Less Fertilizer Through Smart Alliances

Over the course of evolution, plants have developed an elegant strategy to counteract a lack of phosphate in the soil — they form symbiotic relationships with soil fungi. These mycorrhizal fungi efficiently supply their plant partners with phosphate and other essential minerals. Recently, scientists at the Leibniz Institute of Plant Biochemistry (IPB) in Halle, in collaboration with partners at the University of Bonn, discovered a molecular switch that detects the plant's phosphate content and signals whether to initiate or inhibit the symbiosis. This signaling pathway could enable plants to form partnerships with soil fungi even when sufficient phosphate is available. The study, published in the renowned journal Science Advances, offers a potential solution to a long-standing agricultural problem and opens new avenues for reducing fertilizer use. 

Inoculating crops with mycorrhizal fungi is crucial for increasing agricultura

yields. The fine fungal hyphae act like an extended root system, significantly increasing the uptake of minerals and nutrients. Phosphate is particularly vital for plants as it is a key regulator of their energy balance, and they efficiently extract it from the soil through this symbiosis. However, this partnership comes at a cost for plants: they must cede a portion of the carbohydrates they produce to the fungi. "This cost is so high for the plant that it suppresses the symbiosis when sufficient phosphate is available in the soil," explains Martina Ried-Lasi, head of the Symbiosis Signaling research group at the IPB. Despite an adequate phosphate supply, foregoing the symbiosis negatively impacts yields because the fungi also promote the uptake of nitrogen, magnesium, and potassium. "Therefore, agriculture is seeking strategies to maintain mycorrhizal associations in crops regardless of soil phosphate availability," says Gabriel Schaaf of the University of Bonn. "Our study demonstrates a potential approach for specifically promoting such symbioses in the future."

Using the model plant Lotus japonicus, the research team identified the enzyme VIH2 as a key regulator of symbiosis formation. VIH2 controls the production of inositol pyrophosphates, which are signal molecules that indicate phosphate status. When there is little phosphate in the cell, VIH2 produces low amounts of inositol pyrophosphates, signaling the cell to initiate a starvation program. This response includes activating phosphate-deficiency genes, remodeling root architecture, and initiating mycorrhizal symbiosis. When the phosphate supply is adequate, VIH2 produces large amounts of these signaling molecules. Accordingly, the phosphate starvation response is suppressed, and the partnership with mycorrhizal fungi is inhibited.