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

Abstract

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.

Keywords: Cucumber; Ion homeostasis; Rhizosphere microorganisms; Salt stress.

Fig. 1

Root-associated bacterial community composition, structure, and function under salt stress. a Relative abundance of bacteria at the phylum level; the “others” category includes phyla with a relative abundance less than 1%. b Principal coordinate analysis (PCoA) for the microbial community structure and c constrained principal coordinate analysis (CPCoA) for the microbial community structure in the salt stress treatment and d across sampling locations. e A functional prediction of microbial communities using FAPROTAX shows the top 10 functions based on summed mean values. A Kruskal‒Wallis rank sum test was used for multiple group comparisons. The horizontal axis indicates the function name, the vertical axis shows the percentage abundance of each function within the sample, different colours represent different subgroups, and P values are indicated on the far right. An * indicates 0.01 < P ≤ 0.05

Fig. 2

Effects of salt-induced rhizosphere microorganisms (SiRMs) on the growth of cucumber seedlings under salt stress. a Schematic diagram of the experimental design. b Composition of salt-induced rhizosphere bacteria at the genus level. Effects of SiRMs on c plant height, d shoot fresh weight, e root fresh weight, f root length, g root surface area, and h root volume of cucumber seedlings under salt treatment. Duncan's multiple range test was used for multiple comparisons. Different letters denote significant differences between treatments (P < 0.05). All the plants were grown in sterile soil under four treatment conditions: treatment with sterile water (control), treatment with 75 mM NaCl (Na), treatment with SiRM inoculation under nonsaline conditions (Rh), and treatment with SiRM inoculation under 75 mM NaCl conditions (RN)

Fig. 3

Effects of salt-induced rhizosphere microorganisms (SiRMs) on the antioxidant enzyme activity, root activity, and nutrient uptake of cucumber seedlings under salt stress. a Superoxide dismutase (SOD) activity, b peroxidase (POD) activity, c superoxide anion (O₂·⁻) content, d root activity, and e nutrient element content ratio of cucumber seedlings in the rhizosphere microorganism and salt stress treatments. Duncan's multiple range test was used for multiple comparisons. Different letters denote significant differences between treatments (P < 0.05), and asterisks indicate significant differences at the 0.05 level. All the plants were grown in sterile soil under four treatment conditions: treatment with sterile water (control), treatment with 75 mM NaCl (Na), treatment with SiRM inoculation under nonsaline conditions (Rh), and treatment with SiRM inoculation under 75 mM NaCl conditions (RN)

Fig. 4

Effects of salt-induced rhizosphere microorganisms (SiRMs) on the ion content in roots, root H⁺-ATPase activity, and root ion fluxes of cucumber seedlings under salt stress. a Na⁺ content, b K⁺ content, c Ca²⁺ content, d H⁺-ATPase activity, e Na⁺ flux, f K⁺ flux, g H⁺ flux, and h Cl⁻ flux in the roots of cucumber seedlings in the rhizosphere microorganism and salt stress treatments. Positive values represent efflux, negative values represent influx, and different letters in the graphs indicate significant differences between treatments (Duncan's multiple range test, P < 0.05). All the plants were grown in sterile soil under four treatment conditions: treatment with sterile water (control), treatment with 75 mM NaCl (Na), treatment with SiRM inoculation under nonsaline conditions (Rh), and treatment with SiRM inoculation under 75 mM NaCl conditions (RN)

Fig. 5

Effects of salt-induced rhizosphere microorganisms (SiRMs) on gene expression in cucumber seedlings under salt stress. a Volcano plot of differentially expressed genes (DEGs) in the RN treatment vs. the Na treatment. b Heatmap and cluster analysis of DEGs under different treatments. c GO enrichment analysis of DEGs in the RN treatment vs. the Na treatment (top 20). d Functional annotation analysis of DEGs in the RN treatment vs. the Na treatment (top 20). BP: Biological process, CC: Cellular component, MF: Molecular function. RN: Cucumber seedlings inoculated with SiRMs under 75 mM NaCl conditions. Na: Cucumber seedlings treated with 75 mM NaCl

Fig. 6

Effects of salt-induced rhizosphere microorganisms (SiRMs) on the expression of genes related to ion transport in cucumber seedling roots under salt stress. For qRT‒PCR analysis, Actin was used as the reference gene, and gene expression in roots without SiRM inoculation under 75 mM NaCl conditions was defined as 1. The data are presented as the means ± standard errors, and different letters on the graph indicate significant differences between treatments (P < 0.05)