Friends and Enemies in the Plant Kingdom

Interview with Uta Paszkowski, lecturer in plant science

22.5.2013 | The beneficial relationships that plants form with symbiotic fungi, and their ability to avoid parasitic fungi, could be used to improve agricultural crop yields, says Prof. Uta Paszkowski, lecturer at the University of Cambridge.
AcademiaNet: Your research looks at mutually beneficial symbiotic relationships between fungi and crop plants such as rice and maize. Why might a symbiotic relationship between a plant and a fungus be advantageous?

Prof. Uta Paszkowski
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Prof. Uta Paszkowski
Uta Paszkowski: Plants that grow in soil are equipped with a root system that has two major functions. One is anchoring the plant in the soil and the second is taking up water and minerals and nutrients. Both water and nutrients are acquired from the direct environment of the root, the so-called rhizosphere. When the root has depleted the zone in which it is in direct contact, it needs to grow and explore new soil volumes to acquire from this virgin territory more minerals and water. However, when the plant lives in a symbiotic association with a beneficial type of fungus, the fungus forms a network of tiny little hyphae that contribute to soil exploration for nutrient acquisition. Although the fungus needs a certain amount of these newly acquired minerals for itself, it does in fact transfer a large chunk to the plant.

This particular type of symbiotic relationship is evolutionarily extraordinarily old. We have fossil records as well as molecular supporting data that clearly place the evolutionary origin of this symbiosis way back to the occurrence of the earliest land plants. This explains why we find this symbiosis in more than 80 percent of our terrestrial plant species.

Presumably plants must need to be able to identify which fungi are harmful and which ones might be beneficial. How exactly do they go about doing that?

The fungi emit very wide variety of chemical compounds – they could be proteins, secondary metabolites or small molecules - that help the plant to distinguish the positive from the negative neighbour in the soil. The plant needs a fine tuned detection system: specific receptors that would then commence a specific signalling pathway required for the plant to accommodate a beneficial fungus and combat a pathogenic one.

However, there is a continuum between mutualism and parasitism when it comes to the individual components of the symbiotic relationships between plants and fungi. Interestingly, one of the first recognition signals released by the host plant and perceived by the beneficial mycorrhizal fungi is a compound called strigolactone, and this same compound is detected by the parasitic fungi. So the compound that communicates to the beneficial fungus the position of the host roots has the very same function for parasitic plants that eventually kill the host. This is a clear example of the considerably later evolving parasitic plants hijacking an anciently advantageous mechanism to use for their own benefit.
The symbiosis between plant and fungus
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(© Uta Paszkowski)

The symbiosis between plant and fungus | both partners communicate via chemicals, metabolites or molecules.

One area that you have studied in some detail is how plants absorb phosphate through this symbiotic relationship. Why is absorption of phosphate in particular so important?

Nitrogen and phosphate are highly important for optimal development of plants but are usually limited in the soil. With high input crop fields we normally add quite a considerable amount of phosphate fertiliser to guarantee that the plants develop in close to optimal fashion. A plant that lives in association with mycorrhizal fungi typically obtains the vast majority of its inorganic phosphate via the symbiotic fungus. It has been estimated that depending on the plant-fungus combination, the fungus provides between 70 and almost 100 percent of the plant's overall phosphate uptake.

As such arbuscular mycorrhizal symbiosis has the potential to be implemented in sustainable agricultural practices to improve crop yields. But our difficulty at the moment is that we don't really know yet how to quantitatively define this potential. Mycorrhizal relationships are already implemented in low input agricultural settings. So the big question that we are trying to answer is whether and how we can make use of the biofertiliser capacity of symbiosis in modern and more high input agricultural settings, meaning more intensive farming methods.

Your research studies in particular rice and maize. Do you study those two systems because of their importance as food crops?

Yes, although we needed to consider two factors when choosing a model system: the academic and the translational. Academically, the model systems for our daily studies should enable us to answer fundamental biological questions. Only if we can understand the processes that underpin the establishment of such symbioses can we estimate the potential of mycorhizal symbiosis for sustainable crop cultivation. For us this meant plant systems that provide the appropriate genetic resources. But I wanted to combine this with working in plant species that permitted the immediate transfer of laboratory gained knowledge to the field, that were of economic value, and, beyond that, were of value for food security.

Rice terraces in Asia
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Rice terraces in Asia
Because the symbiotic relationship is so well conserved across plant species, there was quite a choice. In theory we could have worked in solanaceous plant species belonging to the nightshade family, like potato or tomato, or alternatively in crop legumes, such as bean or soybean. But looking at the combination of available genetic resources and the importance of the respective plant species for human food security, I think we've settled on two excellent plant systems with the choice of rice and maize.

Dear Prof. Paszkowski, thank you very much for this interesting interview.

Interview: Helen Jaques   (© AcademiaNet)

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