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  1. Abstract

    The eukaryote‐specific ribosomal protein of the small subunit eS6 is phosphorylated through the target of rapamycin (TOR) kinase pathway. Although this phosphorylation event responds dynamically to environmental conditions and has been studied for over 50 years, its biochemical and physiological significance remains controversial and poorly understood. Here, we report data fromArabidopsis thaliana, which indicate that plants expressing only a phospho‐deficient isoform of eS6 grow essentially normally under laboratory conditions. The eS6z (RPS6A) paralog of eS6 functionally rescued a double mutant in bothrps6aandrps6bgenes when expressed at approximately twice the wild‐type dosage. A mutant isoform of eS6z lacking the major six phosphorylatable serine and threonine residues in its carboxyl‐terminal tail also rescued the lethality, rosette growth, and polyribosome loading of the double mutant. This isoform also complemented many mutant phenotypes ofrps6that were newly characterized here, including photosynthetic efficiency, and most of the gene expression defects that were measured by transcriptomics and proteomics. However, compared with plants rescued with a phospho‐enabled version of eS6z, the phospho‐deficient seedlings retained a mild pointed‐leaf phenotype, root growth was reduced, and certain cell cycle‐related mRNAs and ribosome biogenesis proteins were misexpressed. The residual defects of the phospho‐deficient seedlings could be understood as an incomplete rescue of therps6mutant defects. There was little or no evidence for gain‐of‐function defects. As previously published, the phospho‐deficient eS6z also rescued therps6aandrps6bsingle mutants; however, phosphorylation of the eS6y (RPS6B) paralog remained lower than predicted, further underscoring that plants can tolerate phospho‐deficiency of eS6 well. Our data also yield new insights into how plants cope with mutations in essential, duplicated ribosomal protein isoforms.

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  2. Abstract

    The molecular machinery for protein synthesis is profoundly similar between plants and other eukaryotes. Mechanisms of translational gene regulation are embedded into the broader network of RNA‐level processes including RNA quality control and RNA turnover. However, over eons of their separate history, plants acquired new components, dropped others, and generally evolved an alternate way of making the parts list of protein synthesis work. Research over the past 5 years has unveiled how plants utilize translational control to defend themselves against viruses, regulate translation in response to metabolites, and reversibly adjust translation to a wide variety of environmental parameters. Moreover, during seed and pollen development plants make use of RNA granules and other translational controls to underpin developmental transitions between quiescent and metabolically active stages. The economics of resource allocation over the daily light–dark cycle also include controls over cellular protein synthesis. Important new insights into translational control on cytosolic ribosomes continue to emerge from studies of translational control mechanisms in viruses. Finally, sketches of coherent signaling pathways that connect external stimuli with a translational response are emerging, anchored in part around TOR and GCN2 kinase signaling networks. These again reveal some mechanisms that are familiar and others that are different from other eukaryotes, motivating deeper studies on translational control in plants.

    This article is categorized under:

    Translation > Translation Regulation

    RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems

    RNA Interactions with Proteins and Other Molecules > Protein‐RNA Interactions: Functional Implications

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