Microbial activities are the dynamic core of nutrient cycling in organic substrates, and the exploitation of plant growth-promoting rhizobacteria strains contributes to sustainable agricultural development. This study aimed to investigate the effect and mechanism of Bacillus velezensis SX13 in nutrient cycling and plant promotion under different substrate supply conditions. The effects of reduced substrate amount (sCK) and inoculation of SX13 strain under both substrate supply conditions (Bv and sBv) on rhizosphere microenvironment and plant growth were investigated using conventional substrate amount (CK) as a control. Results showed no significant difference in the α-diversity indexes (Chao1 and Shannon) of the rhizospheric microbial community among the four treatments. However, nonmetric multidimensional scaling analysis and principal coordinate analysis revealed that compared with CK treatment, the inoculation of SX13 strain and reduced substrate supply reshaped the β-diversity structure of microbial communities. Furthermore, inoculation with B. velezensis SX13 under both substrate supply conditions increased the abundance of Proteobacteria (1.64%–2.46 %), Acidobacteria (14.09%–43.07 %), and Firmicutes (179.29%–861.29 %). The results of the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the metabolic pathway with the highest abundance of enriched genes was also the pathway with the most enriched differential genes caused by reducing substrate supply or inoculation of B. velezensis SX13. The rhizosphere inoculation of B. velezensis SX13 significantly up-regulated the top genes related to carbohydrate esterases, carbohydrate binding modules, glycoside hydrolases, glycoside transferases, and polysaccharide lyases. As a result, the activities of carbon and nitrogen cycle-related enzymes such as cellobiohydrolase, β-glucosidase, urease, l-leucine amino peptidase, and β-1 4-N-acetylglucosaminidase were increased, which in turn accelerated nutrient cycling. B. velezensis SX13 and its mediated improvement of the rhizospheric microenvironment resulted in the up-regulation of root CsNRT family genes (such as CsNRT1.1, CsNRT1.4a, CsNRT1.4b, CsNRT1.5a, CsNRT1.5b, CsNRT1.5c, and CsNRT1.8), which accelerated nitrogen uptake, accumulation, and utilization efficiency and ultimately improved the yield and quality of cucumber. The effect of SX13 strain was more stable and efficient under conventional substrate supply conditions than under reduced substrate supply conditions.

中文翻译：

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基于宏基因组的 Bacillus velezensis SX13 在不同栽培环境下调控根茎环境和黄瓜生长的机制研究


微生物活动是有机基质中养分循环的动态核心，开发促进植物生长的根际细菌菌株有助于农业可持续发展。本研究旨在探讨不同基质供应条件下 Bacillus velezensis SX13 在养分循环和植物推广中的作用和机制。以常规底物量 （CK） 为对照，研究了在底物供应条件 （Bv 和 sBv） 下减少底物量 （sCK） 和接种 SX13 菌株对根际微环境和植物生长的影响。结果显示，4 种处理间根际微生物群落的α多样性指数 （Chao1 和 Shannon） 差异不显著。然而，非度量多维尺度分析和主坐标分析表明，与 CK 处理相比，SX13 菌株的接种和底物供应的减少重塑了微生物群落的β多样性结构。此外，在两种底物供应条件下接种 B. velezensis SX13 增加了变形菌门 （1.64%–2.46 %）、酸杆菌门 （14.09%–43.07 %） 和厚壁菌门 （179.29%–861.29 %） 的丰度。京都基因与基因组百科全书 （KEGG） 通路富集分析结果显示，富集基因丰度最高的代谢途径也是由于减少 B. velezensis SX13 的底物供应或接种引起的差异基因富集最多的通路。B. velezensis SX13 的根际接种显著上调了与碳水化合物酯酶、碳水化合物结合模块、糖苷水解酶、糖苷转移酶和多糖裂解酶相关的顶级基因。 结果，纤维二糖水解酶、β-葡萄糖苷酶、脲酶、L-亮氨酸氨基肽酶和 β-1-4-N-乙酰氨基葡萄糖苷酶等碳氮循环相关酶的活性增加，进而加速了营养循环。B. velezensis SX13 及其介导的根际微环境改善导致根系 CsNRT 家族基因 （如 CsNRT1.1、CsNRT1.4a、CsNRT1.4b、CsNRT1.5a、CsNRT1.5b、CsNRT1.5c 和 CsNRT1.8）的上调，加速了氮素的吸收、积累和利用效率，最终提高了黄瓜的产量和品质。SX13 菌株在常规衬底供应条件下的效果比在减少衬底供应条件下更稳定和有效。