Combined application of Bacillus amyloliquefaciens and sodium selenite promotes tea seedling growth and selenium uptake by improving soil pH and regulating the rhizosphere bacterial community

Low selenium (Se) fertilizer utilization efficiency in soil hampers agricultural Se biofortification. Microbes involved in the soil Se transport process can improve crops Se biofortification by increasing soil Se availability, while there is a lack of knowledge on the mechanism. To investigate its effect on Se uptake by tea plants and the underlying mechanism within the root-soil bacterial community, Bacillus amyloliquefaciens, a widely used plant growth-promoting bacteria was used, and four treatments including no fertilizer application (control), B. amyloliquefaciens application (BA), sodium selenite application (Se), and application of B. amyloliquefaciens and sodium selenite fertilization (BA+Se) were conducted. The results showed that, compared with the control, the BA+Se treatment significantly improved leaf biomass and total biomass of tea seedlings by 129.1% and 96.4%, respectively. Meanwhile, the BA+Se treatment significantly increased the Se concentrations in the leaf and Se content in the whole tea seedling by 101.4% and 149.5%, respectively, compared with the Se treatment. Additionally, the BA+Se treatment significantly increased the soil exchangeable Se and total available Se concentrations, compared with the Se treatment. Compared with the control, Se treatment significantly increased the expression levels of CsSULTR1;1, CsSULTR1;2, CsPHT1;2a and CsPHT1;2b in tea seedling roots; BA+Se treatment significantly increased the CsSULTR1;1 and CsPHT1;2a expression levels in tea seedling roots. The 16S rRNA of the rhizosphere bacteria indicated that neither B. amyloliquefaciens nor the application of Se fertilizer affected the diversity but altered the structure of the rhizosphere bacterial community. The relative abundance of Chloroflexi, Proteobacteria, and Actinobacteriota showed a downward trend, while the relative abundance of Acidobacteriota showed an upward trend with Se fertilizer addition. Soil pH and total carbon content were important environmental factors affecting the bacterial community, and the BA+Se treatment had the highest soil pH and the most complex symbiotic network. Further analysis indicated that the BA+Se treatment promoted Se fertilizer utilization efficiency by improving soil pH, recruiting the g_Sinomonas, and regulating the abundance of functional genes of Se reductase.