Data_Sheet_1_Inorganic Phosphate Solubilization by Rhizosphere Bacterium Paenibacillus sonchi: Gene Expression and Physiological Functions.docx

Due to the importance of phosphorus (P) in agriculture, crop inoculation with phosphate-solubilizing bacteria is a relevant subject of study. Paenibacillus sonchi genomovar Riograndensis SBR5 is a promising candidate for crop inoculation, as it can fix nitrogen and excrete ammonium at a remarkably high rate. However, its trait of phosphate solubilization (PS) has not yet been studied in detail. Here, differential gene expression and functional analyses were performed to characterize PS in this bacterium. SBR5 was cultivated with two distinct P sources: NaH2PO4 as soluble phosphate source (SPi) and hydroxyapatite as insoluble phosphate source (IPi). Total RNA of SBR5 cultivated in those two conditions was isolated and sequenced, and bacterial growth and product formation were monitored. In the IPi medium, the expression of 68 genes was upregulated, whereas 100 genes were downregulated. Among those, genes involved in carbon metabolism, including those coding for subunits of 2-oxoglutarate dehydrogenase, were identified. Quantitation of organic acids showed that the production of tricarboxylic acid cycle-derived organic acids was reduced in IPi condition, whereas acetate and gluconate were overproduced. Increased concentrations of proline, trehalose, and glycine betaine revealed active osmoprotection during growth in IPi. The cultivation with hydroxyapatite also caused the reduction in the motility of SBR5 cells as a response to Pi depletion at the beginning of its growth. SBR5 was able to solubilize hydroxyapatite, which suggests that this organism is a promising phosphate-solubilizing bacterium. Our findings are the initial step in the elucidation of the PS process in P. sonchi SBR5 and will be a valuable groundwork for further studies of this organism as a plant growth-promoting rhizobacterium.

鉴于磷（phosphorus, P）在农业生产中的重要作用，利用溶磷细菌（phosphate-solubilizing bacteria）开展作物接种研究具有重要的学术与应用价值。索氏类芽孢杆菌（Paenibacillus sonchi）基因组变种Riograndensis SBR5可高效固氮并分泌铵态氮，是极具潜力的作物接种候选菌株，但其溶磷（phosphate solubilization, PS）特性尚未得到系统深入的研究。
本研究通过差异基因表达分析与功能实验，对该菌株的溶磷特性进行解析。实验采用两种不同磷源培养SBR5：以磷酸二氢钠作为可溶性磷源（soluble phosphate source, SPi），以羟基磷灰石作为不溶性磷源（insoluble phosphate source, IPi）。分别提取两种培养条件下SBR5的总RNA并进行转录组测序，同时监测菌株生长与代谢产物积累情况。
结果显示，在不溶性磷源培养基中，共有68个基因上调表达，100个基因下调表达。其中包括编码2-氧戊二酸脱氢酶亚基的基因在内的碳代谢相关基因被显著富集。有机酸定量分析表明，不溶性磷源培养条件下，三羧酸循环来源的有机酸合成量显著降低，而乙酸与葡糖酸的生成量则明显升高。脯氨酸、海藻糖与甜菜碱浓度的升高，表明菌株在不溶性磷源培养基中生长时，会启动活跃的渗透压保护机制以应对胁迫。此外，以羟基磷灰石培养时，SBR5细胞的运动能力出现下降，这是其在生长初期遭遇磷源耗尽时的应激响应。
实验证实SBR5具备溶解羟基磷灰石的能力，表明该菌株是极具潜力的溶磷细菌。本研究结果为解析索氏类芽孢杆菌SBR5的溶磷机制迈出了第一步，同时也为将该菌株开发为植物促生根际菌的后续研究提供了宝贵的研究基础。