论文题目：Bacillus populations restore amino acid metabolism in Mesorhizobium under saline-alkali stress to enhance nitrogen fixation efficiency
论文作者：Jiamin Ai, Mingxia Ren, Ziwei Hao, Yuru Zhai, Entao Wang, Zhenshan Deng, Zhefei Li
论文摘要：The root nodules formed by rhizobia and leguminous plants are specialized structures for nitrogen fixation. However, a large number of non-rhizobial endophytes (NREs) also coexist within the nodules, and their contribution to nitrogen fixation under abiotic stress conditions remains unclear. Here, using the wild leguminous shrub Sophora davidii as model system, we identified an important NRE (Bacillus siamensis BT-9-1) by analyzing keystone taxa within the bacterial cooccurrence network of root nodules. This strain could improve the survival of Mesorhizobium metallidurans YC-39 under saline-alkali stress. A mechanistic investigation revealed that the expression of ilvA, ilvH, and ilvD was downregulated, and the contents of (2S)-isopropylmalate and succinic acid decreased in M. metallidurans YC-39 under saline-alkali conditions, whereas B. siamensis BT-9-1 presented increased accumulation of these metabolites. These findings indicate that B. siamensis BT-9-1 cross-feeds M. metallidurans YC-39 with these metabolites, rescuing the compromised branched-chain amino acid synthesis pathway and the TCA cycle in saline-alkali environments. Eventually, coinoculation with B. siamensis BT-9-1 and M. metallidurans YC-39, along with (2S)-isopropylmalate and succinic acid supplementation, increased nitrogenase activity of the symbionts. Our study reveals a novel mechanism by which non-rhizobial endophyte Bacillus species enhances the growth and nitrogen fixation efficiency of M. metallidurans under saline-alkali stress through the delivery of key metabolites.
根瘤菌和豆科植物形成的根瘤是执行固氮功能的特殊结构。然而，在根瘤内同时存在大量的非根瘤菌内生菌（NREs），它们在非生物胁迫条件下对固氮的贡献尚不清楚。本文以野生豆科白刺花为研究对象，通过分析根瘤内细菌群落共发生网络中的关键类群，鉴定出一株芽孢杆菌。该菌株能提高中慢生根瘤菌YC-39在盐碱胁迫下的存活率。进一步研究表明，盐碱条件下，根瘤菌 YC-39的ilvA、ilvH和ilvD表达下调，(2S)-异丙基苹果酸酯和琥珀酸含量降低，而芽孢杆菌 BT-9-1的这些代谢物合成量增加。这些结果表明，芽孢杆菌 BT-9-1以交叉饲养方式将这些代谢物提供给根瘤菌YC-39，从而恢复了根瘤菌在盐碱环境中受损的支链氨基酸合成途径和TCA循环。最终，共接种芽孢杆菌和根瘤菌剂，并添加(2S)-异丙基苹果酸酯和琥珀酸，显著提高了根瘤的固氮效率。本研究揭示了非根瘤菌内生芽孢杆菌通过向根瘤菌提供关键代谢物来提高其在盐碱胁迫下的生长和共生机制。
论文链接：https://doi.org/10.1093/ismejo/wrag087