The Arabidopsis thaliana F-box gene HAWAIIAN SKIRT is a new player in the microRNA pathway

In Arabidopsis, the F-box HAWAIIAN SKIRT (HWS) protein is important for organ growth. Loss of function of HWS exhibits pleiotropic phenotypes including sepal fusion. To dissect the HWS role, we EMS-mutagenized hws-1 seeds and screened for mutations that suppress hws-1 associated phenotypes. We identified shs-2 and shs-3 (suppressor of hws-2 and 3) mutants in which the sepal fusion phenotype of hws-1 was suppressed. shs-2 and shs-3 (renamed hst-23/hws-1 and hst-24/hws-1) carry transition mutations that result in premature terminations in the plant homolog of Exportin-5 HASTY (HST), known to be important in miRNA biogenesis, function and transport. Genetic crosses between hws-1 and mutant lines for genes in the miRNA pathway, also suppress the phenotypes associated with HWS loss of function, corroborating epistatic relations between the miRNA pathway genes and HWS. In agreement with these data, accumulation of miRNA is modified in HWS loss or gain of function mutants. Our data propose HWS as a new player in the miRNA pathway, important for plant growth

Nucleo-cytoplasmic transport of proteins and RNA in plants

Merkle T. Nucleo-cytoplasmic transport of proteins and RNA in plants. Plant Cell Reports. 2011;30(2):153-176.Transport of macromolecules between the nucleus and the cytoplasm is an essential necessity in eukaryotic cells, since the nuclear envelope separates transcription from translation. In the past few years, an increasing number of components of the plant nuclear transport machinery have been characterised. This progress, although far from being completed, confirmed that the general characteristics of nuclear transport are conserved between plants and other organisms. However, plant-specific components were also identified. Interestingly, several mutants in genes encoding components of the plant nuclear transport machinery were investigated, revealing differential sensitivity of plant-specific pathways to impaired nuclear transport. These findings attracted attention towards plant-specific cargoes that are transported over the nuclear envelope, unravelling connections between nuclear transport and components of signalling and developmental pathways. The current state of research in plants is summarised in comparison to yeast and vertebrate systems, and special emphasis is given to plant nuclear transport mutants

Antiviral Silencing and Suppression of Gene Silencing in Plants

RNA silencing is an evolutionary conserved sequence-specific gene inactivation mechanism that contributes to the control of development, maintains heterochromatin, acts in stress responses, DNA repair and defends against invading nucleic acids like transposons and viruses. In plants RNA silencing functions as one of the main immune systems. RNA silencing process involves the small RNAs and trans factor components like Dicers, Argonautes and RNA-dependent RNA poly- merases. To deal with host antiviral silencing responses viruses evolved mecha- nisms to avoid or counteract this, most notably through expression of viral suppressors of RNA silencing. Due to the overlap between endogenous and antiviral silencing pathways while blocking antiviral pathways viruses also impact endogenous silencing processes. Here we provide an overview of antiviral silencing pathway, host factors implicated in it and the crosstalk between antiviral and endogenous branches of silencing. We summarize the current status of knowledge about the viral counter-defense strategies acting at various steps during virus infection in plants with the focus on representative, well studied silencing suppres- sor proteins. Finally we discuss future challenges of the antiviral silencing and counter-defense research field

A genetics screen highlights emerging roles for CPL3, RST1 and URT1 in RNA metabolism and silencing.

Post-transcriptional gene silencing (PTGS) is a major mechanism regulating gene expression in higher eukaryotes. To identify novel players in PTGS, a forward genetics screen was performed on an Arabidopsis thaliana line overexpressing a strong growth-repressive gene, ETHYLENE RESPONSE FACTOR6 (ERF6). We identified six independent ethyl-methanesulfonate mutants rescuing the dwarfism of ERF6-overexpressing plants as a result of transgene silencing. Among the causative genes, ETHYLENE-INSENSITIVE5, SUPERKILLER2 and HASTY1 have previously been reported to inhibit PTGS. Notably, the three other causative genes have not, to date, been related to PTGS: UTP:RNA-URIDYLYLTRANSFERASE1 (URT1), C-TERMINAL DOMAIN PHOSPHATASE-LIKE3 (CPL3) and RESURRECTION1 (RST1). We show that these genes may participate in protecting the 3' end of transgene transcripts. We present a model in which URT1, CPL3 and RST1 are classified as PTGS suppressors, as compromisation of these genes provokes the accumulation of aberrant transcripts which, in turn, trigger the production of small interfering RNAs, initiating RNA silencing