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This article is a Commentary on Wang et al. (2024), 242: 1043–1054. [ABSTRACT FROM AUTHOR]

Dicer-like (DCL) enzymes play a pivotal role in RNA silencing in plants, processing the long double-stranded RNA (dsRNA) that triggers silencing into the primary short interfering RNAs (siRNAs) that mediate it. The siRNA population can be augmented and silencing amplified via transitivity, an RNA-dependent RNA polymerase (RDR)-dependent pathway that uses the target RNA as substrate to generate secondary siRNAs. Here we report that Arabidopsis DCL2-but not DCL4-is required for transitivity in cell-autonomous, post-transcriptional silencing of transgenes. An insertion mutation in DCL2 blocked sense transgene-induced silencing and eliminated accumulation of the associated RDR-dependent siRNAs. In hairpin transgene-induced silencing, the dcl2 mutation likewise eliminated accumulation of secondary siRNAs and blocked transitive silencing, but did not block silencing mediated by primary siRNAs. Strikingly, in all cases, the dcl2 mutation eliminated accumulation of all secondary siRNAs, including those generated by other DCL enzymes. In contrast, mutations in DCL4 promoted a dramatic shift to transitive silencing in the case of the hairpin transgene and enhanced silencing induced by the sense transgene. Suppression of hairpin and sense transgene silencing by the P1/HC-Pro and P38 viral suppressors was associated with elimination of secondary siRNA accumulation, but the suppressors did not block processing of the stem of the hairpin transcript into primary siRNAs. Thus, these viral suppressors resemble the dcl2 mutation in their effects on siRNA biogenesis. We conclude that DCL2 plays an essential, as opposed to redundant, role in transitive silencing of transgenes and may play a more important role in silencing of viruses than currently thought.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2009.06.031 Byline: Gang Wu (1), Mee Yeon Park (1), Susan R. Conway (1), Jia-Wei Wang (2), Detlef Weigel (2), R. Scott Poethig (1) Keywords: RNA; DEVBIO Abstract: The transition from the juvenile to the adult phase of shoot development in plants is accompanied by changes in vegetative morphology and an increase in reproductive potential. Here, we describe the regulatory mechanism of this transition. We show that miR156 is necessary and sufficient for the expression of the juvenile phase, and regulates the timing of the juvenile-to-adult transition by coordinating the expression of several pathways that control different aspects of this process. miR156 acts by repressing the expression of functionally distinct SPL transcription factors. miR172 acts downstream of miR156 to promote adult epidermal identity. miR156 regulates the expression of miR172 via SPL9 which, redundantly with SPL10, directly promotes the transcription of miR172b. Thus, like the larval-to-adult transition in Caenorhabditis elegans, the juvenile-to-adult transition in Arabidopsis is mediated by sequentially operating miRNAs. miR156 and miR172 are positively regulated by the transcription factors they target, suggesting that negative feedback loops contribute to the stability of the juvenile and adult phases. Author Affiliation: (1) Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA (2) Department of Molecular Biology, Max Planck Institute for Developmental Biology, D-72076 Tubingen, Germany Article History: Received 6 February 2009; Revised 8 May 2009; Accepted 12 June 2009 Article Note: (miscellaneous) Published: August 20, 2009

Genetic analysis is used to define the pathway by which trans-acting siRNAs and other short siRNAs are generated from the loci in Arabidopsis. Results define the core components of a post-transcriptional gene-silencing pathway in Arabidopsis and reveal some of the features that direct transcripts to this pathway.

SGS3 and SGS2/SDE1/RDR6 act in the same pathways as the Argonuate protein, ZIP, and the putative miRNA nuclear export receptor; HST to regulate vegetative phase change and floral development in Arabidopsis is studied. The role of endogenous siRNA in the regulation of gene expression and suggest that PTGS play a central role in the temporal control of shoot development in plants.