RNA-induced silencing complexes (RISCs) play central roles in posttranscriptional gene silencing. In plants, the mechanism of RISC assembly has remained elusive due to the lack of cell-free systems that recapitulate the process. In this report, we demonstrate that plant AGO1 protein synthesized by in vitro translation using an extract of evacuolated tobacco protoplasts incorporates synthetic small interfering RNA (siRNA) and microRNA (miRNA) duplexes to form RISCs that sequester the single-stranded siRNA guide strand and miRNA strand, respectively. The formed RISCs were able to recognize and cleave the complementary target RNAs. In this system, the siRNA duplex was incorporated into HSP90-bound AGO1, and subsequent removal of the passenger strand was triggered by ATP hydrolysis by HSP90. Removal of the siRNA passenger strand required the ribonuclease activity of AGO1, while that of the miRNA star strand did not. Based on these results, the mechanism of plant RISC formation is discussed.

In plants, DNA methylation can be mediated by a class of Argonaute4 (AGO4)-associated heterochromatic siRNAs (hc-siRNAs), through a pathway termed RNA-directed DNA methylation (RdDM). It has been thought that RdDM is solely a nuclear process, as both the biogenesis and functioning of hc-siRNAs take place in the nucleus. In this study, we unexpectedly found that hc-siRNAs are predominantly present in the cytoplasm. We demonstrated that AGO4 is loaded with hc-siRNAs in the cytoplasm and the formation of mature AGO4/siRNA complexes requires HSP90 and the cleavage activity of AGO4. Intriguingly, siRNA binding facilitates the redistribution of AGO4 into the nucleus, likely through inducing conformational change that leads to the exposure of the nuclear localization signal (NLS). Our findings reveal an unsuspected cytoplasmic step in the RdDM pathway. We propose that selective nuclear import of mature AGO4/siRNA complexes is a key regulatory point prior to the effector stage of RdDM.

Posttranscriptional gene silencing is mediated by RNAinduced silencing complexes (RISCs) that contain AGO proteins and single-stranded small RNAs. The assembly of plant AGO1-containing RISCs depends on the molecular chaperone HSP90. Here, we demonstrate that cyclophilin 40 (CYP40), protein phosphatase 5 (PP5), and several other proteins with the tetratricopeptide repeat (TPR) domain associates with AGO1 in an HSP90-dependent manner in extracts of evacuolated tobacco protoplasts (BYL). Intriguingly, CYP40, but not the other TPR proteins, could form a complex with small RNA duplexbound AGO1. Moreover, CYP40 that was synthesized by in-vitro translation using BYL uniquely facilitated binding of small RNA duplexes to AGO1, and as a result, increased the amount of mature RISCs that could cleave target RNAs. CYP40 was not contained in mature RISCs, indicating that the association is transient. Addition of PP5 or cyclophilin-binding drug cyclosporine A prevented the association of endogenous CYP40 with HSP90-AGO1 complex and inhibited RISC assembly. These results suggest that a complex of AGO1, HSP90, CYP40, and a small RNA duplex is a key intermediate of RISC assembly in plants.

