The law of the minimum and how fungi can help

What is it about?

Mycorrhizas are symbiotic relationships between fungi and plant roots. The fungi need the carbon products of photosynthesis from the plant to survive. They return the favour in the form of harvested soil nutrients like phosphorus or nitrogen. Mycorrhizas can be commensal, mutualistic or parasitic, correlating with a neutral, positive or negative outcome for the plant. Commensalism is defined as a symbiosis that benefits the fungi and has no effect (positive or negative) on the plant. According to the Law of the Minimum, plant growth is controlled by the resource that is most limited in supply. In this study, the researchers wanted to find out if the Law of the Minimum is useful for predicting the nature of arbuscular mycorrhizal (AM) symbioses. IMPORTANT EXCHANGES AT THE ROOT Arbuscular mycorrhizas are well known for ameliorating phosphorus limitation in plants, but nitrogen availability can mediate this effect. If light and nitrogen are in abundance, more photosynthesis occurs and symbiotic trade of carbon for phosphorus benefits both plants and fungi. However, nitrogen limitation can influence the symbiotic outcome; studies have been inconsistent as to whether AM fungi can help the plant by passing on the nitrogen it can gather. The researchers hypothesize this is due to the greater need for nitrogen by the fungi than the plant. The mycorrhizal exchange has to be worth the carbon cost. EXPERIMENTING WITH SOIL To test their hypothesis, the researchers looked at mycorrhizal functioning at different sites with different proportions of soil nitrogen and phosphorus, both in shade and in full sunlight. They discovered mutualism in phosphorus-limited systems, meaning both the fungi and plants benefited from the symbiotic exchange of carbon for phosphorus. But in nitrogen-limited systems, the AM symbioses functioned as a commensalism, or even a parasitism, because the fungi clung onto the nitrogen they got from the soil. Also, nitrogen limitation restricted photosynthesis and reduced the amount of carbon that plants could exchange with their fungal partners. Therefore, the Law of Minimum could serve as a guide to manage mycorrhizas. It could be exploited for increasing crop yield or increasing carbon sequestration in AM fungal hyphae.

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