HASTY, the Arabidopsis EXPORTIN5 ortholog, regulates cell-to-cell and vascular microRNA movement.

Plant microRNAs (miRNAs) guide cytosolic post-transcriptional gene silencing of sequence-complementary transcripts within the producing cells, as well as in distant cells and tissues. Here, we used an artificial miRNA-based system (amiRSUL) in Arabidopsis thaliana to explore the still elusive mechanisms of inter-cellular miRNA movement via forward genetics. This screen identified many mutant alleles of HASTY (HST), the ortholog of mammalian EXPORTIN5 (XPO5) with a recently reported role in miRNA biogenesis in Arabidopsis. In both epidermis-peeling and grafting assays, amiRSUL levels were reduced much more substantially in miRNA-recipient tissues than in silencing-emitting tissues. We ascribe this effect to HST controlling cell-to-cell and phloem-mediated movement of the processed amiRSUL, in addition to regulating its biogenesis. While HST is not required for the movement of free GFP or siRNAs, its cell-autonomous expression in amiRSUL-emitting tissues suffices to restore amiRSUL movement independently of its nucleo-cytosolic shuttling activity. By contrast, HST is dispensable for the movement and activity of amiRSUL within recipient tissues. Finally, HST enables movement of endogenous miRNAs that display mostly unaltered steady-state levels in hst mutant tissues. We discuss a role for HST as a hitherto unrecognized regulator of miRNA movement in relation to its recently assigned nuclear function at the nexus of MIRNA transcription and miRNA processing.

A contemporary reassessment of the enhanced transient expression system based on the tombusviral silencing suppressor protein P19.

Transient transgenic expression accelerates pharming and facilitates protein studies in plants. One embodiment of the approach involves leaf infiltration of Agrobacterium strains whose T-DNA is engineered with the gene(s) of interest. However, gene expression during 'agro-infiltration' is intrinsically and universally impeded by the onset of post-transcriptional gene silencing (PTGS). Nearly 20 years ago, a simple method was developed, whereby co-expression of the tombusvirus-encoded P19 protein suppresses PTGS and thus enhances transient gene expression. Yet, how PTGS is activated and suppressed by P19 during the process has remained unclear to date. Here, we address these intertwined questions in a manner also rationalizing how vastly increased protein yields are achieved using a minimal viral replicon as a transient gene expression vector. We also explore, in side-by-side analyses, why some proteins do not accumulate to the expected high levels in the assay, despite vastly increased mRNA levels. We validate that enhanced co-expression of multiple constructs is achieved within the same transformed cells, and illustrate how the P19 system allows rapid protein purification for optimized downstream in vitro applications. Finally, we assess the suitability of the P19 system for subcellular localization studies - an originally unanticipated, yet increasingly popular application - and uncover shortcomings of this specific implement. In revisiting the P19 system using contemporary knowledge, this study sheds light onto its hitherto poorly understood mechanisms while further illustrating its versatility but also some of its limits.

TCTP and CSN4 control cell cycle progression and development by regulating CULLIN1 neddylation in plants and animals.

Translationally Controlled Tumor Protein (TCTP) controls growth by regulating the G1/S transition during cell cycle progression. Our genetic interaction studies show that TCTP fulfills this role by interacting with CSN4, a subunit of the COP9 Signalosome complex, known to influence CULLIN-RING ubiquitin ligases activity by controlling CULLIN (CUL) neddylation status. In agreement with these data, downregulation of CSN4 in Arabidopsis and in tobacco cells leads to delayed G1/S transition comparable to that observed when TCTP is downregulated. Loss-of-function of AtTCTP leads to increased fraction of deneddylated CUL1, suggesting that AtTCTP interferes negatively with COP9 function. Similar defects in cell proliferation and CUL1 neddylation status were observed in Drosophila knockdown for dCSN4 or dTCTP, respectively, demonstrating a conserved mechanism between plants and animals. Together, our data show that CSN4 is the missing factor linking TCTP to the control of cell cycle progression and cell proliferation during organ development and open perspectives towards understanding TCTP's role in organ development and disorders associated with TCTP miss-expression.

