Bacterial cell-free supernatants as metabolite-based biostimulants for alleviating salinity stress in plants

Soil salinity severely limits global crop productivity, affecting approximately 20% of irrigated land under changing climate conditions. It impairs water uptake, disrupts nutrient balance, and induces oxidative stress, leading to significant yield losses, particularly in salt-sensitive crops. This review evaluates the potential of metabolite-rich bacterial cell-free supernatants (CFS) derived from plant growth-promoting bacteria (PGPB) as biostimulants for mitigating salinity stress in crops including soybean, canola, and wheat. CFS contain diverse bioactive compounds, including phytohormones, osmolytes, antioxidants, microbe-to-plant signal compounds and nutrient-solubilizing agents, which enhance plant performance through mechanisms such as modulation of hormonal signaling, activation of antioxidant defenses, and regulation of ion homeostasis. Compared to living-cell microbial inoculants, CFS offer advantages in stability and regulatory acceptance. However, their practical application is constrained by variability in composition, lack of standardized protocols, and limited field validation. Addressing these challenges through targeted research and field studies will be critical to enable the adoption of CFS-based strategies for climate-resilient agriculture.