IBBM, Facultad de Ciencias Exactas, CCT-La Plata CONICET, Universidad Nacional de La Plata, La Plata, Argentina.
Center for Synthetic Microbiology (SYNMIKRO), Department of Biology, Philipps-Universität Marburg, Marburg, Germany.
Appl Environ Microbiol. 2021 Apr 13;87(9). doi: 10.1128/AEM.02989-20.
When subjected to nutritional stress, bacteria modify their amino acid metabolism and cell division activities by means of the stringent response, which is controlled by the Rsh protein in alphaproteobacteria. An important group of alphaproteobacteria are the rhizobia, which fix atmospheric N in symbiosis with legume plants. Although nutritional stress is common for rhizobia while infecting legume roots, the stringent response has scarcely been studied in this group of soil bacteria. In this report, we obtained a mutant with a kanamycin resistance insertion in the gene of , the N-fixing symbiont of soybean. This mutant was defective for type 3 secretion system induction, plant defense suppression at early root infection, and nodulation competition. Furthermore, the mutant produced smaller nodules, although with normal morphology, which led to lower plant biomass production. Soybean () genes and , involved in autoregulation of nodulation, were upregulated in plants inoculated with the mutant under the N-free condition. In addition, when plants were inoculated in the presence of 10 mM NHNO, the mutant produced nodules containing bacteroids, and and were downregulated. The mutant released more auxin to the culture supernatant than the wild type, which might in part explain its symbiotic behavior in the presence of combined N. These results indicate that the stringent response integrates into the plant defense suppression and regulation of nodulation circuits in soybean, perhaps mediated by the type 3 secretion system. The symbiotic N fixation carried out between prokaryotic rhizobia and legume plants performs a substantial contribution to the N cycle in the biosphere. This symbiotic association is initiated when rhizobia infect and penetrate the root hairs, which is followed by the growth and development of root nodules, within which the infective rhizobia are established and protected. Thus, the nodule environment allows the expression and function of the enzyme complex that catalyzes N fixation. However, during early infection, the rhizobia find a harsh environment while penetrating the root hairs. To cope with this nuisance, the rhizobia mount a stress response known as the stringent response. In turn, the plant regulates nodulation in response to the presence of alternative sources of combined N in the surrounding medium. Control of these processes is crucial for a successful symbiosis, and here we show how the rhizobial stringent response may modulate plant defense suppression and the networks of regulation of nodulation.
当受到营养压力时，细菌通过严格反应来改变其氨基酸代谢和细胞分裂活动，该反应由 α 变形菌中的 Rsh 蛋白控制。α 变形菌中的一个重要群体是根瘤菌，它们与豆科植物共生固氮。尽管根瘤菌在感染豆科植物根部时经常受到营养压力的影响，但严格反应在这群土壤细菌中几乎没有被研究过。在本报告中，我们获得了一个在大豆固氮共生体 基因中插入卡那霉素抗性的突变体。该突变体在 III 型分泌系统诱导、早期根感染时的植物防御抑制和结瘤竞争方面存在缺陷。此外，突变体产生的根瘤较小，尽管形态正常，但导致植物生物量产量降低。在无氮条件下接种突变体的植物中，大豆 () 基因 和 ，参与结瘤的自动调节，上调。此外，当植物在 10mM NHNO 存在下接种时，突变体产生含有类菌体的根瘤，并且 和 下调。突变体向培养上清液中释放出比野生型更多的生长素，这可能部分解释了它在存在组合氮的情况下的共生行为。这些结果表明，严格反应整合到大豆中植物防御抑制和结瘤调控电路中，可能通过 III 型分泌系统介导。原核根瘤菌和豆科植物之间进行的共生固氮对生物圈内的氮循环做出了巨大贡献。这种共生关系始于根瘤菌感染并穿透根毛，随后根瘤的生长和发育，其中感染性根瘤菌得以建立和保护。因此，根瘤环境允许催化固氮的酶复合物的表达和功能。然而，在早期感染过程中，根瘤菌在穿透根毛时会遇到恶劣的环境。为了应对这种麻烦，根瘤菌会产生一种应激反应，称为严格反应。反过来，植物会根据周围介质中是否存在其他来源的组合氮来调节结瘤。这些过程的控制对成功的共生至关重要，在这里我们展示了根瘤菌严格反应如何可能调节植物防御抑制和结瘤调控网络。
