The key eukaryotic translation initiation factor 3 (eIF3) (Hinnebusch, 2006) serves as a scaffold for mS6K phosphorylation by mTOR (Ma & Blenis, 2009). The mTORC1 pathway promotes 5′‐cap‐dependent translation via phosphorylation of ribosomal protein S6 kinases (mS6Ks) and eIF4E‐binding proteins (m4E‐BPs) (Ma & Blenis, 2009 Sonenberg & Hinnebusch, 2009). mTORC2 mediates cell metabolism and cytoskeletal organization (Cybulski & Hall, 2009). mTORC1-comprising mTOR, raptor, and mLST8-is sensitive to the immunosuppressant drug rapamycin and regulates cell growth by activating ribosome biogenesis, transcription, and protein synthesis (Hara et al, 2002 Loewith et al, 2002). Mammalian TOR (mTOR) occurs in two structurally and functionally distinct complexes: TOR complex 1 (TORC1) and TOR complex 2 (TORC2) (Zoncu et al, 2011 Shimobayashi & Hall, 2014). Target of rapamycin (TOR) is a main sensor of cell growth in response to nutrients, energy status, and growth factors and is conserved from humans to yeasts and plants. These findings imply a mode of translation control whereby, as an upstream effector of TOR, ROP2 coordinates TOR function in translation reinitiation pathways in response to auxin. TOR inactivation abolishes ROP2 regulation of translation reinitiation, but not its effects on cytoskeleton or intracellular trafficking. CA‐ ROP2 is efficient in loading u ORF‐containing mRNAs onto polysomes and stimulates translation in protoplasts, and both processes are sensitive to TOR inhibitor AZD‐8055. GFP‐ TOR can associate with endosome‐like structures in ROP2‐overexpressing plants, indicating that endosomes mediate ROP2 effects on TOR activation. TOR activation in response to auxin is abolished in ROP‐deficient rop2 rop6 ROP4 RNAi plants. ROP2 physically interacts with and, when GTP‐bound, activates TOR in vitro. Plants with high levels of active ROP2, including those expressing constitutively active ROP2 ( CA‐ ROP2), contain high levels of active TOR. Here, we report that the small GTPase ROP2, if activated by the phytohormone auxin, promotes activation of TOR, and thus translation reinitiation of u ORF‐containing mRNAs.

Target of rapamycin ( TOR) promotes reinitiation at upstream ORFs (u ORFs) in genes that play important roles in stem cell regulation and organogenesis in plants.

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