Dynamics of the rice yellow mottle disease in western Burkina Faso: Epidemic monitoring, spatio-temporal variation of viral diversity, and pathogenicity in a disease hotspot.

The rice yellow mottle virus (RYMV) is a model in plant virus molecular epidemiology, with the reconstruction of historical introduction routes at the scale of the African continent. However, information on patterns of viral prevalence and viral diversity over multiple years at a local scale remains scarce, in spite of potential implications for crop protection. Here, we describe a 5-year (2015-9) monitoring of RYMV prevalence in six sites from western Burkina Faso (geographic areas of Bama, Banzon, and Karfiguela). It confirmed one irrigated site as a disease hotspot and also found one rainfed lowland (RL) site with occasional high prevalence levels. Within the studied fields, a pattern of disease aggregation was evidenced at a 5-m distance, as expected for a mechanically transmitted virus. Next, we monitored RYMV genetic diversity in the irrigated disease hotspot site, revealing a high viral diversity, with the current coexistence of various distinct genetic groups at the site scale (ca. 520 ha) and also within various specific fields (25 m side). One genetic lineage, named S1bzn, is the most recently emerged group and increased in frequency over the studied period (from 20 per cent or less in 2015-6 to more than 65 per cent in 2019). Its genome results from a recombination between two other lineages (S1wa and S1ca). Finally, experimental work revealed that three rice varieties commonly cultivated in Burkina Faso were not different in terms of resistance level, and we also found no significant effect of RYMV genetic groups on symptom expression and viral load. We found, however, that infection outcome depended on the specific RYMV isolate, with two isolates from the lineage S1bzn accumulating at the highest level at early infections. Overall, this study documents a case of high viral prevalence, high viral diversity, and co-occurrence of divergent genetic lineages at a small geographic scale. A recently emerged lineage, which comprises viral isolates inducing severe symptoms and high accumulation under controlled conditions, could be recently rising through natural selection. Following up the monitoring of RYMV diversity is required to confirm this trend and further understand the factors driving the local maintenance of viral diversity.

A Flexible and Original Architecture of Two Unrelated Zinc Fingers Underlies the Role of the Multitask P1 in RYMV Spread.

Viruses of the sobemovirus genus are plant viruses, most of which generate very important agricultural and financial losses. Among them, the rice yellow mottle virus (RYMV) is one of the most damaging pathogens devastating rice fields in Africa. RYMV infectivity and propagation rely on its protein P1, identified as a key movement and potential long-distance RNA silencing suppressor. Here we describe P1's complete 3D structure and dynamics obtained by an integrative approach combining X-Ray crystallography and NMR spectroscopy. We show that P1 is organized in two semi-independent and topologically unrelated domains, each harboring an original zinc finger. The two domains exhibit different affinities for zinc and sensitivities to oxidoreduction conditions, making the C-terminal P1 region a potential labile sensor of the plant redox status. An additional level of regulation resides on the capacity of P1 to oligomerize through its N-terminal domain. Coupling P1 structure information with site-directed mutagenesis and plant functional assays, we identified key residues in each zinc domain essential for infectivity and spread in rice tissues. Altogether, our results provide the first complete structure of a sobemoviral P1 movement protein and highlight structural and dynamical properties that may serve RYMV functions to infect and invade its host plant.

Complete genome sequence of a novel marafivirus infecting pearl millet in Burkina Faso.

Pearl millet (Pennisetum glaucum (L.) R. Br.) is a staple food that is widely cultivated in sub-Saharan Africa. In August 2018, a survey was conducted in the main producing regions of Burkina Faso, and leaf samples were analyzed using virion-associated nucleic acid (VANA)-based metagenomic approach and Illumina sequencing. A new virus, tentatively named "Pennisetum glaucum marafivirus" (PGMV), was detected, and its complete nucleotide sequence of 6364 nucleotides was determined. The sequence contains a large open reading frame (ORF) encoding a polyprotein of 224.2 kDa with five domains (methyltransferase, papain-like protease, helicase, RNA-dependent RNA polymerase, and coat proteins), typical of marafiviruses. Additionally, a characteristic conserved marafibox domain was detected in the genome. The nucleotide sequence of the complete PGMV genome shares 68.5% identity with that of sorghum bicolor marafivirus, and its coat protein shares 58.5% identity with that of oat blue dwarf virus. Phylogenetic analysis confirmed that the pearl millet virus is unambiguously grouped with members of the genus Marafivirus in the family Tymoviridae. This is the first report on the occurrence of a marafivirus in pearl millet.

