Pol II-directed short RNAs suppress the nuclear export of mRNA

Enhanced Synthesis of Foreign Nuclear Protein Stimulates Viral Reproduction via the Induction of γ-Thionin Expression

Plants are a promising platform for recombinant protein production. Here we propose a novel approach to increase the level of viral vector-mediated recombinant protein synthesis. This approach is based on the hypothesis that antiviral protection is weakened during the antibacterial cellular response. We suggested that introduced to the cell foreign nuclear localized proteins, including effectors such as bacterial nucleomodulins, can interfere with the import of cellular nuclear proteins and launch antibacterial defense reactions, creating favorable conditions for cytoplasmic virus reproduction. Here, we performed synthesis of an artificial nuclear protein—red fluorescent protein (mRFP) fused with a nuclear localization sequence (NLS)—in plant cells as a mimetic of a bacterial effector. Superproduction of mRFP:NLS induced Nicotiana benthamiana γ-thionin (NbγThio) mRNA accumulation. Both NLS-containing protein synthesis and increased NbγThio expression stimulated reproduction of the viral vector based on the genome of crucifer-infecting tobacco mosaic virus (crTMV) in N. benthamiana leaves. We isolated the NbγThio gene promoter (Pr^γThio) and showed that Pr^γThio activity sharply increased in response to massive synthesis of GFP fused with NLS. We conclude that NLS-induced Pr^γThio activation and increased accumulation of Nbγthio mRNA led to the stimulation of GFP expression from crTMV: GFP vector in the transient expression system.

Long non-coding RNAs coordinate cellular responses to stress

Following the initial discovery of the heat shock RNA omega (hsrω) gene of Drosophila melanogaster to be non-coding (nc) and also inducible by cell stress, other stress-inducible long non-coding RNAs (lncRNA) have been described in diverse organisms. In view of the rapid sequence divergence of lncRNAs, present knowledge of stress trasncriptome is limited and fragmented. Several known stress-related lncRNAs, associated with specific nuclear speckled domains or nucleolus, provide structural base for sequestering diverse RNA-processing/regulatory proteins. Others have roles in transcriptional or translational inhibition during stress or in signaling pathways; functions of several other lncRNAs are not yet known. Most stress-related lncRNAs act primarily by modulating activity of the proteins to which they bind or by sequestering specific sets of proteins away from the active pool. A common emerging theme is that a given lncRNA targets one or more protein/s with key role/s in the cascade of events triggered by the stress and therefore has a widespread integrative effect. Since proteins associate with RNA through short sequence motifs, the overall base sequence of functionally similar ncRNAs is often not conserved except for specific motifs. The rapid evolvability of ncRNA sequences provides elegant modules for adaptability to changing environment as binding of one or the other protein to ncRNA can alter its structure and functions in distinct ways. Thus the stress-related lncRNAs act as hubs in the cellular networks to coordinate activities of the members within and between different networks to maintain cellular homeostasis for survival or to trigger cell death.

A new viral vector exploiting RNA polymerase I-mediated transcription

We have developed a new viral vector system exploiting RNA-polymerase I transcription. The vector is based on the crucifer-infecting tobacco mosaic virus (crTMV) cDNA inserted into the rRNA transcriptional cassette (promoter and terminator). To visualize reproduction of the vector, the coat protein gene was replaced with the gene encoding green fluorescent protein (GFP) resulting in a Pr(rRNA)-crTMV-GFP construct. Our results showed that agroinjection of Nicotiana benthamiana leaves with this vector results in GFP production from uncapped crTMV-GFP RNA because RNA polymerase I mediates synthesis of rRNA lacking a cap. Coexpression of the crTMV 122 kDa capping protein gene and the silencing suppressor encoded by the tomato bushy stunt virus p19 gene stimulated virus-directed GFP production more than 100-fold. We conclude that the Pol I promoter can be used to drive transcription in a transient expression system.

Airborne signals from a wounded leaf facilitate viral spreading and induce antibacterial resistance in neighboring plants

Many plants release airborne volatile compounds in response to wounding due to pathogenic assault. These compounds serve as plant defenses and are involved in plant signaling. Here, we study the effects of pectin methylesterase (PME)-generated methanol release from wounded plants ("emitters") on the defensive reactions of neighboring "receiver" plants. Plant leaf wounding resulted in the synthesis of PME and a spike in methanol released into the air. Gaseous methanol or vapors from wounded PME-transgenic plants induced resistance to the bacterial pathogen Ralstonia solanacearum in the leaves of non-wounded neighboring "receiver" plants. In experiments with different volatile organic compounds, gaseous methanol was the only airborne factor that could induce antibacterial resistance in neighboring plants. In an effort to understand the mechanisms by which methanol stimulates the antibacterial resistance of "receiver" plants, we constructed forward and reverse suppression subtractive hybridization cDNA libraries from Nicotiana benthamiana plants exposed to methanol. We identified multiple methanol-inducible genes (MIGs), most of which are involved in defense or cell-to-cell trafficking. We then isolated the most affected genes for further analysis: β-1,3-glucanase (BG), a previously unidentified gene (MIG-21), and non-cell-autonomous pathway protein (NCAPP). Experiments with Tobacco mosaic virus (TMV) and a vector encoding two tandem copies of green fluorescent protein as a tracer of cell-to-cell movement showed the increased gating capacity of plasmodesmata in the presence of BG, MIG-21, and NCAPP. The increased gating capacity is accompanied by enhanced TMV reproduction in the "receivers". Overall, our data indicate that methanol emitted by a wounded plant acts as a signal that enhances antibacterial resistance and facilitates viral spread in neighboring plants.

Top 10 plant viruses in molecular plant pathology

Many scientists, if not all, feel that their particular plant virus should appear in any list of the most important plant viruses. However, to our knowledge, no such list exists. The aim of this review was to survey all plant virologists with an association with Molecular Plant Pathology and ask them to nominate which plant viruses they would place in a 'Top 10' based on scientific/economic importance. The survey generated more than 250 votes from the international community, and allowed the generation of a Top 10 plant virus list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Tobacco mosaic virus, (2) Tomato spotted wilt virus, (3) Tomato yellow leaf curl virus, (4) Cucumber mosaic virus, (5) Potato virus Y, (6) Cauliflower mosaic virus, (7) African cassava mosaic virus, (8) Plum pox virus, (9) Brome mosaic virus and (10) Potato virus X, with honourable mentions for viruses just missing out on the Top 10, including Citrus tristeza virus, Barley yellow dwarf virus, Potato leafroll virus and Tomato bushy stunt virus. This review article presents a short review on each virus of the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant virology community, as well as laying down a benchmark, as it will be interesting to see in future years how perceptions change and which viruses enter and leave the Top 10.