Chemical enhancers of posttranscriptional gene silencing in Arabidopsis.

RNAi mediated by small-interfering RNAs (siRNAs) operates via transcriptional (TGS) and posttranscriptional gene silencing (PTGS). In Arabidopsis thaliana, TGS relies on DICER-LIKE-3 (DCL3)-dependent 24-nt siRNAs loaded into AGO4-clade ARGONAUTE effector proteins. PTGS operates via DCL4-dependent 21-nt siRNAs loaded into AGO1-clade proteins. We set up and validated a medium-throughput, semi-automatized procedure enabling chemical screening, in a 96-well in vitro format, of Arabidopsis transgenic seedlings expressing an inverted-repeat construct from the phloem companion cells. The ensuing quantitative PTGS phenotype was exploited to identify molecules, which, upon topical application, either inhibit or enhance siRNA biogenesis/activities. The vast majority of identified modifiers were enhancers, among which Sortin1, Isoxazolone, and [5-(3,4-dichlorophenyl)furan-2-yl]-piperidine-1-ylmethanethione (DFPM) provided the most robust and consistent results, including upon their application onto soil-grown plants in which their effect was nonautonomous and long lasting. The three molecules increased the RNAi potency of the inverted-repeat construct, in large part by enhancing 21-nt siRNA accumulation and loading into AGO1, and concomitantly reducing AGO4 and DCL3 levels in planta. A similar, albeit not identical effect, was observed on 22-nt siRNAs produced from a naturally occurring inverted-repeat locus, demonstrating that the molecules also enhance endogenous PTGS. In standardized assays conducted in seedling extracts, the three enhancers selectively increased DCL4-mediated processing of in vitro-synthesized double-stranded RNAs, indicating the targeting of a hitherto unknown PTGS component probably independent of the DCL4-cofactor DOUBLE-STRANDED RNA-BINDING 4 (DRB4). This study establishes the proof-of-concept that RNAi efficacy can be modulated by chemicals in a whole organism. Their potential applications and the associated future research are discussed.

AUXIN RESPONSE FACTOR8 regulates Arabidopsis petal growth by interacting with the bHLH transcription factor BIGPETALp.

Plant organ growth and final size are determined by coordinated cell proliferation and expansion. The BIGPETALp (BPEp) basic helix-loop-helix (bHLH) transcription factor was shown to limit Arabidopsis thaliana petal growth by influencing cell expansion. We demonstrate here that BPEp interacts with AUXIN RESPONSE FACTOR8 (ARF8) to affect petal growth. This interaction is mediated through the BPEp C-terminal domain (SD(BPEp)) and the C-terminal domain of ARF8. Site-directed mutagenesis identified an amino acid consensus motif in SD(BPEp) that is critical for mediating BPEp-ARF8 interaction. This motif shares sequence similarity with motif III of ARF and AUXIN/INDOLE-3-ACETIC ACID proteins. Petals of arf8 mutants are significantly larger than those of the wild type due to increased cell number and increased cell expansion. bpe arf8 double mutant analyses show that during early petal development stages, ARF8 and BPEp work synergistically to limit mitotic growth. During late stages, ARF8 and BPEp interact to limit cell expansion. The alterations in cell division and cell expansion observed in arf8 and/or bpe mutants are associated with a change in expression of early auxin-responsive genes. The data provide evidence of an interaction between an ARF and a bHLH transcription factor and of its biological significance in regulating petal growth, with local auxin levels likely influencing such a biological function.

Translationally controlled tumor protein is a conserved mitotic growth integrator in animals and plants.

