Small RNA and mRNA sequencing Arabidopsis thaliana WT and hyl1-2 seedlings in light or dark illumination conditions.

Light is the most influential and essential environment stimuli for a plant growth. As a response to a deficient light source plants re-program their developmental programs to adjust their growth in search for a light source. Such adaptive trait is orchestrated by a fine equilibrium of gene expression. Here, we shown that plants rely on a massive shutdown of microRNAs (miRNAs) biogenesis during long dark deprivation followed by a quick recovery after light restoration to accomplish a proper response to dark-light transition. In order to achieve such response the miRNA biogenesis factor HYPONASTIC LEAVES 1 (HYL1) is actively degraded during light deprivation in the cell cytoplasm while an inactive pool of phosphorylated protein remains protected inside the nucleus. Upon light restoration a quick dephosphorylation of the HYL1 reserve pool lead to the reactivation of miRNA biogenesis and de-etiolation response. Our findings thus define an unexpected massive regulatory phenomenon involving the plant silencing machinery during plant light response.