title: DCL2- and RDR6-dependent transitive silencing of SMXL4 and SMXL5 in Arabidopsis dcl4 mutants causes defective phloem transport and carbohydrate over-accumulation.
abstract: DICER-LIKE (DCL) enzymes process double-stranded RNA into small RNAs that act as regulators of gene expression. Arabidopsis DCL4 and DCL2 each allow the post-transcriptional gene silencing (PTGS) of viruses and transgenes, but primary PTGS-prone DCL4 outcompetes transitive PTGS-prone DCL2 in wild-type plants. This hierarchy likely prevents DCL2 having any detrimental effects on endogenous genes. Indeed, dcl4 mutants exhibit developmental defects and increased sensitivity to genotoxic stress. In this study, the mechanism underlying dcl4 defects was investigated using genetic, biochemical and high-throughput sequencing approaches. We show that the purple phenotype of dcl4 leaves correlates with carbohydrate over-accumulation and defective phloem transport, and depends on the activity of SUPPRESSOR OF GENE SILENCING 3, RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) and DCL2. This phenotype correlates with the downregulation of two genes expressed in the apex and the vasculature, SMAX1-LIKE 4 (SMXL4) and SMXL5, and the accumulation of DCL2- and RDR6-dependent small interfering RNAs derived from these two genes. Supporting a causal effect, smxl4 smxl5 double mutants exhibit leaf pigmentation, enhanced starch accumulation and defective phloem transport, similar to dcl4 plants. Overall, this study elucidates the detrimental action of DCL2 when DCL4 is absent, and indicates that DCL4 outcompeting DCL2 in wild-type plants is crucial to prevent the degradation of endogenous transcripts by DCL2- and RDR6-dependent transitive PTGS.
authors.affiliation: Agricultural Biotechnology Research Center, Academia Sinica, Taipei, 11529, Taiwan.
authors.forename: Yu-Yi
authors.initials: YY
authors.lastname: Wu
authors.affiliation: Agricultural Biotechnology Research Center, Academia Sinica, Taipei, 11529, Taiwan.
authors.forename: Bo-Han
authors.initials: BH
authors.lastname: Hou
authors.affiliation: Agricultural Biotechnology Research Center, Academia Sinica, Taipei, 11529, Taiwan.
authors.forename: Wen-Chi
authors.initials: WC
authors.lastname: Lee
authors.affiliation: Agricultural Biotechnology Research Center, Academia Sinica, Taipei, 11529, Taiwan.
authors.forename: Shin-Hua
authors.initials: SH
authors.lastname: Lu
authors.affiliation: Agricultural Biotechnology Research Center, Academia Sinica, Taipei, 11529, Taiwan.
authors.forename: Chen-Jui
authors.initials: CJ
authors.lastname: Yang
authors.affiliation: Institut Jean-Pierre Bourgin, UMR 1318, INRA, SPS Saclay Plant Sciences, Versailles, France.
authors.forename: Hervé
authors.initials: H
authors.lastname: Vaucheret
authors.affiliation: Agricultural Biotechnology Research Center, Academia Sinica, Taipei, 11529, Taiwan.
authors.forename: Ho-Ming
authors.identifier: http://orcid.org/0000-0001-5979-0121
authors.initials: HM
authors.lastname: Chen
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D050260:Carbohydrate Metabolism / Q000235:genetics / Q000502:physiology; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D052585:Phloem / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D012324:RNA-Dependent RNA Polymerase / Q000235:genetics / Q000378:metabolism*; D043244:Ribonuclease III / Q000235:genetics / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D018797:Cell Cycle Proteins; C514575:RDR6 protein, Arabidopsis; D012324:RNA-Dependent RNA Polymerase; C504982:DCL4 protein, Arabidopsis; C511919:DCL2 protein, Arabidopsis; D043244:Ribonuclease III
keywords: 
  Arabidopsis thaliana
  ; 22-nt small RNA; DICER-LIKE 2; DICER-LIKE 4; SMAX1-LIKE 4; SMAX1-LIKE 5; post-transcriptional gene silencing