Arabidopsis tonoplast proteins TIP1 and TIP2 interact with the cucumber mosaic virus 1a replication protein

Comparative proteome profiling of susceptible and resistant rice cultivars identified an arginase involved in rice defense against Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo), the causative agent of bacterial blight, is one of the major threats to rice productivity. Yet, the molecular mechanism of rice-Xoo interaction is elusive. Here, we report comparative proteome profiles of Xoo susceptible (Dongjin) and resistant (Hwayeong) cultivars of rice in response to two-time points (3 and 6 days) of Xoo infection. Low-abundance proteins were enriched using a protamine sulfate (PS) precipitation method and isolated proteins were quantified by a label-free quantitative analysis, leading to the identification of 3846 proteins. Of these, 1128 proteins were significantly changed between mock and Xoo infected plants of Dongjin and Hwayeong cultivars. Based on the abundance pattern and functions of the identified proteins, a total of 23 candidate proteins were shortlisted that potentially participate in plant defense against Xoo in the resistant cultivar. Of these candidate proteins, a mitochondrial arginase-1 showed Hwayeong specific abundance and was significantly accumulated following Xoo inoculation. Overexpression of arginase 1 (OsArg 1) in susceptible rice cultivar (Dongjin) resulted in enhanced tolerance against Xoo as compared to the wild-type. In addition, expression analysis of defense-related genes encoding PR1, glucanase I, and chitinase II by qRT-PCR showed their enhanced expression in the overexpression lines as compared to wild-type. Taken together, our results uncover the proteome changes in the rice cultivars and highlight the functions of OsARG1 in plant defense against Xoo.

Regulation of salt-stressed sunflower (Helianthus annuus) seedling's water status by the coordinated action of Na+/K+ accumulation, nitric oxide, and aquaporin expression

Among abiotic stresses, salt stress is a major threat to crop production all over the world. Present work demonstrates the profuse accumulation of Na+ in 2-day-old, dark-grown sunflower (Helianthus annuus L.) seedlings roots in response to salt stress (NaCl). The pattern of K+ accumulation in response to salt stress is similar to that of Na+ but on relatively lower scale. Application of nitric oxide (NO) donor (DETA) scales down Na+ accumulation in salt-stressed seedlings. The impact of NO donor on K+ accumulation is, however, different in control and salt-stressed seedling roots. In control seedlings, it enhances K+ accumulation, whereas, it gets reduced in salt-stressed seedlings. Specialised channels called 'aquaporins' (AQPs) play a major role maintaining the water status and transport across plant parts under salt-stress. Thus, accumulation of plasma-membrane intrinsic proteins (PIPs) and tonoplast-intrinsic proteins (TIPs), localised on plasma-membrane and vacuolar-membrane, respectively was undertaken in 2-day-old, dark-grown seedling roots. Salt stress increased the abundance of these isoforms, whereas, NO application resulted in decreased accumulation of PIP2 and TIP1. PIP1 and TIP2 isoforms remained undetectable. Present work thus, puts forward a correlation between AQP expression and ions (Na+ and K+) homeostasis in response to salt stress and NO.

Significance of solute specificity, expression, and gating mechanism of tonoplast intrinsic protein during development and stress response in plants

Tonoplast intrinsic proteins (TIPs), belonging to the aquaporin family, are transmembrane channels located mostly at the tonoplast of plant cells. The TIPs are known to transport water and many other small solutes such as ammonia, urea, hydrogen peroxide, and glycerol. In the present review, phylogenetic distribution, structure, transport dynamics, gating mechanism, sub-cellular localization, tissue-specific expression, and co-expression of TIPs are discussed to define their versatile role in plants. Based on the phylogenetic distribution, TIPs are classified into five distinct groups with aromatic-arginine (Ar/R) selectivity filters, typical pore-morphology, and tissue-specific gene expression patterns. The tissue-specific expression of TIPs is conserved among diverse plant species, more particularly for TIP3s, which are expressed exclusively in seeds. Studying TIP3 evolution will help to understand seed development and germination. The solute specificity of TIPs plays an imperative role in physiological processes like stomatal movement and vacuolar sequestration as well as in alleviating environmental stress. TIPs also play an important role in growth and developmental processes like radicle protrusion, anther dehiscence, seed germination, cell elongation, and expansion. The gating mechanism of TIPs regulates the solute flow in response to external signals, which helps to maintain the physiological functions of the cell. The information provided in this review is a base to explore TIP's potential in crop improvement programs.