An atypical class of non-coding small RNAs is produced in rice leaves upon bacterial infection.

Non-coding small RNAs (sRNA) act as mediators of gene silencing and regulate plant growth, development and stress responses. Early insights into plant sRNAs established a role in antiviral defense and they are now extensively studied across plant-microbe interactions. Here, sRNA sequencing discovered a class of sRNA in rice (Oryza sativa) specifically associated with foliar diseases caused by Xanthomonas oryzae bacteria. Xanthomonas-induced small RNAs (xisRNAs) loci were distinctively upregulated in response to diverse virulent strains at an early stage of infection producing a single duplex of 20-22 nt sRNAs. xisRNAs production was dependent on the Type III secretion system, a major bacterial virulence factor for host colonization. xisRNA loci overlap with annotated transcripts sequences, with about half of them encoding protein kinase domain proteins. A number of the corresponding rice cis-genes have documented functions in immune signaling and xisRNA loci predominantly coincide with the coding sequence of a conserved kinase motif. xisRNAs exhibit features of small interfering RNAs and their biosynthesis depend on canonical components OsDCL1 and OsHEN1. xisRNA induction possibly mediates post-transcriptional gene silencing but they do not broadly suppress cis-genes expression on the basis of mRNA-seq data. Overall, our results identify a group of unusual sRNAs with a potential role in plant-microbe interactions.

Optimized RNA-Silencing Strategies for Rice Ragged Stunt Virus Resistance in Rice.

Rice ragged stunt virus (RRSV) is one of the most damaging viruses of the rice culture area in south and far-eastern Asia. To date, no genetic resistance has been identified and only expensive and non-environmentally friendly chemical treatments are deployed to fight this important disease. Non-chemical approaches based on RNA-silencing have been developed as resistance strategies against viruses. Here, we optimized classical miRNA and siRNA-based strategies to obtain efficient and durable resistance to RRSV. miRNA-based strategies are involved in producing artificial miRNA (amiR) targeting viral genomes in plants. Classically, only one amiR is produced from a single construct. We demonstrated for the first time that two amiRs targeting conserved regions of RRSV genomes could be transgenically produced in Nicotiana benthamiana and in rice for a single precursor. Transgenic rice plants producing either one or two amiR were produced. Despite efficient amiR accumulations, miRNA-based strategies with single or double amiRs failed to achieve efficient RRSV resistance in transformed rice plants. This suggests that this strategy may not be adapted to RRSV, which could rapidly evolve to counteract them. Another RNA-silencing-based method for viral resistance concerns producing several viral siRNAs targeting a viral fragment. These viral siRNAs are produced from an inverted repeat construct carrying the targeted viral fragment. Here, we optimized the inverted repeat construct using a chimeric fragment carrying conserved sequences of three different RRSV genes instead of one. Of the three selected homozygous transgenic plants, one failed to accumulate the expected siRNA. The two other ones accumulated siRNAs from either one or three fragments. A strong reduction of RRSV symptoms was observed only in transgenic plants expressing siRNAs. We consequently demonstrated, for the first time, an efficient and environmentally friendly resistance to RRSV in rice based on the siRNA-mediated strategy.

Spatiotemporal Survey of Multiple Rice Diseases in Irrigated Areas Compared to Rainfed Lowlands in the Western Burkina Faso.

