Synergistic regulation of physiological, nutritional, cellular, and molecular responses in Pennisetum glaucum by humic acid and Serendipita indica under salinity stress.

Soil salinity is a major constraint to agricultural productivity, disrupting plant growth through osmotic stress, ion toxicity, oxidative damage, and impaired cellular homeostasis. Sustainable strategies that integrate beneficial microorganisms with natural biostimulants may provide an effective approach to enhancing crop resilience under saline conditions. This study investigated the individual and synergistic effects of the root endophytic fungus Serendipita indica and humic acid (HA) on salinity tolerance in Pennisetum glaucum grown under 0, 100, and 200 mM NaCl. Plant responses were evaluated through integrated morphological, physiological, biochemical, ultrastructural, and molecular analyses. Salinity markedly impaired growth, photosynthetic performance, nutrient acquisition, metabolite accumulation, antioxidant capacity, membrane stability, and chloroplast integrity while suppressing the expression of aquaporin (PgPIP1.1 and PgTIP1.1) and salt overly sensitive (PgSOS1 and PgSOS2) genes. Both S. indica and HA alleviated these adverse effects; however, their combined application consistently produced the greatest improvement across all biological levels. The synergistic treatment enhanced root architecture, biomass accumulation, photosynthetic efficiency, osmotic adjustment, antioxidant defense, and nutrient assimilation while preserving chloroplast ultrastructure and plasma membrane integrity. Moreover, co-application markedly upregulated genes involved in water transport and ion homeostasis, indicating coordinated molecular regulation underlying enhanced salt tolerance. Collectively, these findings demonstrate that the integration of S. indica with HA confers superior salinity resilience through coordinated physiological, cellular, and transcriptional reprogramming rather than individual stress mitigation mechanisms. This work provides mechanistic evidence supporting microbial-biostimulant synergy as a sustainable strategy for improving pearl millet performance in salt-affected agroecosystems.