The external potassium availability determines the effect of the arbuscular mycorrhizal fungus Rhizophagus irregularis on salinity tolerance in soybeans

Abstract

Arbuscular mycorrhizal (AM) fungi improve water and nutrient acquisition of most land plants. Additionally, they can help plants to alleviate abiotic stresses, such as salinity which causes a major threat for many crop species. Potassium (K + ) plays a major role in plant tolerance to salinity, and we recently demonstrated, by using rubidium (Rb + ) as a proxy, that AM fungi can directly transfer K + to their host plant. Here, we first investigated the impact of K + availability on soybean root development upon salinity. Then, using two-compartment systems, we also inoculated soybean plants with the AM fungus Rhizophagus irregularis , grew them in various K + and sodium (Na + ) regimes, and used Rb + to track K + movements. Root development parameters, biomass, colonization rate, and nutrient concentrations were assessed in AM and non-mycorrhizal plants. Our results show that soybean root development was significantly affected by NaCl treatments, rather than K + availability. Additionally, although the AM symbiosis was drastically reduced by high salinity, it improved K + concentrations and prevented Na + accumulation in inoculated plants, mainly under limiting K + conditions. Rb + transport was observed only when the plants were in demand for K + , but was inhibited by high salinity. Finally, we also show that the addition of NaCl slightly influences the availability of K + and Rb + . This report shows the combined impact of K + availability and AM symbiosis on soybean tolerance to salinity and discusses the limitations of using Rb + as a proxy for K + upon increasing salinity conditions.

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