Cucumber Mosaic Virus 1a Protein Interacts with the Tobacco SHE1 Transcription Factor and Partitions between the Nucleus and the Tonoplast Membrane

The transcription factor SHE1 was identified as an interacting partner with the cucumber mosaic virus (CMV) 1a protein in the yeast two-hybrid system, by a pull-down assay, and via bimolecular fluorescent complementation. Using fluorescent-tagged proteins and confocal microscopy, the CMV 1a protein itself was found distributed predominantly between the nucleus and the tonoplast membrane, although it was also found in speckles in the cytoplasm. The SHE1 protein was localized in the nucleus, but in the presence of the CMV 1a protein was partitioned between the nucleus and the tonoplast membrane. SHE1 expression was induced by infection of tobacco with four tested viruses: CMV, tobacco mosaic virus, potato virus X and potato virus Y. Transgenic tobacco expressing the CMV 1a protein showed constitutive expression of SHE1, indicating that the CMV 1a protein may be responsible for its induction. However, previously, such plants also were shown to have less resistance to local and systemic movement of tobacco mosaic virus (TMV) expressing the green fluorescent protein, suggesting that the CMV 1a protein may act to prevent the function of the SHE1 protein. SHE1 is a member of the AP2/ERF class of transcription factors and is conserved in sequence in several Nicotiana species, although two clades of SHE1 could be discerned, including both different Nicotiana species and cultivars of tobacco, varying by the presence of particular insertions or deletions.

Three-dimensional reconstruction and comparison of vacuolar membranes in response to viral infection

The vacuole is a unique plant organelle that plays an important role in maintaining cellular homeostasis under various environmental stress conditions. However, the effects of biotic stress on vacuole structure has not been examined using three-dimensional (3D) visualization. Here, we performed 3D electron tomography to compare the ultrastructural changes in the vacuole during infection with different viruses. The 3D models revealed that vacuoles are remodeled in cells infected with cucumber mosaic virus (CMV) or tobacco necrosis virus A Chinese isolate (TNV-AC ), resulting in the formation of spherules at the periphery of the vacuole. These spherules contain neck-like channels that connect their interior with the cytosol. Confocal microscopy of CMV replication proteins 1a and 2a and TNV-AC auxiliary replication protein p23 showed that all of these proteins localize to the tonoplast. Electron microscopy revealed that the expression of these replication proteins alone is sufficient to induce spherule formation on the tonoplast, suggesting that these proteins play prominent roles in inducing vacuolar membrane remodeling. This is the first report of the 3D structures of viral replication factories built on the tonoplasts. These findings contribute to our understanding of vacuole biogenesis under normal conditions and during assembly of plant (+) RNA virus replication complexes.

Soybean antiviral immunity conferred by dsRNase targets the viral replication complex

Eukaryotic positive-strand RNA viruses replicate their genomes in membranous compartments formed in a host cell, which sequesters the dsRNA replication intermediate from antiviral immune surveillance. Here, we find that soybean has developed a way to overcome this sequestration. We report the positional cloning of the broad-spectrum soybean mosaic virus resistance gene Rsv4, which encodes an RNase H family protein with dsRNA-degrading activity. An active-site mutant of Rsv4 is incapable of inhibiting virus multiplication and is associated with an active viral RNA polymerase complex in infected cells. These results suggest that Rsv4 enters the viral replication compartment and degrades viral dsRNA. Inspired by this model, we design three plant-gene-derived dsRNases that can inhibit the multiplication of the respective target viruses. These findings suggest a method for developing crops resistant to any target positive-strand RNA virus by fusion of endogenous host genes.

Arabidopsis tonoplast intrinsic protein and vacuolar H+-adenosinetriphosphatase reflect vacuole dynamics during development of syncytia induced by the beet cyst nematode Heterodera schachtii

Plant parasitic cyst nematodes induce specific hypermetabolic syncytial nurse cell structures in host roots. A characteristic feature of syncytia is the lack of the central vacuole and the formation of numerous small and larger vesicles. We show that these structures are formed de novo via widening of ER cisternae during the entire development of syncytium, whereas in advanced stages of syncytium development, larger vacuoles are also formed via fusion of vesicles/tubules surrounding organelle-free pre-vacuole regions. Immunogold transmission electron microscopy of syncytia localised the vacuolar markers E subunit of vacuolar H⁺-adenosinetriphosphatase (V-ATPase) complex and tonoplast intrinsic protein (γ-TIP1;1) mostly in membranes surrounding syncytial vesicles, thus indicating that these structures are vacuoles and that some of them have a lytic character. To study the function of syncytial vacuoles, changes in expression of AtVHA-B1, AtVHA-B2 and AtVHA-B3 (coding for isoforms of subunit B of V-ATPase), and TIP1;1 and TIP1;2 (coding for γ-TIP proteins) genes were analysed. RT-qPCR revealed significant downregulation of AtVHA-B2, TIP1;1 and TIP1;2 at the examined stages of syncytium development compared to uninfected roots. Expression of VHA-B1 and VHA-B3 decreased at 3 dpi but reached the level of control at 7 dpi. These results were confirmed for TIP1;1 by monitoring At-γ-TIP-YFP reporter construct expression. Infection test conducted on tip1;1 mutant plants showed formation of larger syncytia and higher numbers of females in comparison to wild-type plants indicating that reduced levels or lack of TIP1;1 protein promote nematode development.

