Effects of dark septate endophyte on root growth, physiology and transcriptome of Ammopiptanthus mongolicus seedlings under drought stress

As the only evergreen relict species in the desert environment of western China, Ammopiptanthus mongolicus (Leguminosae) roots is colonized with dark septate endophytes (DSE), but the potential of DSE to alleviate the adverse effects of drought on seedling roots remains uncertain. This study examined the effects of DSE on root growth, physiology and transcriptome of A. mongolicus under drought stress. Drought drastically reduced root biomass by 47.7%, while all DSE strains established positive symbiosis with A.mongolicus, with G.hyphopodioides having the most pronounced promoting effect. Inoculation with G. hyphopodioides alleviated drought stress injury by increasing CAT activity, AsA content and soluble sugar content in the roots, with a significant reduction in MDA accumulation by 97.7%. G. hyphopodioides also significantly increased zeatin and brassinosteroid contents, which in turn regulated the root structure and increased root activity, resulting in a 208.6% increase in root biomass. Transcriptome analysis screened 1246 differentially expressed genes (542 up-regulated and 704 down-regulated) between G. hyphopodioides inoculation under drought treatment, mainly associated with phenylpropanoid biosynthesis, ascorbic acid and aldehyde metabolism, hormone synthesis and signalling, sucrose and starch metabolism, and vitamin B6 metabolism, and further investigated and identified key potential genes and transcription factors (DREB, ERF, NAC, MYB, C2H2). These findings reveal the physiological and molecular mechanisms by which DSE symbiosis improves the drought resistance of A. mongolicus seedlings, providing valuable guidance on the use of DSE resources to promote ecological construction and production of desert plants.