Research progress on the evaluation, screening, and mechanisms of salinity tolerance in Azolla

Soil salinization is a major challenge to global agriculture and ecosystems. Screening and breeding salt-tolerant plants is a key strategy for achieving the sustainable utilization of saline-alkali land. Azolla is a group of aquatic ferns with significant ecological and agronomic value. The obligate symbiotic system formed by Azolla and the nitrogen-fixing cyanobacteria has considerable potential for the sustainable remediation of saline-alkali environments. We synthesized recent advances in salinity-tolerance evaluation, screening of salt-tolerant germplasm, and the underlying physiological and molecular mechanisms in Azolla. The available studies have preliminarily established a multidimensional evaluation system based on growth, morphological, physiological, and biochemical indicators, revealing significant differences in salinity tolerance among various germplasms. The salt-tolerance mechanisms of Azolla involve coordinated response across multiple levels, including regulation of cellular ion homeostasis, osmotic adjustment and metabolic remodeling, enhanced antioxidant defenses, and adaptive adjustments in host-cyanobacteria symbiotic interactions. Meanwhile, we identified the limitations in current research, including the lack of unified evaluation criteria, an incomplete understanding of the genetic basis of salt tolerance, and limited exploration of combined stresses and their mechanisms. Future studies should integrate gene editing, synthetic biology, and host-cyanobacteria symbiotic interactions, establish a coordinated screening system for salt-tolerant germplasm and symbiotic cyanobacteria, strengthen research on combined stresses, and systematically evaluate their field application potential and ecological benefits.