Description:
<jats:title>Summary</jats:title><jats:p><jats:list list-type="bullet">
<jats:list-item><jats:p>Diatoms are eukaryotic microalgae responsible for nearly half of the marine productivity. RNA interference (RNAi) is a mechanism of regulation of gene expression mediated by small RNAs (sRNAs) processed by the endoribonuclease Dicer (DCR). To date, the mechanism and physiological role of RNAi in diatoms are unknown.</jats:p></jats:list-item>
<jats:list-item><jats:p>We mined diatom genomes and transcriptomes for key RNAi effectors and retraced their phylogenetic history. We generated DCR knockout lines in the model diatom species <jats:italic>Phaeodactylum tricornutum</jats:italic> and analyzed their mRNA and sRNA populations, repression‐associated histone marks, and acclimatory response to nitrogen starvation.</jats:p></jats:list-item>
<jats:list-item><jats:p>Diatoms presented a diversification of key RNAi effectors whose distribution across species suggests the presence of distinct RNAi pathways. <jats:italic>P. tricornutum</jats:italic> DCR was found to process 26–31‐nt‐long double‐stranded sRNAs originating mostly from transposons covered by repression‐associated epigenetic marks. In parallel, <jats:italic>P. tricornutum</jats:italic> DCR was necessary for the maintenance of the repression‐associated histone marks H3K9me2/3 and H3K27me3. Finally, PtDCR‐KO lines presented a compromised recovery post nitrogen starvation suggesting a role for <jats:italic>P. tricornutum</jats:italic> DCR in the acclimation to nutrient stress.</jats:p></jats:list-item>
<jats:list-item><jats:p>Our study characterized the molecular function of the single DCR homolog of <jats:italic>P. tricornutum</jats:italic> suggesting an association between RNAi and heterochromatin maintenance in this model diatom species.</jats:p></jats:list-item>
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