P-384 - The endophytic fungus Colletotrichum tofieldiae enhances plant growth under nitrogen-limited conditions by facilitating nitrogen assimilation and recruiting beneficial bacteria to the roots

The endophytic fungus Colletotrichum tofieldiae (Ct4) colonizes Arabidopsis thaliana roots and promotes plant growth by transferring phosphorus to the host under phosphorus deficiency. Here, we reveal that Ct4 can promote plant growth under nitrogen (N) deficiency in field and laboratory conditions. In gnotobiotic systems simulating N deficiency, transcriptomic and genetic analyses revealed that Ct4 induces plant nitrate transporters and improves plant nitrate uptake. Disrupting these transporter genes resulted in the weakening of the PGP effect while the N provision from Ct hyphae persisted, suggesting the transporter contribution to maximize plant growth while being dispensable for N transfer. Disruption of Ct4’s secondary N sources (i.e., nitrate) utilization in the nit2 mutant partially impaired plant growth promotion (PGP), and the supplement of ammonium can restore PGP. Further, Ct4 also shifted the plant rhizospheric bacterial community, and a Paraburkholderia strain (A10) isolated from Ct4-colonized roots showed synergistic PGP with Ct4, especially in media containing amino acids. RNA sequencing revealed that A10 upregulates plant ammonium and amino acid transporters, some of which were further enhanced in Ct4-A10 co-colonized roots. Collectively, our data suggests a dual role of Ct4: enhancing nitrate acquisition and selectively enriching rhizospheric bacteria that aid in utilizing diverse nitrogen sources to improve plant resilience under N-limited conditions.