The Beneficial Fungus Modulates Amino Acid Homeostasis in Arabidopsis Under Nitrogen Starvation

Non-mycorrhizal but beneficial fungi often mitigate (a)biotic stress-related traits in host plants. The underlying molecular mechanisms are mostly still unknown, as in the interaction between the endophytic growth-promoting soil fungus and . Here, abiotic stress in the form of nitrogen (N) deficiency was used to investigate the effects of the fungus on colonized plants. In particular, the hypothesis was investigated that fungal infection could influence N deficiency via an interaction with the high-affinity nitrate transporter NRT2.4, which is induced by N deficiency. For this purpose, Arabidopsis wild-type knock-out and NRT2.4 reporter lines were grown on media with different nitrate concentrations with or without colonization. We used chemical analysis methods to determine the amino acids and phytohormones. Experimental evidence suggests that the fungus does not modulate expression under N starvation. Instead, alleviates N starvation in other ways: The fungus supplies nitrogen (N) to the N-starved plant. The presence of the fungus restores the plants' amino acid homeostasis, which was out of balance due to N deficiency, and causes a strong accumulation of branched-chain amino acids. We conclude that the plant does not need to invest in defense and resources for growth are maintained, which in turn benefits the fungus, suggesting that this interaction should be considered a mutualistic symbiosis.