Exploring the application of signaling compounds and soil amendments to modulate plant–microbe interactions for improved plant salinity tolerance

Soil salinity presents major constraint for symbiotic root-microbe interactions and overall crop productivity. While compost amendments are effective, availability is limited in drylands where salinity is prevalent. As an alternative approach, we investigated the foliar application of signaling compounds. In this study, cowpea plants were evaluated under the treatments of foliar application of strigolactones (SL), salicylic acid (SA), SL and SA or SLSA, and coumarins (CMR), under soil amendment with gypsum (GP) and compost (CMP), and unamended natural saline soil as the control soil (CS) treatment. The microbiome structure in the rhizosphere, roots, leaves, and seeds were assessed, along with root traits, nodulation, arbuscular mycorrhizal (AMF) colonization, nutrient concentrations, and biomass yield. Several treatments exhibited positive effects, with CMP and SLSA yielding the most comprehensive improvements. The SLSA outperformed CMP in enhancing nodulation and AMF colonization. Only CMP induced a significant shift in the rhizosphere and root microbiome structure, while the leaf and seed microbiomes remained stable. An AMF Rhizophagus in the rhizosphere and AMF Diversispora in the root endosphere of SLSA treatment, and Actinobacteria and Streptomyces in the root endosphere of the SLSA treatment showed increased abundance. The increased abundance of several microbial groups with SLSA indicated their effectiveness as beneficial keystone taxa, which appears to be an influential factor for impacting salinity tolerance in the legume. Overall, these findings confirm that the application of signaling compounds is a viable strategy for enhancing symbiotic root-microbe interactions and improving plant productivity in saline soils.