Phosphate transport of fungi to plant roots requires proton pump
Max Planck Institute of Molecular Plant PhysiologyArbuscular mycorrhizal structures in a root stained in blue and magnified with a light microscope. Phosphorous (P) is a component of DNA and plays an important role in energy metabolism; therefore it is essential for all organisms. Plants are able to take it up from the soil in the form of salts, namely phosphates. But in many soils phosphate is already depleted and the world’s phosphate resources, which can be used to produce fertilizer, are declining. Nevertheless, crop plants need an optimal P-supply to gain high yields. To overcome this problem, a special community of plants and fungi could become more important in the future. About 80% of all land plants live in a kind of marriage with arbuscular mycorrhizal fungi. This relationship secures the plants’ phosphate nutrition while the fungi are rewarded with sugars. Scientists around Franziska Krajinski from the Max Planck Institute of Molecular Plant Physiology recently discovered that a special proton pump facilitates the transport of fungal phosphate into the plant. (Plant Cell, DOI: 10.1105/tpc.113.120436).
It is all about give-and-take
This relationship, or better symbiosis, is an ancient story of success; arbuscular mycorrhizal fungi (AM fungi) already supported plants in the initial colonization of land over 400 million years ago. In contrast to other fungi, like the yellow boletus, AM fungi are not visible above-ground. They enter the roots of plants with their hyphae and build treelike structures called arbuscules. This name derives from the Latin phrase “arbusculus”, meaning “little tree”.
When one partner lives inside the other, this is called an endosymbiosis and, as in all well working relationships, this symbiosis positively affects both partners. The plant receives phosphate from the fungus in exchange for sugars.
Nothing works without energy
The scientists around Franziska Krajinski from the MPI-MP are interested in the transport processes between the AM fungus Rhizophagus irregularis and the barrel clover Medicago truncatula. Although AM fungi live inside the root cells of their symbiotic partners, both are always separated from each other by two membranes – the fungal membrane and the so-called periarbuscular membrane, on the plant side. Phosphate has to cross those barriers on its way from the fungus to the root cell. In the periarbuscular membrane, this is facilitated by certain proteins, that transport their cargo from the fungus to the plant like little trucks and just like the real trucks they need energy to do their job. “But, proteins cannot stop at a petrol station to refuel with energy. They have to use other resources”, Daniela Sieh comments on the current research. “We wanted to unravel the energy source of phosphate transport. Luckily, we could refer to older studies, where we identified a gene in barrel clover, which encodes a proton pump”, Prof. Franziska Krajinski adds.
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