Cesium could be used as a proxy for potassium in mycorrhizal Medicago truncatula

Arbuscular mycorrhizal (AM) fungi interact with the roots of most land plants and help them to acquire various mineral resources from the soil, including potassium (K⁺). However, tracking K⁺ movement in AM symbiosis remains challenging. Recently, we reported that rubidium can be used as a proxy for K⁺ in mycorrhizal Medicago truncatula. In the present work, we investigated the possibility of using cesium (Cs⁺) as another proxy for K⁺ in AM symbiosis. Plants were placed in growing systems that include a separate compartment only accessible to the AM fungus Rhizophagus irregularis isolate 09 and in which various amounts of cesium chloride (0 mM, 0.5 mM, 1.5 mM, or 3.75 mM) were supplied. Plants were watered with sufficient K⁺ or K⁺-free nutrient solutions, and shoot and root biomass, fungal colonization, and K⁺ and Cs⁺ concentrations were recorded seven weeks after inoculation. Our results indicate that Cs⁺ accumulated in plant tissues only when K⁺ was present in the nutrient solution and when the highest concentration of Cs⁺ was used in the fungal compartment. Consequently, we conclude that Cs⁺ could be used as a proxy for K⁺ in AM symbiosis, but with serious limitations.