Rhizosphere Microecological Mechanism Of Selenium Enhancing Phosphorus Uptake In Citrus

The application of phosphate fertilizer(P)has become the primary agronomic measure for enhancing citrus yield and quality.However,citrus exhibits low efficiency in P absorption,leading to excessive fertilization in orchards to ensure adequate P supply.This practice is prone to causing P loss and water eutrophication issues.Therefore,improving P uptake by citrus has emerged as a pressing concern in the field of ecological environment.There exists a plethora of microorganisms associated with P cycling in the rhizosphere of plants,which can exert an influence on the processes of P form,transport and transformation.Selenium(Se),as a beneficial element in plants,has the potential to enhance nutrient uptake.However,there is limited research on whether Se can promote P absorption and utilization by citrus rhizosphere microorganisms.Furthermore,the mechanism of Se regulation on citrus rhizosphere microorganisms remains poorly understood.In light of this,we established a Se-CitrusP test system to elucidate the promoting effect of Se on P absorption and utilization in citrus,analyze the impact of Se on rhizosphere microorganisms and root exudates of citrus,and explore their correlation mechanism.Key findings are as follows:1.Se in soil can enhance the growth of Poncirus trifoliate(L.)Raf roots and promote P absorption and accumulation in Poncirus trifoliate(L.)Raf Raf.Compared with the control group,there was a 23.4% increase in P accumulation in the root system of Poncirus trifoliate(L.)Raf.The results of microbiome sequencing revealed that Se significantly altered the structure and composition of the rhizosphere microbial community in Poncirus trifoliate(L.)Raf,with a greater impact on bacterial communities than fungi.Se supplementation significantly increased the relative abundance of Actinobacteria and Proteobacteria by 39.32% and 7.66%,respectively.Moreover,beneficial bacteria related to nutrient uptake and plant growth such as Streptomyces,Micrococcaceae,Cupriavidus,Arthrobotrys were also significantly enriched by Se.The PICRUSt2 analysis revealed that Se supplementation promoted metabolic pathways associated with nutrient uptake and amino acid metabolism in Poncirus trifoliate(L.)Raf rhizosphere,Additionally,the relative abundance of functional genes closely related to soil P cycling,such as phosphatase and inorganic P transporters,was also increased.2.In a hydroponic experiment,Se significantly altered the morphology of Poncirus trifoliate(L.)Raf.The main characteristics were increased number of lateral roots,larger root diameter and larger root volume.The root biomass of Poncirus trifoliate(L.)Raf increased with rising Se concentration.Liquid chromatography-mass spectrometry(LC-MS)was employed to further investigate the impact of Se on root metabolism of Poncirus trifoliate(L.)Raf.The results showed that there were 12 upregulated substances and 2 down-regulated substances under the action of Se.Se significantly increased the contents of L-pipecolic acid,cyclopentylglycine,cycloleucine,inositol,L-proline,probetaine,allitol and other amino acids,organic acids and sugar alcohols in the root exudates of Poncirus trifoliate(L.)Raf.These differential metabolites were mainly enriched in amino acid biosynthesis,lysine biosynthesis,lysine degradation,arginine biosynthesis,ABC transporters,secondary metabolite biosynthesis and other metabolic pathways.In summary,Se exerted a more pronounced impact on the metabolic pathways associated with amino acids in the rhizosphere of Poncirus trifoliate(L.)Raf.3.Bacteria present in the rhizosphere soil of Poncirus trifoliate(L.)were isolated and identified based on microbiome analysis,followed by functional verification.Sequence alignment analysis revealed that certain isolates not only exhibited proficient dissolved P capabilities,but also belonged to the core OTU,such as Streptomyces FXJ1.553 and Mesorhizobium plurifarium.Furthermore,their relative abundance was significantly upregulated following Se treatment.The up-regulated metabolites in the root exudates exhibited varying degrees of promotion on bacterial growth.Overall,Se enriched microbial groups were more stimulated by metabolite nutrition.At the same time,some bacteria are attracted by proline,cyclopentylglycine,piperidinic acid,inositol and other signaling molecules to form chemotaxis.By establishing a synthetic microbial community in pot experiments,it was observed that the microbiota exhibiting positive responses to Se had superior effects on enhancing P accumulation of root.4.Se enhanced P uptake and accumulation in citrus and reduced P loss from soil.Compared to the control group,the combination of sodium selenite with calcium and magnesium or chitin resulted in a reduction of P loss by 31.26% and 38.32%,respectively,which was superior to that achieved by calcium and magnesium or chitin alone.Moreover,plant P accumulation under the treatment of sodium selenite was also significantly increased.Through metagenomic analysis,we have observed changes in the composition of microorganisms and functional genes induced by Se.Specifically,there was an increase in the number of microbial species present on the soil surface layer,as well as a rise in the relative abundance of P-solubilizing microorganisms such as Pseudomonas,Bacillus,Rhizobium and Streptomyces.Furthermore,it was noted an elevation in the relative abundance of Pst SAC-a functional gene that plays a crucial role in regulating P transport within soil’s P cycle.P metabolism-related genes,such as ppa,pho D and pho R,as well as phosphate-solubilizing microorganisms like rhizobia and Streptomyces,are believed to play a pivotal role in the cycling of soil P.In conclusion,Se could improve P uptake and accumulation in citrus and reduce P loss in soil.Se primarily influences the exudation of proline and cycloleucine in root exudates via the amino acid metabolic pathway,thereby recruiting and enriching beneficial microbial communities associated with plant growth and P absorption.This enhances the abundance of P metabolism function genes in citrus rhizosphere,ultimately promoting citrus root development as well as improving P absorption and utilization.The results have provided a novel concept and theoretical foundation for the application of Se in regulating citrus rhizosphere processes,thereby achieving efficient nutrient uptake by citrus plants.