Rhizosphere microorganisms mediate ion homeostasis in cucumber seedlings: a new strategy to improve plant salt tolerance.

Background: Soil salinization is a formidable challenge for vegetable production, primarily because of the detrimental effects of ion toxicity. Rhizosphere microorganisms promote plant growth and bolster salt tolerance, but the extent to which microbial communities can increase plant resilience by regulating ion homeostasis under salt stress remains underexplored. The goal of this study was to enrich microbial communities from the rhizosphere of salt-stressed cucumber seedlings and identify their impact on ion balance and plant growth under saline conditions.

Results: Salt stress induced significant alterations in the composition, structure, and function of the root-associated microbial community. Compared with a 75 mM NaCl treatment alone, inoculation with salt-induced rhizosphere microorganisms (SiRMs) under the same conditions significantly increased the growth of cucumber seedlings; plant height increased by 61.3%, and the fresh weights of the shoots and roots increased by 45.3% and 38.9%, respectively. Moreover, superoxide dismutase (SOD) activity increased by 4.1%, and peroxidase (POD) activity and superoxide anion (O₂·^-) content decreased by 10.5% and 3.7%, respectively. In the roots, stems, and leaves of cucumber seedlings treated with SiRMs and 75 mM NaCl, the Na⁺ content was significantly reduced by 15.8%, 18.9%, and 9.7%, respectively. Conversely, the K⁺ content significantly increased by 32.7%, 16.9%, and 28.8%, respectively. Under salt stress conditions, inoculation with SiRMs significantly increased the rate of Na⁺ expulsion in the roots of cucumber seedlings by 18.3%, but the K⁺ expulsion rate decreased by 76.7%. These dynamic changes are attributed to the upregulation of genes such as CsHKT1, CsHAK5, and CsCHX18;4.

Conclusions: Enrichment with SiRMs played a pivotal role in maintaining ion homeostasis and significantly enhanced the salt tolerance of cucumber seedlings. These findings highlight the potential for microbial-assisted strategies to mitigate the adverse effects of soil salinity and provide valuable insights into the complex interplay between the microbial community and plant resilience from the perspective of ion balance.