trans-acting small interfering RNAs (tasiRNAs) are plant-specific endogenous siRNAs produced via a unique pathway whose first step is the microRNA (miRNA)-programmed RNA-induced silencing complex (RISC)-mediated cleavage of tasiRNA gene (TAS) transcripts. One of the products is subsequently transformed into tasiRNAs by a pathway that requires several factors including SUPPRESSOR OF GENE SILENCING3 (SGS3) and RNA-DEPENDENT RNA POLYMERASE6. Here, using in vitro assembled ARGONAUTE (AGO)1-RISCs, we show that SGS3 is recruited onto RISCs only when they bind target RNA. Following cleavage by miRNA173 (miR173)-programmed RISC, SGS3 was found in complexes containing cleaved TAS2 RNA and RISC. The 3′ cleavage fragment (the source of tasiRNAs) was protected from degradation in this complex. Depletion of SGS3 did not affect TAS2 RNA cleavage by miR173-programmed RISC, but did affect the stability of the 3′ cleavage fragment. When the 3′ nucleotide of 22-nt miR173 was deleted or the corresponding nucleotide in TAS2 RNA was mutated, the complex was not observed and the 3′ cleavage fragment was degraded. Importantly, these changes in miR173 or TAS2 RNA are known to lead to a loss of tasiRNA production in vivo. These results suggest that (i) SGS3 associates with AGO1-RISC via the doublestranded RNA formed by the 3′-terminal nucleotides of 22-nt miR173 and corresponding target RNA, which probably protrudes from the AGO1-RISC molecular surface, (ii) SGS3 protects the 3′ cleavage fragment of TAS2 RNA from degradation, and (iii) the observed SGS3-dependent stabilization of the 3′ fragment of TAS2 RNA is key to tasiRNA production.RNA silencing | secondary siRNA | transitivity I n RNA silencing, 20-to 30-nt small RNAs (sRNAs) mediate sequence-specific gene regulation. trans-acting siRNAs (tasiRNAs), a class of plant-specific endogenous small interfering RNAs (siRNAs), posttranscriptionally regulate mRNAs that have complementary sequences (1-4). tasiRNA biogenesis is initiated by the microRNA (miRNA)-directed cleavage of a transcript from the tasiRNA gene (TAS) loci (5). There are eight TAS loci in Arabidopsis thaliana (TAS1a-c, TAS2, TAS3a-c, and TAS4). The production of tasiRNAs from TAS1a-c and TAS2 is triggered by miRNA173 (miR173). miRNA390 (miR390) and miRNA828 (miR828) initiate tasiRNA production from TAS3a-c and TAS4 transcripts, respectively (3, 4, 6). The processing of the TAS3 transcript is distinct from that of other TAS transcripts, because it requires two miR390 target sites that are specifically recognized by an ARGONAUTE (AGO)7-containing RNA-induced silencing complex (RISC) (7,8). In contrast, other TAS primary transcripts have single miRNA target sites, are cleaved by AGO1-containing RISCs, and produce tasiRNAs from 3′ cleavage fragments. Previous studies using engineered MIRNA genes showed that the primary miRNAs need to be 22 nt in length to trigger tasiRNA or secondary siRNA production, whereas the targets of these 22-nt miRNAs do not have any determinants beyond the presence of target sites (9-12). This implies...

In , ARGONAUTE1 (AGO1) plays a central role in microRNA (miRNA) and small interfering RNA (siRNA)-mediated silencing and is a key component in antiviral responses. The polerovirus F-box P0 protein triggers AGO1 degradation as a viral counterdefense. Here, we identified a motif in AGO1 that is required for its interaction with the S phase kinase-associated protein1-cullin 1-F-box protein (SCF) P0 (SCF) complex and subsequent degradation. The AGO1 P0 degron is conserved and confers P0-mediated degradation to other AGO proteins. Interestingly, the degron motif is localized in the DUF1785 domain of AGO1, in which a single point mutation (, obtained by forward genetic screening) compromises recognition by SCF Recapitulating formation of the RNA-induced silencing complex in a cell-free system revealed that this mutation impairs RNA unwinding, leading to stalled forms of AGO1 still bound to double-stranded RNAs. In vivo, the DUF1785 is required for unwinding perfectly matched siRNA duplexes, but is mostly dispensable for unwinding imperfectly matched miRNA duplexes. Consequently, its mutation nearly abolishes phased siRNA production and sense transgene posttranscriptional gene silencing. Overall, our work sheds new light on the mode of AGO1 recognition by P0 and the in vivo function of DUF1785 in RNA silencing.

The molecular and physiological mechanisms behind the maturation and maintenance of N 2 -fixing nodules during development of symbiosis between rhizobia and legumes still remain unclear, although the early events of symbiosis are relatively well understood. Azorhizobium caulinodans ORS571 is a microsymbiont of the tropical legume Sesbania rostrata, forming N 2 -fixing nodules not only on the roots but also on the stems. In this study, 10,080 transposon-inserted mutants of A. caulinodans ORS571 were individually inoculated onto the stems of S. rostrata, and those mutants that induced ineffective stem nodules, as displayed by halted development at various stages, were selected. From repeated observations on stem nodulation, 108 Tn5 mutants were selected and categorized into seven nodulation types based on size and N 2 fixation activity. Tn5 insertions of some mutants were found in the well-known nodulation, nitrogen fixation, and symbiosis-related genes, such as nod, nif, and fix, respectively, lipopolysaccharide synthesis-related genes, C 4 metabolism-related genes, and so on. However, other genes have not been reported to have roles in legume-rhizobium symbiosis. The list of newly identified symbiosis-related genes will present clues to aid in understanding the maturation and maintenance mechanisms of nodules.Symbiosis between rhizobia and legumes results in the formation of nitrogen-fixing nodules. The symbiotic interaction begins with the induction of bacterial nod genes by flavonoids secreted from the plant roots (11). The nod genes encode proteins that synthesize nodulation factor (Nod factor), which initiates many of the developmental changes seen in the host plant early in the nodulation process (11,26,55). After the initial exchange and bacterial attachment at the surface, cortical cells begin dividing to form the nodule primordia. Bacteria penetrate the developing nodule primordia via host-derived infection threads (11,26,55). Upon release from the infection threads, bacteria invade the plant cell cytoplasm, where they differentiate into bacteroids and provide ammonium to the host plant by reducing atmospheric dinitrogen in exchange for carbon and amino acid compounds (18,54,58).It is deduced that multiple stages exist in the establishment of complete nitrogen-fixing symbiosis and that signal exchange between rhizobia and legumes might occur at each stage. The finding that the bacterial Nod factor switches on the nodulation program in the plant and the characterization of the plant genes of the Nod factor receptor or perception complexes have revealed a remarkable early event in the process of nodule development (23,46,47,60,75). However, the molecular and physiological mechanisms behind the maturation and maintenance of nodules still remain unclear. The transcriptional analysis of this process has been well-described in several legumerhizobium symbiosis systems, and the results have revealed that drastic transcriptional changes occur during nodule development (2,5,7,76). However, to our knowledge, large...

