The genome of Arabidopsis thaliana encodes approximately 260 copper (Cu)‐dependent proteins, which includes enzymes in central pathways of photosynthesis, respiration and responses to environmental stress. Under Cu‐deficient growth conditions, Squamosa promoter binding Protein‐Like 7 ( SPL 7) activates the transcription of genes encoding Cu acquisition systems, and it mediates a metabolic reorganization to economize on Cu. The transcription factor SPL 7 groups among comparably large proteins in the SPL family, which additionally comprises a second group of small SPL proteins targeted by mi RNA 156 with roles in plant development. SPL 7 shares extended regions of sequence homology with SPL 1 and SPL 12. Therefore, we investigated the possibility of a functional overlap between these three members of the group of large SPL family proteins. We compared the spl1 spl12 double mutant and the spl1 spl7 spl12 triple mutant with both the wild type and the spl7 single mutant under normal and Cu‐deficient growth conditions. Biomass production, chlorophyll content and tissue elemental composition at the seedling stage, as well as plant and flower morphology during reproductive stages, confirmed the involvement of SPL 7, but provided no indication for important roles of SPL 1 or SPL 12 in the acclimation of Arabidopsis to Cu deficiency. Furthermore, we analyzed the effects of zinc (Zn) deficiency on the same set of mutants. Different from what is known in the green alga Chlamydomonas reinhardtii , Arabidopsis did not activate Cu deficiency responses under Zn deficiency, and there was no Cu overaccumulation in either shoot or root tissues of Zn‐deficient wild type plants. Known Zn deficiency responses were unaltered in spl7 , spl1 spl12 and spl1 spl7 spl12 mutants. We observed that CuZn SOD activity is strongly downregulated in Zn‐deficient A. thaliana , in association with an about 94% reduction in the abundance of the CSD 2 transcript, a known target of miR398. However, different from the known Cu deficiency responses of Arabidopsis, this Zn deficiency response was independent of SPL 7 and not associated with an upregulation of MIR 398b primary transcript levels. Our data suggest that there is no conservation in A. thaliana of the crosstalk between Zn and Cu homeostasis mediated by the single SPL family protein CRR 1 of Chl...

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Diphthamide, a post-translationally modified histidine residue of eukaryotic TRANSLATION ELONGATION FACTOR2 (eEF2), is the human host cell-sensitizing target of diphtheria toxin. Diphthamide biosynthesis depends on the 4Fe-4S-cluster protein Dph1 catalyzing the first committed step, as well as Dph2 to Dph7, in yeast and mammals. Here we show that diphthamide modification of eEF2 is conserved in Arabidopsis thaliana and requires AtDPH1. Ribosomal −1 frameshifting-error rates are increased in Arabidopsis dph1 mutants, similar to yeast and mice. Compared to the wild type, shorter roots and smaller rosettes of dph1 mutants result from fewer formed cells. TARGET OF RAPAMYCIN (TOR) kinase activity is attenuated, and autophagy is activated, in dph1 mutants. Under abiotic stress diphthamide-unmodified eEF2 accumulates in wild-type seedlings, most strongly upon heavy metal excess, which is conserved in human cells. In summary, our results suggest that diphthamide contributes to the functionality of the translational machinery monitored by plants to regulate growth.

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The genome of Arabidopsis thaliana encodes approximately 260 copper (Cu)-dependent proteins, which include enzymes in central pathways of photosynthesis, respiration and responses to environmental stress. Under Cu-deficient growth conditions, Squamosa promoter binding Protein-Like 7 (SPL7) activates the transcription of genes encoding Cu acquisition systems and mediates a metabolic reorganization to economize on Cu. The transcription factor SPL7 groups among comparably large proteins in the SPL family, which additionally comprises a second group of small SPL proteins targeted by miRNA156 with roles in plant development. SPL7 shares extended regions of sequence homology with SPL1 and SPL12. Therefore, we investigated the possibility of a functional overlap between these three members of the group of large SPL family proteins. We compared the spl1 spl12 double mutant and the spl1 spl7 spl12 triple mutant with the wild type and the spl7 single mutant under normal and Cu-deficient growth conditions. Biomass production, chlorophyll content and tissue elemental composition at the seedling stage, as well as plant and flower morphology during reproductive stages, confirmed the involvement of SPL7, but provided no indication for important roles of SPL1 or SPL12, in the acclimation of Arabidopsis to Cu deficiency. Furthermore, we analyzed the effects of zinc (Zn) deficiency on the same set of mutants. Different from what is known in the green alga Chlamydomonas reinhardtii, Arabidopsis did not activate Cu deficiency responses under Zn deficiency, and there was no Cu overaccumulation in either shoot or root tissues of Zn-deficient wild-type plants. Known Zn deficiency responses were unaltered in spl7, spl1 spl12 and spl1 spl7 spl12 mutants. We observed that CuZnSOD activity is strongly downregulated in Zn-deficient A. thaliana, in association with an about 94% reduction in the abundance of the CDS2 transcript, a known target of miR398. However, different from the known Cu deficiency responses of Arabidopsis, this Zn deficiency response was independent of SPL7 and not associated with an upregulation of MIR398b primary transcript levels. Our data suggest that there is no conservation in A. thaliana of the crosstalk between Zn and Cu homeostasis mediated by the single SPL family protein CRR1 in Chlamydomonas. In the future, resolving how the specificity of SPL protein activation and recognition of target gene promoters is achieved will advance our understanding of the specific functions of different SPL family proteins in either Cu deficiency response regulation or growth and development of land plants.