Specification of Leaf Polarity in Arabidopsis via the trans-Acting siRNA Pathway

Abstract

Plants leaves develop proximodistal, dorsoventral (adaxial-abaxial), and mediolateral patterns following initiation. The Myb domain gene PHANTASTICA (PHAN) is required for adaxial fate in many plants , but the Arabidopsis ortholog ASYMMETRIC LEAVES1 (AS1) has milder effects, suggesting that alternate or redundant pathways exist . We describe enhancers of as1 with more elongate and dissected leaves. As well as RDR6, an RNA-dependent RNA polymerase previously proposed to influence as1 through microRNA , these enhancers disrupt ARGONAUTE7 (AGO7)/ZIPPY, SUPPRESSOR OF GENE SILENCING3 (SGS3), and DICER-LIKE4 (DCL4), which instead regulate trans-acting small interfering RNA (ta-siRNA) . Microarray analysis revealed that the AUXIN RESPONSE FACTOR genes ETTIN (ETT)/ARF3 and ARF4 were upregulated in ago7, whereas FILAMENTOUS FLOWER (FIL) was upregulated only in as1 ago7 double mutants. RDR6 and SGS3 likewise repress these genes, which specify abaxial fate . We show that the trans-acting siRNA gene TAS3, which targets ETT and ARF4, is expressed in the adaxial domain, and ett as1 ago7 triple mutants resemble as1. Thus FIL is downregulated redundantly by AS1 and by TAS3, acting through ETT, revealing a role for ta-siRNA in leaf polarity. RDR6 and DCL4 are required for systemic silencing, perhaps implicating ta-siRNA as a mobile signal.

Dual Effects of miR156-TargetedSPLGenes andCYP78A5/KLUHon Plastochron Length and Organ Size inArabidopsis thaliana

“…Conversely, inactivation of the TAS3 pathway causes an acceleration of phase change. However, leaf initiation rates are not altered in unpublished results), all of which are affected in TAS3-dependent phase change (Hunter et al, 2003(Hunter et al, , 2006Xie et al, 2005;Adenot et al, 2006;Fahlgren et al, 2006;Garcia et al, 2006). As with meristem size, the relationship between phase change and plastochron length is complex.…”

Organ Polarity in Plants Is Specified through the Opposing Activity of Two Distinct Small Regulatory RNAs

“…For instance, whereas the Arabidopsis ta-siRNA pathway represses FIL expression (Li et al 2005;Garcia et al 2006;Xu et al 2006), members of the maize yabby gene family closely related to FIL are positively regulated by lbl1 (Juarez et al 2004a). Additionally, although expression analysis suggests that ta-siRNAs contribute to the regulation of miR165/166 in Arabidopsis (Li et al 2005;Xu et al 2006), disruption of the ta-siRNA pathway affects expression of specific MIR165/166 family members only during later stages of leaf development (F. Nogueira et al, unpubl.).…”

“…The differential reliance on the ta-siRNA pathway for adaxial/abaxial patterning in part reflects redundancy of the pathway in Arabidopsis with ASYMMETRIC LEAVES1 (AS1) and AS2. Double mutants affecting the ta-siRNA pathway as well as AS1 or AS2 function develop weakly abaxialized leaves (Li et al 2005;Garcia et al 2006;Xu et al 2006).…”

Hibiscus chlorotic ringspot virus coat protein inhibits trans-acting small interfering RNA biogenesis in Arabidopsis

“…These data further support our hypothesis that HCRSV CP interferes with the ta-siRNA pathway. TAS3 specifies leaf polarity by targeting ARF3 and ARF4, which in turn downregulate the FILAMENTOUS FLOWER (FIL) gene (Garcia et al, 2006). Based on the developmental roles of TAS target genes, it is possible that these varied ta-siRNA targets may be crucial for the abnormal development in CP6.…”