Multiple constraints affect rice yields in West Africa. Among these constraints are viral, bacterial, and fungal pathogens. We aimed to describe the spatiotemporal patterns of occurrence and incidence of multiple rice diseases in farmers' fields in contrasting rice growing systems in the western Burkina Faso. For this purpose, we selected a set of three pairs of sites, each comprising an irrigated area and a neighboring rainfed lowland, and studied them over four consecutive years. We first performed interviews with the rice farmers to better characterize the management practices at the different sites. This study revealed that the transplanting of rice and the possibility of growing rice twice a year are restricted to irrigated areas, while other practices, such as the use of registered rice cultivars, fertilization, and pesticides, are not specific but differ between the two rice growing systems. Then, we performed symptom observations at these study sites to monitor the following four diseases: yellow mottle disease, Bacterial Leaf Streak (BLS), rice leaf blast, and brown spot. The infection rates were found to be higher in irrigated areas than in rainfed lowlands, both when analyzing all observed symptoms together (any of the four diseases) and when specifically considering each of the two diseases: BLS and rice leaf blast. Brown spot was particularly prevalent in all six study sites, while yellow mottle disease was particularly structured geographically. Various diseases were frequently found together in the same field (co-occurrence) or even on the same plant (coinfection), especially in irrigated areas.

Characterization of virus species associated with sweetpotato virus diseases in Burkina Faso.

Sweetpotato (Ipomoea batatas) production in sub-Saharan Africa is severely affected by viral diseases caused by several interacting viruses, including sweet potato feathery mottle virus (SPFMV), sweet potato chlorotic stunt virus (SPCSV), and sweet potato leaf curl virus (SPLCV). However, the aetiology of viral symptoms on sweetpotato is rarely established in most countries in Africa. Here, we aimed to investigate and characterize the incidence of sweetpotato viruses in Burkina Faso. We performed a countrywide survey in 18 districts of Burkina Faso and collected 600 plants, with and without symptoms, from 80 fields. Viral strains were identified using nitrocellulose membrane-ELISA, PCR, and reverse transcription-PCR. Three scions from each of 50 selected plants with symptoms were grafted to healthy Ipomoea setosa and then serological and molecular tests were performed on the 150 recorded samples. Three viruses were detected: 24% of samples were positive for SPFMV, 18% for SPLCV, and 2% for SPCSV. Across all diagnostic tests, 40% of all plant samples were virus-negative. Coinfections were found in 16% of samples. Partial sequences were obtained, including 13 that matched SPFMV, one that matched SPLCV, and one that matched SPCSV. All identified SPFMV isolates belonged to either phylogroup B or A-II. The SPCSV-positive isolates had 98% gene sequence homology with SPCSV-West Africa for the coat protein. Begomovirus-positive isolates clustered with SPLCV-United States. This first study of sweetpotato viral diseases in Burkina Faso indicates widespread occurrence and suggests a need for further epidemiological investigations, breeding programmes focused on virus-resistant varieties, and improved farming practices to control disease spread.

Design of a new multiplex PCR assay for rice pathogenic bacteria detection and its application to infer disease incidence and detect co-infection in rice fields in Burkina Faso.

Crop diseases are responsible for considerable yield losses worldwide and particularly in sub-Saharan Africa. To implement efficient disease control measures, detection of the pathogens and understanding pathogen spatio-temporal dynamics is crucial and requires the use of molecular detection tools, especially to distinguish different pathogens causing more or less similar symptoms. We report here the design a new molecular diagnostic tool able to simultaneously detect five bacterial taxa causing important diseases on rice in Africa: (1) Pseudomonas fuscovaginae, (2) Xanthomonas oryzae, (3) Burkholderia glumae and Burkholderia gladioli, (4) Sphingomonas and (5) Pantoea species. This new detection tool consists of a multiplex PCR, which is cost effective and easily applicable. Validation of the method is presented through its application on a global collection of bacterial strains. Moreover, sensitivity assessment for the detection of all five bacteria is reported to be at 0.5 ng DNA by µl. As a proof of concept, we applied the new molecular detection method to a set of 256 rice leaves collected from 16 fields in two irrigated areas in western Burkina Faso. Our results show high levels of Sphingomonas spp. (up to 100% of tested samples in one field), with significant variation in the incidence between the two sampled sites. Xanthomonas oryzae incidence levels were mostly congruent with bacterial leaf streak (BLS) and bacterial leaf blight (BLB) symptom observations in the field. Low levels of Pantoea spp. were found while none of the 256 analysed samples was positive for Burkholderia or Pseudomonas fuscovaginae. Finally, many samples (up to 37.5% in one studied field) were positive for more than one bacterium (co-infection). Documenting co-infection levels are important because of their drastic consequences on epidemiology, evolution of pathogen populations and yield losses. The newly designed multiplex PCR for multiple bacterial pathogens of rice is a significant improvement for disease monitoring in the field, thus contributing to efficient disease control and food safety.

