TOR and S6K1 promote translation reinitiation of uORF-containing mRNAs via phosphorylation of eIF3h.

This paper merits attention on two levels, by first documenting a broader role for target-of-rapamycin (TOR) kinase in promoting expression of mRNAs containing small upstream open reading frames (μORFs), but also by providing evidence that this form of regulation may be an essential element of auxin responses in plants. In recent years, the eukaryotic TOR kinase signalling pathway has been shown to integrate complex sets of cues into coherent growth responses, with up-regulation of translation a prominent aspect of its growth enabling outputs. In earlier work, Ryabova and colleagues described how cauliflower mosaic virus uses TOR kinase as a means to enable re-initiation of translation for polycistronic viral RNAs in plants {1}. In the current work, the group extends this analysis to show that TOR plays a similar role in enabling the re-initiation of translation for endogenous plant mRNAs that contain μORFs upstream of major protein-coding ORFs. Notably, many mRNAs that encode auxin response factors (ARFs) contain such μORFs. Re-initiation of translation downstream of these μORFs, to enable translation of ARF encoding ORFs, was shown to depend on TOR activity in a manner that specifically depended on ribosomal S6kinase1 and eIF3. Together, these results suggest a novel regulatory paradigm in which growth-enabling signalling outputs of TOR are mediated, at least in part, via a rapid translational reinitiation mechanism.