Auxin Signaling in Regulation of Plant Translation Reinitiation

Abstract

The mRNA translation machinery directs protein production, and thus cell growth, according to prevailing cellular and environmental conditions. The target of rapamycin (TOR) signaling pathway-a major growth-related pathway-plays a pivotal role in optimizing protein synthesis in mammals, while its deregulation triggers uncontrolled cell proliferation and the development of severe diseases. In plants, several signaling pathways sensitive to environmental changes, hormones, and pathogens have been implicated in post-transcriptional control, and thus far phytohormones have attracted most attention as TOR upstream regulators in plants. Recent data have suggested that the coordinated actions of the phytohormone auxin, Rho-like small GTPases (ROPs) from plants, and TOR signaling contribute to translation regulation of mRNAs that harbor upstream open reading frames (uORFs) within their 5'-untranslated regions (5'-UTRs). This review will summarize recent advances in translational regulation of a specific set of uORF-containing mRNAs that encode regulatory proteins-transcription factors, protein kinases and other cellular controllers-and how their control can impact plant growth and development.

Keywords: S6K1; endosomes; signal transduction; small GTPases ROPs; target of rapamycin TOR; translation-reinitiation.

FIGURE 1

Evolutionary conservation of upstream TOR signaling pathway in plants and animals. The main inputs upstream of TOR are depicted. TOR, Raptor and Lst8 form the core of the TORC1 complex and are conserved in all eukaryotes. Small GTPases represent the best described regulators of TOR kinase. See text for details and abbreviations.

FIGURE 2

Auxin signaling pathways within the cytoplasm. Auxin signal is recognized via an as yet uncharacterized receptor(s) in the target cells, and transmitted to the cytosol. Auxin mediates recycling of small GTPases—ROP2-GDP to ROP2-GTP—by several GEFs. ROP2 interacts directly with TOR and activates TOR kinase if ROP2 is bound to GTP. TOR activation could occur upon complex formation with ROP2 on earlier endosomes. ROP2 then dissociates from TOR and requires recycling. Active TOR is targeted to eIF3-containing preinitiation complexes and polysomes, where it promotes translation reinitiation of uORF-mRNAs.