Nicotiana benthamiana is a suitable transient system for high-level expression of an active inhibitor of cotton boll weevil α-amylase.

BACKGROUND: Insect resistance in crops represents a main challenge for agriculture. Transgenic approaches based on proteins displaying insect resistance properties are widely used as efficient breeding strategies. To extend the spectrum of targeted pathogens and overtake the development of resistance, molecular evolution strategies have been used on genes encoding these proteins to generate thousands of variants with new or improved functions. The cotton boll weevil (Anthonomus grandis) is one of the major pests of cotton in the Americas. An α-amylase inhibitor (α-AIC3) variant previously developed via molecular evolution strategy showed inhibitory activity against A. grandis α-amylase (AGA). RESULTS: We produced in a few days considerable amounts of α-AIC3 using an optimised transient heterologous expression system in Nicotiana benthamiana. This high α-AIC3 accumulation allowed its structural and functional characterizations. We demonstrated via MALDI-TOF MS/MS technique that the protein was processed as expected. It could inhibit up to 100% of AGA biological activity whereas it did not act on α-amylase of two non-pathogenic insects. These data confirmed that N. benthamiana is a suitable and simple system for high-level production of biologically active α-AIC3. Based on other benefits such as economic, health and environmental that need to be considerate, our data suggested that α-AIC3 could be a very promising candidate for the production of transgenic crops resistant to cotton boll weevil without lethal effect on at least two non-pathogenic insects. CONCLUSIONS: We propose this expression system can be complementary to molecular evolution strategies to identify the most promising variants before starting long-lasting stable transgenic programs.

Virus-based pharmaceutical production in plants: an opportunity to reduce health problems in Africa.

BACKGROUND: Developing African countries face health problems that they struggle to solve. The major causes of this situation are high therapeutic and logistical costs. Plant-made therapeutics are easy to produce due to the lack of the safety considerations associated with traditional fermenter-based expression platforms, such as mammalian cells. Plant biosystems are easy to scale up and inexpensive, and they do not require refrigeration or a sophisticated medical infrastructure. These advantages provide an opportunity for plant-made pharmaceuticals to counteract diseases for which medicines were previously inaccessible to people in countries with few resources. MAIN BODY: The techniques needed for plant-based therapeutic production are currently available. Viral expression vectors based on plant viruses have greatly enhanced plant-made therapeutic production and have been exploited to produce a variety of proteins of industrial, pharmaceutical and agribusiness interest. Some neglected tropical diseases occurring exclusively in the developing world have found solutions through plant bioreactor technology. Plant viral expression vectors have been reported in the production of therapeutics against these diseases occurring exclusively in the third world, and some virus-derived antigens produced in plants exhibit appropriate antigenicity and immunogenicity. However, all advances in the use of plants as bioreactors have been made by companies in Europe and America. The developing world is still far from acquiring this technology, although plant viral expression vectors may provide crucial help to overcome neglected diseases. CONCLUSION: Today, interest in these tools is rising, and viral amplicons made in and for Africa are in progress. This review describes the biotechnological advances in the field of plant bioreactors, highlights factors restricting access to this technology by those who need it most and proposes a solution to overcome these limitations.

Optimized transitory ectopic expression of promastigote surface antigen protein in Nicotiana benthamiana, a potential anti-leishmaniasis vaccine candidate.

In recent years, plants have been shown to be an efficient alternative expression system for high-value pharmaceuticals such as vaccines. However, constitutive expression of recombinant protein remains uncertain on their level of production and biological activity. To overcome these problems, transitory expression systems have been developed. Here, a series of experiments were performed to determine the most effective conditions to enhance vaccine antigen transient accumulation in Nicotiana benthamiana leaves using the promastigote surface antigen (PSA) from the parasitic protozoan Leishmania infantum. This protein has been previously identified as the major antigen of a licensed canine anti-leishmaniasis vaccine. The classical prokaryote Escherichia coli biosystem failed in accumulating PSA. Consequently, the standard plant system based on N. benthamiana has been optimized for the production of putatively active PSA. First, the RNA silencing defense mechanism set up by the plant against PSA ectopic expression was abolished by using three viral suppressors acting at different steps of the RNA silencing pathway. Then, we demonstrated that the signal peptide at the N-terminal side of the PSA is required for its accumulation. The PSA ER signaling and retention with the PSA signal peptide and the KDEL motif, respectively were optimized to significantly increase its accumulation. Finally, we demonstrate that the production of recombinant PSA in N. benthamiana leaves allows the conservation of its immunogenic property. These approaches demonstrate that based on these optimizations, plant based systems can be used to effectively produce the biological active PSA protein.

