Inoculation With Salt‐Tolerant Rhizobia and AMF Regulates K⁺/Na⁺ Homeostasis in <i>Robinia pseudoacacia</i> L. Alleviating Salinity

Soil salinisation and concomitant soil degradation pose a major threat on plant performance and terrestrial ecosystem sustainability. Rhizobia and arbuscular mycorrhizal fungi (AMF) were reported to mitigate negative effects of adversity in their host plants. Here, we exposed two cultivars of Robinia seedlings (salt-tolerant cultivar Lu155 and salt-sensitive cultivar DB) inoculated with salt-tolerant and salt-sensitive rhizobia and/or AMF to salt treatment. The results showed that individual inoculation with salt-tolerant rhizobia greatly reduced Na⁺ and increased K⁺ contents and K⁺/Na⁺ ratios in leaves and roots upon salt exposure. Co-inoculation of salt-tolerant rhizobium and AMF greatly decreased Na⁺ contents in leaves and increased K⁺ contents and Na⁺/K⁺ ratios in roots and leaves of Robinia Lu155 more than individual inoculations. Inoculation with salt-tolerant rhizobium up-regulated the expression of genes involved in K⁺ and Na⁺ homeostasis (RpSOS1, RpHKT1, RpNHX1 and RpSKOR) in roots and leaves of the Lu155 Robinia cultivar, particularly upon co-inoculated with AMF. Apparently, the positive effects of salt-tolerant rhizobia and additive effect of co-inoculation of salt-tolerant rhizobia and AMF on K⁺ contents and K⁺/Na⁺ homeostasis of Robinia plants under salt exposure are mediated by upregulated expression of transporter genes in Robinia roots.