Endophytic fungi isolated from the roots of the coastal halophyte <i>Sesuvium portulacastrum</i> around Hainan Island enhance salt tolerance in maize through regulating K<sup>+</sup>/Na<sup>+</sup> homeostasis

Plant endophytic fungi (EF) play crucial roles in enhancing plant resilience to saline conditions. In this study, the halophyte <i>Sesuvium portulacastrum</i> was collected from coastal saline zones adjacent to Hainan Island. Root EF was isolated employing the tissue block method, followed by comprehensive identification and diversity analysis. Salt-tolerant strains were subsequently identified, and pot experiments were conducted to examine their effects on maize growth under saline conditions. A total of 426 cultivable EF isolates were obtained from the roots of <i>S. portulacastrum</i> sampled across 20 distinct locations, categorized into 112 operational taxonomic units (OTUs). Notable differences in EF species distribution were observed across the sampled regions. Screening on potato dextrose agar (PDA) plates supplemented with 0.75 M NaCl revealed that eight EF isolates exhibited significant salt tolerance, with enhanced growth compared to controls. Among these, <i>Fusarium incarnatum</i> strain LG-BZ-9 was shown to not only promote maize growth but also bolster its salt tolerance. Under salt stress conditions, the application of strain LG-BZ-9 led to increased fresh weight, plant height, and leaf chlorophyll content in maize seedlings. Furthermore, a substantial reduction in Na⁺ concentration within maize roots and shoots was observed, accompanied by an increase in K⁺ concentration, resulting in a higher K⁺/Na⁺ ratio. The EF strain isolated in this research effectively enhanced salt tolerance and stimulated maize growth by modulating ion homeostasis. These findings offer a theoretical basis for leveraging beneficial microorganisms to improve crop salt tolerance and augment yields in saline soils.