Virus-Bacteria Rice Co-Infection in Africa: Field Estimation, Reciprocal Effects, Molecular Mechanisms, and Evolutionary Implications.

Simultaneous infection of a single plant by various pathogen species is increasingly recognized as an important modulator of host resistance and a driver of pathogen evolution. Because plants in agro-ecosystems are the target of a multitude of pathogenic microbes, co-infection could be frequent, and consequently important to consider. This is particularly true for rapidly intensifying crops, such as rice in Africa. This study investigated potential interactions between pathogens causing two of the major rice diseases in Africa: the Rice yellow mottle virus (RYMV) and the bacterium Xanthomonas oryzae pathovar oryzicola (Xoc) in order to: 1/ document virus-bacteria co-infection in rice in the field, 2/ explore experimentally their consequences in terms of symptom development and pathogen multiplication, 3/ test the hypothesis of underlying molecular mechanisms of interactions and 4/ explore potential evolutionary consequences. Field surveys in Burkina Faso revealed that a significant proportion of rice fields were simultaneously affected by the two diseases. Co-infection leads to an increase in bacterial specific symptoms, while a decrease in viral load is observed compared to the mono-infected mock. The lack of effect found when using a bacterial mutant for an effector specifically inducing expression of a small RNA regulatory protein, HEN1, as well as a viral genotype-specific effect, both suggest a role for gene silencing mechanisms mediating the within-plant interaction between RYMV and Xoc. Potential implications for pathogen evolution could not be inferred because genotype-specific effects were found only for pathogens originating from different countries, and consequently not meeting in the agrosystem. We argue that pathogen-pathogen-host interactions certainly deserve more attention, both from a theoretical and applied point of view.

OsMADS26 Negatively Regulates Resistance to Pathogens and Drought Tolerance in Rice.

Functional analyses of MADS-box transcription factors in plants have unraveled their role in major developmental programs (e.g. flowering and floral organ identity) as well as stress-related developmental processes, such as abscission, fruit ripening, and senescence. Overexpression of the rice (Oryza sativa) MADS26 gene in rice has revealed a possible function related to stress response. Here, we show that OsMADS26-down-regulated plants exhibit enhanced resistance against two major rice pathogens: Magnaporthe oryzae and Xanthomonas oryzae. Despite this enhanced resistance to biotic stresses, OsMADS26-down-regulated plants also displayed enhanced tolerance to water deficit. These phenotypes were observed in both controlled and field conditions. Interestingly, alteration of OsMADS26 expression does not have a strong impact on plant development. Gene expression profiling revealed that a majority of genes misregulated in overexpresser and down-regulated OsMADS26 lines compared with control plants are associated to biotic or abiotic stress response. Altogether, our data indicate that OsMADS26 acts as an upstream regulator of stress-associated genes and thereby, a hub to modulate the response to various stresses in the rice plant.

P2 of Rice grassy stunt virus (RGSV) and p6 and p9 of Rice ragged stunt virus (RRSV) isolates from Vietnam exert suppressor activity on the RNA silencing pathway.