The growth of an organism and its size determination require the tight regulation of cell proliferation and cell growth. However, the mechanisms and regulatory networks that control and integrate these processes remain poorly understood. Here, we address the biological role of Arabidopsis translationally controlled tumor protein (AtTCTP) and test its shared functions in animals and plants. The data support a role of plant AtTCTP as a positive regulator of mitotic growth by specifically controlling the duration of the cell cycle. We show that, in contrast to animal TCTP, plant AtTCTP is not implicated in regulating postmitotic growth. Consistent with this finding, plant AtTCTP can fully rescue cell proliferation defects in Drosophila loss of function for dTCTP. Furthermore, Drosophila dTCTP is able to fully rescue cell proliferation defects in Arabidopsis tctp knockouts. Our data provide evidence that TCTP function in regulating cell division is part of a conserved growth regulatory pathway shared between plants and animals. The study also suggests that, although the cell division machinery is shared in all multicellular organisms to control growth, cell expansion can be uncoupled from cell division in plants but not in animals.

Jasmonate controls late development stages of petal growth in Arabidopsis thaliana.

In Arabidopsis, four homeotic gene classes, A, B, C and E, are required for the patterning of floral organs. However, very little is known about how the activity of these master genes is translated into regulatory processes leading to specific growth patterns and the formation of organs with specific shapes and sizes. Previously we showed that the transcript variant BPEp encodes a bHLH transcription factor that is involved in limiting petal size by controlling post-mitotic cell expansion. Here we show that the phytohormone jasmonate is required for control of BPEp expression. Expression of BPEp was negatively regulated in opr3 mutant flowers that are deficient in jasmonate synthesis. Moreover, the expression of BPEp was restored in opr3 flowers following exogenous jasmonate treatments. Expression of the second transcript variant BPEub, which originates from the same gene as BPEp via an alternative splicing event, was not affected, indicating that BPEp accumulation triggered by jasmonate occurs at the post-transcriptional level. Consistent with these data, opr3 exhibited an increase in petal size as a result of increased cell size, as well as a modified vein pattern, phenotypes that are similar to those of the bpe-1 mutant. Furthermore, exogenous treatments with jasmonate rescued petal phenotypes associated with loss of function of OPR3. Our data demonstrate that jasmonate signaling downstream of OPR3 is involved in the control of cell expansion and in limiting petal size, and that BPEp is a downstream target that functions as a component mediating jasmonate signaling during petal growth.

Movement and differential consumption of short interfering RNA duplexes underlie mobile RNA interference.

In RNA interference (RNAi), the RNase III Dicer processes long double-stranded RNA (dsRNA) into short interfering RNA (siRNA), which, when loaded into ARGONAUTE (AGO) family proteins, execute gene silencing1. Remarkably, RNAi can act non-cell autonomously2,3: it is graft transmissible4-7, and plasmodesmata-associated proteins modulate its cell-to-cell spread8,9. Nonetheless, the molecular mechanisms involved remain ill defined, probably reflecting a disparity of experimental settings. Among other caveats, these almost invariably cause artificially enhanced movement via transitivity, whereby primary RNAi-target transcripts are converted into further dsRNA sources of secondary siRNA5,10,11. Whether siRNA mobility naturally requires transitivity and whether it entails the same or distinct signals for cell-to-cell versus long-distance movement remains unclear, as does the identity of the mobile signalling molecules themselves. Movement of long single-stranded RNA, dsRNA, free/AGO-bound secondary siRNA or primary siRNA have all been advocated12-15; however, an entity necessary and sufficient for all known manifestations of plant mobile RNAi remains to be ascertained. Here, we show that the same primary RNAi signal endows both vasculature-to-epidermis and long-distance silencing movement from three distinct RNAi sources. The mobile entities are AGO-free primary siRNA duplexes spreading length and sequence independently. However, their movement is accompanied by selective siRNA depletion reflecting the AGO repertoires of traversed cell types. Coupling movement with this AGO-mediated consumption process creates qualitatively distinct silencing territories, potentially enabling unlimited spatial gene regulation patterns well beyond those granted by mere gradients.