Arbuscular mycorrhizal fungi (AMF) can transfer inorganic nitrogen (N) from the soil to host plants to cope with drought stress, with arginine synthesis and NH4+ transport being pivotal processes. However, the regulatory mechanism underlying these processes remains unclear. Here, we found that drought stress upregulated expression of genes involved in the N transfer pathway and putrescine and glutathione synthesis in the mycorrhizal structures of Rhizophagus irregularis within alfalfa (Medicago sativa) roots, i.e., carbamoyl-phosphate synthase (RiCPSI), arginase (RiCARI), urease (RiURE), ornithine decarboxylase (RiODC), and glutamate-cysteine ligase (RiGCL). Furthermore, we confirmed that RiCPSI is a carbamoyl phosphate synthase. Silencing RiCARI via host-induced gene silencing inhibited arbuscule formation, suppressed putrescine and glutathione synthesis, and altered arginine metabolism within R. irregularis-plant symbiosis, leading to a substantial reduction in the drought tolerance of M. sativa. Conversely, silencing RiCPSI decreased arginine, putrescine, and glutathione synthesis in R. irregularis but did not adversely affect NH4+ transfer from fungi to the host plant and drought tolerance of M. sativa. Interestingly, overexpressing RiCPSI via our host-induced gene overexpressing system enhanced arginine, putrescine, and glutathione synthesis in R. irregularis, reduced arbuscule abundance, and improved drought tolerance of M. sativa. Our findings demonstrate that, under drought stress, R. irregularis-plant symbiosis facilitates improved NH4+ transfer from AMF to the host plant. This is accompanied by increased arginine, putrescine, and glutathione synthesis within R. irregularis, driven by the upregulation of RiCPSI and RiCARI expression in mycorrhizal structures within the roots. These molecular adjustments collectively contribute to enhanced drought tolerance in R. irregularis-plant symbiosis.

中文翻译：

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Rhizophagus irregularis 调控 RiCPSI 和 RiCARI 表达以影响植物的耐旱性


丛枝菌根真菌 （AMF） 可以将无机氮 （N） 从土壤转移到寄主植物以应对干旱胁迫，其中精氨酸合成和 NH4+ 运输是关键过程。然而，这些过程背后的调节机制仍不清楚。在这里，我们发现干旱胁迫上调了紫花苜蓿根内不规则根中涉及氮转移途径和腐胺合成的基因的表达，即氨基甲酰磷酸合酶 （RiCPSI）、精氨酸酶 （RiCARI）、脲酶 （RiURE）、鸟氨酸脱羧酶 （RiODC） 和谷氨酸-半胱氨酸连接酶 （RiGCL）。此外，我们证实 RiCPSI 是一种氨基甲酰磷酸合酶。通过宿主诱导的基因沉默沉默 RiCARI 抑制了丛枝形成，抑制了腐胺和谷胱甘肽的合成，并改变了 R. irregularis-plant 共生体内的精氨酸代谢，导致 M. sativa 的耐旱性显着降低。相反，沉默 RiCPSI 减少了 R. irregularis 中精氨酸、腐胺和谷胱甘肽的合成，但不会对 NH4+ 从真菌转移到寄主植物和苜蓿的耐旱性产生不利影响。有趣的是，通过我们的宿主诱导的基因过表达系统过表达 RiCPSI 增强了 R. irregularis 中的精氨酸、腐胺和谷胱甘肽合成，降低了丛枝丰度，并提高了 M. sativa 的耐旱性。我们的研究结果表明，在干旱胁迫下，R. irregularis-plant 共生促进了 NH4+ 从 AMF 到寄主植物的转移。这伴随着 R 内精氨酸、腐胺和谷胱甘肽合成的增加。 不规则，由根内菌根结构中 RiCPSI 和 RiCARI 表达的上调驱动。这些分子调整共同有助于增强 R. irregularis-plant 共生的耐旱性。