In Vietnam, the two main viruses that cause disease in rice are the Rice grassy stunt virus (RGSV) and the Rice ragged stunt virus (RRSV). Outbreaks of these two viruses have dramatically decreased rice production in Vietnam. Because natural resistance genes are unknown, an RNAi strategy may be an alternative method to develop resistance to RGSV and RRSV. However, this strategy will be efficient only if putative silencing suppressors encoded by the two viruses are neutralized. To identify these suppressors, we used the classical green fluorescent protein (GFP) agroinfiltration method in Nicotiana benthamiana. Then, we investigated the effects of viral candidate proteins on GFP expression and GFP siRNA accumulation and their interference with the short- or long-range signal of silencing. RGSV genes s2gp1, s5gp2, and s6gp1 and RRSV genes s5gp1, s6gp1, s9gp1, and s10gp1 were selected for viral silencing suppressor investigation according to their small molecular weight, the presence of cysteines, or the presence of a GW motif in related protein products. We confirmed that protein p6 of RRSV displays mild silencing suppressor activity and affects long-range silencing by delaying the systemic silencing signal. In addition, we identified two new silencing suppressors that displayed mild activity: p2 of RGSV and p9 of RRSV.

Sites under positive selection modulate the RNA silencing suppressor activity of rice yellow mottle virus movement protein P1.

RNA silencing is a eukaryotic mechanism for RNA-based gene regulation that plays an essential role in diverse biological processes, such as defence against viral infections. The P1 of rice yellow mottle virus (RYMV) is a movement protein and displays RNA silencing suppression activity with variable efficiency, depending on the origin of the isolates. In this study, the positive selection pressure acting on the P1 protein gene was assessed. A site-by-site analysis of the dN/dS ratio was performed and 18 positively selected sites were identified. Four of these were mutated, and the ability to suppress RNA silencing was evaluated for the resulting mutants in a transient expression assay. All mutations affected quantitatively RNA silencing suppression, one caused a significant decrease in the activity and three significantly increased it. This work demonstrates, for what is to the best of our knowledge the first time, that the RYMV gene encoding the P1 RNA silencing suppressor is under adaptive evolution.

The RYMV-encoded viral suppressor of RNA silencing P1 is a zinc-binding protein with redox-dependent flexibility.

Viral suppressors of RNA interference (VSRs) target host gene silencing pathways, thereby operating important roles in the viral cycle and in host cells, in which they counteract host innate immune responses. However, the molecular mechanisms of VSRs are poorly understood. We provide here biochemical and biophysical features of the dual suppressor/activator VSR P1 protein encoded by the rice yellow mottle virus. In silico analyses of P1 suggested common features with zinc finger proteins and native mass spectrometry unambiguously confirmed that recombinant P1 binds reversibly two zinc atoms, each with a different strength. Additionally, we demonstrate that the reaction of P1 with H2O2 leads to zinc release, disulfide bond formation, and protein oligomerization. A reversible protein modification by redox alterations has only been described for a limited number of zinc finger proteins and has never been reported for VSRs. Those reported here for P1 might be a general feature of Cys-rich VSRs and could be a key regulatory mechanism for the control of RNA silencing.

The rice yellow mottle virus P1 protein exhibits dual functions to suppress and activate gene silencing.

In plants RNA silencing is a host defense mechanism against viral infection, in which double-strand RNA is processed into 21-24-nt short interfering RNA (siRNA). Silencing spreads from cell to cell and systemically through a sequence-specific signal to limit the propagation of the virus. To counteract this defense mechanism, viruses encode suppressors of silencing. The P1 protein encoded by the rice yellow mottle virus (RYMV) displays suppression activity with variable efficiency, according to the isolates that they originated from. Here, we show that P1 proteins from two RYMV isolates displaying contrasting suppression strength reduced local silencing induced by single-strand and double-strand RNA in Nicotiana benthamiana leaves. This suppression was associated with a slight and a severe reduction in 21- and 24-nt siRNA accumulation, respectively. Unexpectedly, cell-to-cell movement and systemic propagation of silencing were enhanced in P1-expressing Nicotiana plants. When transgenically expressed in rice, P1 proteins induced specific deregulation of DCL4-dependent endogenous siRNA pathways, whereas the other endogenous pathways were not affected. As DCL4-dependent pathways play a key role in rice development, the expression of P1 viral proteins was associated with the same severe developmental defects in spikelets as in dcl4 mutants. Overall, our results demonstrate that a single viral protein displays multiple effects on both endogenous and exogenous silencing, not only in a suppressive but also in an enhancive manner. This suggests that P1 proteins play a key role in maintaining a subtle equilibrium between defense and counter-defense mechanisms, to insure efficient virus multiplication and the preservation of host integrity.

