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.

Just like the transport proteins mentioned above, this proton pump is localized in the periarbuscular membrane. There, it transports protons - small positively charged hydrogen ions – into the space between the periarbuscular and the fungal membrane. This leads to a higher concentration of protons on the outside of the plant cell than on the inside, a so-called proton gradient. The protons on the outside serve as energy source for the transport of phosphate into the plant cell.

No proton pump – no phosphate

To prove that this proton pump is required to transport phosphate, the scientists generated Medicago truncatula mutants that have a non-functional version of the respective gene. Thus, the proton pump cannot be synthesized correctly. The symbiotic phosphate uptake and the growth rate of those mutants were compared to wild type plants. Roots of both plants - mutant and wild type - were equally colonized by Rhizophagus irregularis. Nevertheless, under phosphate deprivation the wild type plants grew better than the mutants due to the extra P-supply from the fungus.

The scientists also compared the phosphate transport to different parts of the plants. Wild type plants incorporated fungal phosphate in roots and shoots, as usual for mycorrhizal symbiosis. But, this pattern of phosphate incorporation could not be observed for the mutants. “We discovered that the proton pump is essential for phosphate transport”, Franziska Krajinski says, “The mutants could not grow on phosphate depleted soil, although they were colonized by the fungus”. Considering the declining phosphate resources, it is crucial to build a better understanding of symbiotic processes. The use of mycorrhizal products as replacement for mineral fertilizers is restricted to organic farming at the moment. Though, this will probably get increasingly important for the nutrition of crop plants and our own nutrition as well in the future.

More information, Prof. Dr. Krajinski's Website: http://www.mpimp-golm.mpg.de/8316/2krajinski

Original publication: http://www.plantcell.org/content/early/2014/04/29/tpc.113.120436.abstract?sid=0e