Diverse Nitrogen Signals Activate Convergent ROP2-TOR Signaling in Arabidopsis

The evolutionarily conserved TOR (Target-of-Rapamycin) kinase coordinates cellular and organismal growth in all eukaryotes. In yeasts and animals, amino acids (AAs) are key upstream signals for TOR activation, but how nitrogen-related nutrients regulate TOR signaling in autotrophic plants is poorly understood. Here, we discovered that inorganic nitrate and ammonium function as primary nitrogen signals to activate TOR and stimulate cell proliferation in the leaf primordium for true leaf development, which is uncoupled from nitrogen-assimilation and the nitrate sensor NRT1.1 (Nitrate-Transporter-1.1). We further identified that total 15 proteinogenic AAs are able to activate Arabidopsis TOR, and the first AAs generated from plant specific nitrogen assimilation (glutamine), sulfur assimilation (cysteine) and glycolate cycle (glycine) exhibit the highest potency for TOR-activation. Interestingly, nitrate, ammonium and glutamine all activate the small GTPase ROP2, and constitutively active ROP2 restores TOR activation and cell proliferation in leaf primordium under nitrogen-starvation conditions. Together, our findings suggest that specific evolutionary adaptations of the nitrogen-TOR signaling pathway might have occurred in plant lineages, and ROP2 is a hub to sense and integrate diverse nitrogen signals and auxin hormone signal to mediate TOR-activated plant growth and development.