A swap transaction with mutual benefit

Plants and fungi live in close interactions for millions of years and thereby adapted to each other over time. Fungi of the arbuscular mycorrhizal fungi class live in symbiosis with plants living on the land and made it their task to establish trading systems with their host plants. Some land plants live in regions with nutrient depleted soil, which leads to phosphate deficiency. At this point, the mycorrhizal fungi take action, because they specialized on taking up huge amounts of phosphate from nutrient depleted soils, which they can further share with their host plant.

Millions of years ago mycorrhizal fungi lost their ability to produce important fatty acids on their own. Fatty acids are an important part of each cell, because they are building blocks of the cell membrane and responsible for energy storage. Without a membrane or energy, a cell cannot survive. Plants perform photosynthesis, which results in high amounts of carbohydrates. From these carbohydrates, the plant produces fatty acids, which are traded with the fungus in return for phosphate (see Figure 1).

This trading system has been proven to be successful for millions of years and it also benefits fungi which are no arbuscular mycorrhizal fungi. Excess nutrients are traded against the ones, which are hard to gather. To make sure only their symbiotic partner receives the nutrients, the fungi grow partially inside the plant. Inside, especially the arbuscular mycorrhizal fungi, form special, highly branched structures to generate the biggest possible surface area for the nutrient exchange (see Figure 2). These structures are called arbuscules (latin; arbusculum = little tree) and they gave the arbuscular mycorrhizal fungi their name.

Figure 1: Overview of the nutrient exchange between a plant cell (green) and an arbuscular mycorrhizal fungus (grey). Between the cells is an interaction space where all nutrients can be exported to be then taken up by the symbiotic partner. Phosphate is taken up by the fungus from the soil and then transported to the fungal cells inside the plant. The plant produces carbohydrates via photosynthesis, which then are transformed into fatty acids with the help of several enzymes.

We as scientist are very interested in understanding the mechanisms underlying this nutrient exchange between plants and fungi to make it usable for our purposes. If we understand a system well enough, we could start to alter it. For example, we could make the fungus not only deliver phosphate to the plant but also other important nutrients for plant growth. This way a farmer could lower the use of synthetic fertilizers since the fungus would partially take over the job of properly fertilizing the plant. In addition, the fungus could be optimized for each plant and environmental conditions to ensure optimal nutrition.

Figure 2: On the left, you can see an arbuscule (blue tinted) inside a plant root cell. It reminds of a tree with a highly branched crown. In the middle, our model plant Arabidopsis thaliana is. On the right, a microscopy picture of our fungus Colletotrichum tofieldiae (green) is shown, which grows around and inside an Arabidopsis root cell (red).

To get one step closer to this goal, me and my colleague Hang Lu try to mimic the arbuscular nutrient exchange from the mycorrhizal symbiosis in a plant and fungus that do not use this system. To do so we utilize the plant Arabidopsis thaliana (short: Arabidopsis). Arabidopsis is a so-called model plant, which is used in many different research areas because of its high versatility. It is also not able to establish a stable nutrient exchange with mycorrhizal fungi, which makes it a good fit for our purpose. As a fungus we use Colletotrichum tofieldiae (short: Colletotrichum), because it is built similar to mycorrhizal fungi but it belongs to a group from which some plant pathogens are known. In comparisons to its close relatives, Colletotrichum is beneficial for Arabidopsis. Arabidopsis and Colletotrichum naturally interact in a similar way like mycorrhizal fungi and their host plants but they use a different system, which we do not fully understand yet.

Our goal is to establish the processes from Figure 1 in our new model system. My colleague Hang is going to genetically modify Arabidopsis to make it capable of producing fatty acids (ß-MAGs), which can be used by the fungus. My task will be the modification of Colletotrichum to make it take up more phosphate, which can be delivered to the plant and to take up more fatty acids from Arabidopsis. To do so I will introduce genes from other organisms, which encode for the export of phosphate and the import of fatty acids. But before I first have to switch off the fatty acid biosynthesis of Colletotrichum to generate a dependency of the fungus on the plant, like it is the case for mycorrhizal fungi.

If all these modifications are successfully established in Arabidopsis and Colletotrichum they should be able to maintain a constantly stable nutrient exchange, which mimics the mycorrhizal symbiosis. If this base is set many possibilities open up to further modify the system. For example, more nutrient transport processes can be added to generate a universal “fertilizer fungus”.

Planter's Punch

Under the heading Planter’s Punch we present each month one special aspect of the CEPLAS research programme. All contributions are prepared by our young researchers.

About the author

Svenja Hermanns is a doctoral researcher at the University of Cologne in the group of Prof. Bucher. Her research focuses on the construction of a synthetic interaction between plants and fungi. The main goal is to alter the nutrient exchange between the two organisms in order to mimic the common arbuscular mycorrhizal symbiosis.