Identification of nuclear dicing bodies containing proteins for microRNA biogenesis in living Arabidopsis plants

MicroRNAs (miRNAs) are important for regulating gene expression in muticellular organisms. MiRNA processing is a two-step process. In animal cells, the first step is nuclear and the second step cytoplasmic, whereas in plant cells, both steps occur in the nucleus via the enzyme Dicer-like1 (DCL1) [1, 2] and other proteins including the zinc-finger-domain protein Serrate (SE) [3, 4] and a double-stranded RNA (dsRNA) binding-domain protein, Hyponastic Leaves1 (HYL1) [5-7]. Furthermore, plant miRNAs are methylated by Hua Enhancer (HEN1) at their 3' ends [8] and loaded onto Argonaute1 (AGO1) [9]. However, little is known about the cellular basis of miRNA biogenesis. Using live-cell imaging, we show here that DCL1 and HYL1 colocalize in discrete nuclear bodies in addition to being present in a low-level diffuse nucleoplasmic distribution. These bodies, which we refer to as nuclear dicing bodies (D-bodies), differ from Cajal bodies [10, 11]. A mutated DCL1 with impaired function in miRNA processing falls to target to D-bodies, and an introduced primary (pri)-miRNA transcript is recruited to D-bodies. Furthermore, bimolecular fluorescence complementation (BiFC) shows that DCL1, HYL1, and SE interact in D-bodies. On the basis of these data, we propose that D-bodies are crucial for orchestrating pri-miRNA processing and/or storage/assembly of miRNA-processing complexes in the nuclei of plant cells.