ORCID ID: 0000-0002-4178-9242 (T.M.).trans-Acting small interfering RNAs (tasiRNAs) participate in the regulation of organ morphogenesis and determination of developmental timing in plants by down-regulating target genes through mRNA cleavage. The production of tasiRNAs is triggered by microRNA173 (miR173) and other specific microRNA-mediated cleavage of 59-capped and 39-polyadenylated primary TAS transcripts (pri-TASs). Although pri-TASs are not thought to encode functional proteins, they contain multiple short open reading frames (ORFs). For example, the primary TAS2 transcript (pri-TAS2) contains 11 short ORFs, and the third ORF from the 59 terminus (ORF3) encompasses the miR173 target site. Here, we show that nonsense mutations in ORF3 of pri-TAS2 upstream of the miR173 recognition site suppress tasiRNA accumulation and that ORF3 is translated in vitro. Glycerol gradient centrifugation analysis of Arabidopsis (Arabidopsis thaliana) plant extracts revealed that pri-TAS2 and its miR173-cleaved 59 and 39 fragments are fractionated together in the polysome fractions. These and previous results suggest that the 39 fragment of pri-TAS2, which is a source of tasiRNAs, forms a huge complex containing SGS3, miR173-programmed AGO1 RNA-induced silencing complex, the 59 fragment, and ribosomes. This complex overaccumulated, moderately accumulated, and did not accumulate in rdr6, sde5, and sgs3 mutants, respectively. The sgs3 sde5 and rdr6 sde5 double mutants showed phenotypes similar to those of sgs3 and sde5 single mutants, respectively, with regard to the TAS2-related RNA accumulation, suggesting that the complex is formed in an SGS3-dependent manner, somehow modified and stabilized by SDE5, and becomes competent for RDR6 action. Ribosomes in this complex likely play an important role in this process.

In eukaryotes, the RNase-III Dicer often produces length/sequence microRNA (miRNA) variants, called "isomiRs", owing to intrinsic structural/sequence determinants of the miRNA precursors (pre-miRNAs). In this study, we combined biophysics, genetics and biochemistry approaches to study Arabidopsis miR168, the key feedback regulator of central plant silencing effector protein ARGONAUTE1 (AGO1). We identified a motif conserved among plant pre-miR168 orthologs, which enables flexible internal base-pairing underlying at least three metastable structural configurations. These configurations promote alternative, accurate Dicer cleavage events generating length and structural isomiR168 variants with distinctive AGO sorting properties and modes of action. Among these isomiR168s, a duplex with a 22-nt guide strand exhibits strikingly preferential affinity for AGO10, the closest AGO1 paralog. The 22-nt miR168-AGO10 complex antagonizes AGO1 accumulation in part via "transitive RNAi", a silencing-amplification process, to maintain appropriate AGO1 cellular homeostasis. Furthermore, we found that the tombusviral P19 silencing-suppressor protein displays markedly weaker affinity for the 22-nt form among its isomiR168 cargoes, thereby promoting AGO10-directed suppression of AGO1-mediated antiviral silencing. Taken together, these findings indicate that structural flexibility, a previously overlooked property of pre-miRNAs, considerably increases the versatility and regulatory potential of individual MIRNA genes, and that some pathogens might have evolved the capacity or mechanisms to usurp this property.