Su-yang Tang, Peng-fei Liu, Fu Chen, Si-ying Tang, Xian-fu Zhou, Bing-gan Lou
Introduction Glycyrrhiza uralensis Fisch. is a medicinal plant commonly cultivated in salinized soils, where environmental stress suppresses the accumulation of pharmaceutically active components. To date, only limited studies have examined whether Piriformospora indica , a root endophytic fungus with growth-promoting and stress-alleviating properties, can improve the salt tolerance and medicinal quality of G. uralensis , particularly at the physiological and transcriptional levels. Methods In this study, we successfully established a symbiotic system between G. uralensis and P. indica . To evaluate responses to salt stress, P. indica -inoculated and non-inoculated plants were subjected to NaCl treatments at 0, 100, 200, 300, and 350 mM, with 18 biological replicates per treatment. Colonization by P. indica was confirmed through microscopic examination and molecular identification. Growth phenotypes, antioxidant enzyme activities, membrane lipid peroxidation levels, chlorophyll-related indices, and the accumulation of key medicinal components were systema4tically quantified in symbiotic G. uralensis across different growth stages. Results Inoculation with P. indica significantly increased plant height, root length, and dry weight by 27.8%, 25.5%, and 52.2%, respectively. The symbiotic association enhanced the activities of the antioxidant enzymes superoxide dismutase (SOD) and peroxidase (POD) by 48.4% and 27.5%, respectively. Although malondialdehyde (MDA) content initially increased by 16.5% due to early fungal colonization, the canopy SPAD value simultaneously increased by 20.3%. These findings suggest that P. indica colonization is associated with differential oxidative stress responses between roots and shoots. Furthermore, under the high salt concentration of 300 mM NaCl, the contents of liquiritin and glycyrrhizic acid were markedly increased by 124.7% and 62.5%, respectively. P. indica enhanced the accumulation of secondary metabolites by modulating key rate-limiting enzyme genes rather than indiscriminately activating entire metabolic pathways. For example, the symbiont significantly upregulated GuHMGR in triterpenoid saponin biosynthesis and GuCHR in flavonoid biosynthesis. These transcriptional changes may contribute to alleviating salt-induced constraints on secondary metabolite accumulation. Discussion In conclusion, P. indica colonization significantly improved growth performance, stress resistance, and medicinal compound accumulation in G. uralensis under salt stress. This study provides a theoretical foundation for improving the cultivation quality of G. uralensis in saline soils.