Alleviating Effect Of Silica Nanoparticles On Salt-stressed Soybean And Its Microbiological Mechanism

Salinity is one of the important abiotic stresses limiting crop growth and productivity improvement worldwide.The microbiome plays a key role in plant growth and development,health and defense against biotic and abiotic stresses.Silica nanoparticles（SiO₂ NPs）has great potential in alleviating plant salt stress,but whether and how it can mediate plant-microbe interactions to improve plant salt adaptation is still unclear.Therefore,in this study,soybean was used as the test plant,and SiO₂ NPs were applied by foliar spray and soil application.Integrating plant growth and physiology,plant microbiome and root metabolome,the mechanism of SiO₂ NPs-mediated microbial response in soybean adaptation to salinity stress was explored.The main findings are as follows:1.Both foliar and soil application of SiO₂ NPs can promote the growth of soybean under salinity,such as increasing shoots and roots biomass,improving root morphology and structure（root length,root surface area,root volume and root tip number）,enhancing photosynthetic parameters（photosynthetic rate,transpiration rate,stomatal conductance and intercellular CO₂concentration）.Moreover,SiO₂ NPs can also inhibit the absorption of Na⁺by roots,promote the accumulation of K⁺,and maintain Na⁺/K⁺homeostasis in shoots and roots.2.In terms of the effect of foliar spraying of SiO₂ NPs on soybean bacterial microbiome under saline condition,the species diversity of bacterial communities in leaves,roots and rhizosphere had no significant changes,but the species richness of bacterial communities in roots increased significantly.SiO₂ NPs not only changed the structure of the endophytic bacterial community in soybean leaves and roots,but also affected the rhizosphere bacterial community.The co-occurrence network results showed that SiO₂ NPs increased the complexity and stability of the bacterial co-occurrence network under salt stress.After foliar application of SiO₂ NPs,the orders Bacillales and Burkholderiales were enriched in leaves,the genera Pseudarthrobacter,Bacillus and Variovorax were enriched in roots,and the genera Flavisolibacter,Lysobacter and Sphingomonas were enriched in rhizosphere.Spearman correlation analysis demonstrated that there was a significant positive correlation between the bacterial genera enriched in root endosphere and the parameters related to soybean growth（shoots fresh weight,roots fresh weight,root length,root surface area,root volume and root tips）under salinity stress,while the bacterial genera enriched in the rhizosphere were relatively weakly correlated with soybean growth.3.After the application of SiO₂ NPs to the soil,the species diversity of the bacterial community in the root endosphere and rhizosphere of soybean under salinity was not significantly affected,but the species richness of the bacterial community increased.SiO₂ NPs amendment had a strong effect on the bacterial community structure in the rhizosphere,but a relatively weak effect on the bacterial community structure in the root endosphere.Moreover,SiO₂ NPs amendment improved the stability of the bacterial co-occurrence network by increasing the complexity of the bacterial network in the root endosphere and the negative link ratio of the bacterial network in the rhizosphere under salt stress.LEf Se analysis showed that the endophytic bacteria Amycolatopsis,Pedomicrobium and Virgisporangiun were enriched under SiO₂ NPs treatment.In the rhizosphere,the relative abundance of Acidibacter,Haliangium and Roseisolibacter were significantly up-regulated by soil application of SiO₂NPs.Correlation analysis revealed that there were significant positive correlations between soybean growth-related parameters under saline condition and the microorganisms enriched in the root endosphere and rhizosphere.4.Through untargeted metabolomics analysis,we found that soil application of SiO₂ NPs altered the composition of soybean root exudates under salinity stress,with the relative contents of 163 root exudates increasing and 104 root exudates decreasing.These differential metabolites include carbohydrates,organic acids,amino acids,alkaloids and flavonoids,etc.Enrichment analysis showed that SiO₂ NPs significantly affected multiple metabolic pathways in soybean roots under salinity,including tryptophan metabolism,arginine and proline metabolism,purine metabolism,indole alkaloid biosynthesis,flavonoid biosynthesis,etc.Correlation analysis suggested that the up-regulation of some key metabolites caused by soil application of SiO₂ NPs was significantly positively correlated with the enrichment of microbial taxa.In summary,both foliar and soil application of SiO₂ NPs can directly improve soybean growth in saline soils by regulating physiological and biochemical responses,and can also mediate plant-microbe interactions to indirectly enhance plant salt adaptation.Moreover,the response patterns of microbial communities were different under the two application methods.Specifically,foliar spraying of SiO₂ NPs mainly helps soybeans resist salinity stress by enriching some beneficial endophytic bacteria in roots,while soil application of SiO₂ NPs can promote growth and salt tolerance by increasing the abundance of beneficial bacterial taxa in the root endosphere and rhizosphere.In addition,SiO₂ NPs mediated the response changes of root exudates,which also played an important role in the plant-microbe interaction against salt stress.This study elucidates the mechanism of SiO₂ NPs-mediated plant-microbe interaction to mitigate plant salinity stress,and provides a theoretical basis for the rational use of SiO₂NPs to improve plant salt adaptability.