RESEARCH HIGHLIGHTS NON-CODING RNA The other face of PIWI Spermiogenesis involves gradual with 3ʹUTRs of the target mRNAs; Credit: S. Bradbrook/Springer Nature Limited chromatin compaction and trans- reporter protein levels but not cription shut-down. mRNAs that mRNA levels increased, implicating activation. Translation of hundreds are transcribed in spermatocytes translation in reporter activation. of mRNAs co-targeted by piRNA and early-round spermatids are Activation of the target-mRNA and HuR was dependent on MIWI, stored as translationally inactive reporters required piRNA–3ʹUTR indicating that they are direct targets ribonucleoproteins until later during base-pairing and 3ʹUTR binding by of this selective mechanism of spermiogenesis, when their trans- functional MIWI. Screening for translation activation. lation is activated, but how this MIWI-interacting proteins revealed The proteins encoded by activation occurs is largely unknown. that eukaryotic translation initiation two of the five original target PIWI proteins and PIWI-interacting factor 3f (eIF3f) directly interacted mRNAs are essential for sperm RNAs (piRNAs) are essential for with MIWI and was also required MIWI– acrosome formation. Indeed, gametogenesis as they suppress for reporter activation. The activated piRNAs severe acrosome defects were found the expression of transposons and 3ʹUTRs included AU-rich elements … in MIWI-depleted spermatids mRNAs. Dai et al. now show that (AREs) that are bound by HuR, interact with owing to considerable decrease in mouse PIWI (MIWI)–piRNAs which is an RNA-binding protein eIF3f–HuR and the levels of the two proteins. are the core of a complex required known to interact with another other proteins Thus, although MIWI–piRNAs for selective mRNA translation translation factor, eIF4G3, for trans- are mostly known for gene silencing, in spermatids. lation in spermatids. Together with to select a in round spermatids they interact MIWI–piRNAs target transcripts the poly(A) binding protein PABPC1, subset of with eIF3f–HuR and other proteins for silencing through base-pairing they formed the MIWI–piRNA– ARE-containing to select a subset of ARE-containing between the piRNA and the mRNA eIF3f–HuR–eIF4G3–PABPC1 mRNAs for mRNAs for translation activation. 3ʹ untranslated region (3ʹUTR). complex. Working in cells derived from MIWI association with translation Eytan Zlotorynski mouse spermatocytes, the authors eIF3f–HuR was restricted to round activation ORIGINAL ARTICLE Dai, P. et al. A translation- were surprised to find five piRNAs spermatids, thus defining the stage activating function of MIWI/piRNA during mouse spermiogenesis. Cell 179, 1566–1581 (2019) that strongly upregulated reporters of mRNA-specific translation GENOME EDITING d e t Plant gene editing improved i m i L e Gene editing in plants is usually achieved formed meristem- like structures that r u t by modifying the genome of cultured then differentiated into small shoots a N r cells and then regenerating whole plants that could be transferred to a rooting e g n i by exposing the modified cultured medium, and then soil, to obtain full r p S flowering plants (from which seeds could / cells to growth hormones. This method y b s r is inefficient, limited to a few species, be harvested). Using a similar method on e m tomato seedlings, transient expression m and can also lead to undesired genome u S . V aberrations. Maher et al. now report of Wus2, ipt and STM enabled the : it d that it is possible to overcome the tissue regeneration of whole tomato plants. re C culture bottleneck by modifying somatic This protocol was repeated using cells in whole plants and inducing the transgenic seedlings that constitutively modify agronomically important species formation of shoot meristems — the it is possible expressed the nuclease Cas9. But in such as potato and grape. plant stem cell niches that give rise to all addition to developmental regulators, Thus, this study reports the successful the above- ground organs of the plant. … to generate a single guide RNA (sgRNA, which targets generation of gene-edited plants First, the authors developed a gene- edited the Cas9 nuclease to a specific sequence) without the need for time-consuming protocol (a high-throughput platform) shoots by was also expressed transiently to induce and inefficient sterile culture methods. to generate de novo meristems on targeted mutations. This led to the Importantly, the developed high- the leaves of Nicotiana benthamiana transiently gene ration of target gene-edited throughput platform will enable the in vitro- grown seedlings by expressing transforming shoot meristems that developed into method to be tested on different the developmental regulators Wus2 and stems of whole plants. plant species. The authors could then show that it Kim Baumann STM using Agrobacterium- mediated soil- grown transient transformation. This transient is possible to apply the same method ORIGINAL ARTICLE Maher, M. F. et al. Plant gene expression led to the formation of plants to generate gene-edited shoots by editing through de novo induction of meritems. callus- like growths. Even though many transiently trans forming stems of soil- Nat. Biotechnol. https://doi.org/10.1038/s41587- 019-0337-2 (2019) calli remained undifferentiated, some grown plants. Moreover, they could 66 | FEBRUARY 2020 | VOLUME 21 www.nature.com/nrm.