TNO1 is involved in salt tolerance and vacuolar trafficking in Arabidopsis

The Arabidopsis (Arabidopsis thaliana) soluble N-ethylmaleimide-sensitive factor attachment protein receptor SYP41 is involved in vesicle fusion at the trans-Golgi network (TGN) and interacts with AtVPS45, SYP61, and VTI12. These proteins are involved in diverse cellular processes, including vacuole biogenesis and stress tolerance. A previously uncharacterized protein, named TNO1 (for TGN-localized SYP41-interacting protein), was identified by coimmunoprecipitation as a SYP41-interacting protein. TNO1 was found to localize to the TGN by immunofluorescence microscopy. A tno1 mutant showed increased sensitivity to high concentrations of NaCl, KCl, and LiCl and also to mannitol-induced osmotic stress. Localization of SYP61, which is involved in the salt stress response, was disrupted in the tno1 mutant. Vacuolar proteins were partially secreted to the apoplast in the tno1 mutant, suggesting that TNO1 is required for efficient protein trafficking to the vacuole. The tno1 mutant had delayed formation of the brefeldin A (BFA) compartment in cotyledons upon application of BFA, suggesting less efficient membrane fusion processes in the mutant. Unlike most TGN proteins, TNO1 does not relocate to the BFA compartment upon BFA treatment. These data demonstrate that TNO1 is involved in vacuolar trafficking and salt tolerance, potentially via roles in vesicle fusion and in maintaining TGN structure or identity.