around the actively absorbing roots. These depletion zones effectively limit Pi uptake in non-mycorrhizal plants. The symbiotic association formed between plants and vesicular-arbuscular (VA) mycorrhizal fungi allows plants to access Pi beyond the depletion zone

Coupling autotrophic in vitro plant cultivation system to scanning electron microscope to study plant-fungal interactions

“…Arbuscules has been acclaimed as the major site for the exchange of minerals (in particular phosphorus) and carbohydrates between both partners in the AM symbiosis (Rosewarne et al, 1999;Rausch et al, 2001). Using a similar in vitro autotrophic cultivation system, Dupré de Boulois et al (2006) demonstrated the transport of P from a hyphal compartment to a root compartment and subsequently to the plant shoot cells via the fungal interface, while the transport of carbohydrates from plant to the AM fungus was investigated by Voets et al (2008).…”

Signal and nutrient exchange at biotrophic plant–fungus interfaces

“…A high-affinity phosphate transporter from Glomus seems to be responsible for the uptake of phosphate by hyphae growing in the soil [44]. The Lycopersicon esculentum PHOSPHATE TRANSPORTER1 (LePT1)-encoded transporter from tomato also appears to be involved in symbiotic phosphate translocation, as indicated by the high levels of LePT1 transcripts that were detected by in situ hybridization in arbusculated cells [45]. The site of carbohydrate translocation towards AM fungi is unclear.…”

Untitled

“…Supporting evidence includes: i) ATP-hydrolyzing activity, possibly P-ATPase, was localized on periarbuscular membrane of the host (Gianinazzi-Pearson et al, 1991) and ii) High-affinity Pi transporters (Rosewarne et al, 1999;Rausch et al, 2001) and P-ATPase (Gianinazzi-Pearson et al, 2000) genes are expressed in arbuscule-containing host cells.These observations suggest that Pi is actively taken up by the plant through the periarbuscular membrane. iii) ATP-hydrolyzing activity on the fungal plasma membrane of arbuscules is absent (Gianinazzi-Pearson et al, 1991) and iv) the fungal P transporter gene GvPT is not expressed within the roots (Harrison and Van Buuren, 1995).…”