[Plant viruses RNA silencing suppressors: characterization and mechanism of action]. / Les suppresseurs du RNA silencing des phytovirus: caractérisation et mode d'action.

RNA silencing or Post-transcriptional gene silencing (PTGS) in plants is a fundamental defence mechanism against viruses, transgenes and transposons. Most viruses, if not all, are able to overcome RNA-silencing through the production of so-called "silencing suppressors" with counterdefence ability". This strategy is well known for plant and animal viruses. Silencing suppressor proteins block the host RNA silencing by targeting different steps of the silencing pathway. In this review, we will focus on the major silencing suppressor proteins encoded by plant viruses and on the methods used to identify and characterize the molecular bases of silencing suppression.

Identification of precursor transcripts for 6 novel miRNAs expands the diversity on the genomic organisation and expression of miRNA genes in rice.

BACKGROUND: The plant miRNAs represent an important class of endogenous small RNAs that guide cleavage of an mRNA target or repress its translation to control development and adaptation to stresses. MiRNAs are nuclear-encoded genes transcribed by RNA polymerase II, producing a primary precursor that is subsequently processed by DCL1 an RNase III Dicer-like protein. In rice hundreds of miRNAs have been described or predicted, but little is known on their genes and precursors which are important criteria to distinguish them from siRNAs. Here we develop a combination of experimental approaches to detect novel miRNAs in rice, identify their precursor transcripts and genes and predict or validate their mRNA targets. RESULTS: We produced four cDNA libraries from small RNA fractions extracted from distinct rice tissues. By in silico analysis we selected 6 potential novel miRNAs, and confirmed that their expression requires OsDCL1. We predicted their targets and used 5'RACE to validate cleavage for three of them, targeting a PPR, an SPX domain protein and a GT-like transcription factor respectively. In addition, we identified precursor transcripts for the 6 miRNAs expressed in rice, showing that these precursors can be efficiently processed using a transient expression assay in transfected Nicotiana benthamiana leaves. Most interestingly, we describe two precursors producing tandem miRNAs, but in distinct arrays. We focus on one of them encoding osa-miR159a.2, a novel miRNA produced from the same stem-loop structure encoding the conserved osa-miR159a.1. We show that this dual osa-miR159a.2-osa-miR159a.1 structure is conserved in distant rice species and maize. Finally we show that the predicted mRNA target of osa-miR159a.2 encoding a GT-like transcription factor is cleaved in vivo at the expected site. CONCLUSION: The combination of approaches developed here identified six novel miRNAs expressed in rice which can be clearly distinguished from siRNAs. Importantly, we show that two miRNAs can be produced from a single precursor, either from tandem stem-loops or tandemly arrayed in a single stem-loop. This suggests that processing of these precursors could be an important regulatory step to produce one or more functional miRNAs in plants and perhaps coordinate cleavage of distinct targets in the same plant tissue.

Biological and molecular characterization of a putative new sobemovirus infecting Imperata cylindrica and maize in Africa.

A new virus was isolated from both the grass Imperata cylindrica and maize plants that had yellow mottle symptoms in Burkina Faso, West Africa. The virus has isometric particles ca. 32 nm in diameter. The experimental host range was restricted to Rottboellia exaltata. Virions were isolated from leaves of systemically infected maize plants. Koch's postulates were completed by mechanically inoculating uninfected Imperata or maize with either purified virus or sap from infected Imperata plants. Virion preparations were used to produce a specific polyclonal antiserum, and an enzyme-linked immunosorbent assay test was set up. The full genome of the virus was sequenced, and it comprised 4,547 nucleotides. Phylogenetic studies indicated that the virus is closely related to rice yellow mottle virus, a sobemovirus that infects monocotyledons in Africa, and is more distantly related to cocksfoot mottle virus, another sobemovirus that infects monocotyledons. Although the virus can infect R. exaltata experimentally, it differs from Rottboellia yellow mottle virus, a member of a tentative species of the genus Sobemovirus that also infects monocotyledons in Africa. Particle morphology, serological properties, genomic organization, and phylogenetic analysis are all consistent with assignment of the new virus to the genus Sobemovirus. The name Imperata yellow mottle virus is proposed.