Reprogramming Plant Salt Tolerance: A Systems Biology Perspective on Algae-Derived Elicitors and Biostimulants

Soil salinisation poses a formidable threat to global agricultural productivity. It induces osmotic stress, ion toxicity and systemic oxidative damage in crops. Breeding salt-tolerant varieties is time-consuming. This creates an urgent need for rapid and sustainable agronomic interventions. Algae and their bioactive extracts, such as polysaccharides, phytohormones and polyols, act as potent biostimulants and biofertilizers. They effectively function as exogenous elicitors. This review synthesises the multidimensional interactions between algae and plant defence systems under salt stress. We evaluate how compounds derived from algae orchestrate plant tolerance to salt. They bypass or synergise with endogenous signalling pathways to reprogramme transcriptional networks. We delineate the mechanisms underpinning ionic homoeostasis mediated by algae. These include SOS pathway modulation, osmotic adjustment, reactive oxygen species scavenging, and hormonal crosstalk through pathways dependent on or independent of ABA. Furthermore, we critically compare algae extracts with other biostimulant classes and dissect their crop-specific efficacies under multifactorial combined stress scenarios. The focus then shifts from molecular paradigms to agricultural applications. Integrating algae into soil microbiomes and broader soil remediation strategies significantly enhances rhizosphere health. Translating these advances from controlled laboratory conditions to efficacy at the field scale remains a prominent bottleneck. We highlight current knowledge gaps and propose future trajectories, including multiple omics integration, synthetic biology, precision agriculture integration, and nanoscale delivery systems. These approaches will help fully harness the potential of the algae and plant holobiont for agriculture adapted to climate change.