Exploring the Roles of Aquaporins in Plant⁻Microbe Interactions

Aquaporins (AQPs) are membrane channel proteins regulating the flux of water and other various small solutes across membranes. Significant progress has been made in understanding the roles of AQPs in plants' physiological processes, and now their activities in various plant⁻microbe interactions are receiving more attention. This review summarizes the various roles of different AQPs during interactions with microbes which have positive and negative consequences on the host plants. In positive plant⁻microbe interactions involving rhizobia, arbuscular mycorrhizae (AM), and plant growth-promoting rhizobacteria (PGPR), AQPs play important roles in nitrogen fixation, nutrient transport, improving water status, and increasing abiotic stress tolerance. For negative interactions resulting in pathogenesis, AQPs help plants resist infections by preventing pathogen ingress by influencing stomata opening and influencing defensive signaling pathways, especially through regulating systemic acquired resistance. Interactions with bacterial or viral pathogens can be directly perturbed through direct interaction of AQPs with harpins or replicase. However, whilst these observations indicate the importance of AQPs, further work is needed to develop a fuller mechanistic understanding of their functions.

Molecular Modeling for Better Understanding of Cucumovirus Pathology

Cucumber mosaic virus (CMV) is a small RNA virus capable of infecting a wide variety of plant species. The high economic losses due to the CMV infection made this virus a relevant subject of scientific studies, which were further facilitated by the small size of the viral genome. Hence, CMV also became a model organism to investigate the molecular mechanism of pathogenesis. All viral functions are dependent on intra- and intermolecular interactions between nucleic acids and proteins of the virus and the host. This review summarizes the recent data on molecular determinants of such interactions. A particular emphasis is given to the results obtained by utilizing molecular-based planning and modeling techniques.

Transcriptomics reveals multiple resistance mechanisms against cotton leaf curl disease in a naturally immune cotton species, Gossypium arboreum

Cotton leaf curl disease (CLCuD), caused by cotton leaf curl viruses (CLCuVs), is among the most devastating diseases in cotton. While the widely cultivated cotton species Gossypium hirsutum is generally susceptible, the diploid species G. arboreum is a natural source for resistance against CLCuD. However, the influence of CLCuD on the G. arboreum transcriptome and the interaction of CLCuD with G. arboreum remains to be elucidated. Here we have used an RNA-Seq based study to analyze differential gene expression in G. arboreum under CLCuD infestation. G. arboreum plants were infested by graft inoculation using a CLCuD infected scion of G. hirsutum. CLCuD infested asymptomatic and symptomatic plants were analyzed with RNA-seq using an Illumina HiSeq. 2500. Data analysis revealed 1062 differentially expressed genes (DEGs) in G. arboreum. We selected 17 genes for qPCR to validate RNA-Seq data. We identified several genes involved in disease resistance and pathogen defense. Furthermore, a weighted gene co-expression network was constructed from the RNA-Seq dataset that indicated 50 hub genes, most of which are involved in transport processes and might have a role in the defense response of G. arboreum against CLCuD. This fundamental study will improve the understanding of virus-host interaction and identification of important genes involved in G. arboreum tolerance against CLCuD.

The interaction between endogenous 30S ribosomal subunit protein S11 and Cucumber mosaic virus LS2b protein affects viral replication, infection and gene silencing suppressor activity

Cucumber mosaic virus (CMV) is a model virus for plant-virus protein interaction and mechanism research because of its wide distribution, high-level of replication and simple genome structure. The 2b protein is a multifunctional protein encoded by CMV that suppresses RNA silencing-based antiviral defense and contributes to CMV virulence in host plants. In this report, 12 host proteins were identified as CMV LS2b binding partners using the yeast two-hybrid screen system from the Arabidopsis thaliana cDNA library. Among the host proteins, 30S ribosomal subunit protein S11 (RPS11) was selected for further studies. The interaction between LS2b and full-length RPS11 was confirmed using the yeast two-hybrid system. Bimolecular fluorescence complementation (BIFC) assays observed by confocal laser microscopy and Glutathione S-transferase (GST) pull-down assays were used to verify the interaction between endogenous NbRPS11 and viral CMVLS2b both in vivo and in vitro. TRV-based gene silencing vector was used to knockdown NbRPS11 transcription, and immunoblot analysis revealed a decline in infectious viral RNA replication and a decrease in CMV infection in RPS11 down-regulated Nicotiana benthamiana plants. Thus, the knockdown of RPS11 likely inhibited CMV replication and accumulation. The gene silencing suppressor activity of CMV2b protein was reduced by the RPS11 knockdown. This study demonstrated that the function of viral LS2b protein was remarkably affected by the interaction with host RPS11 protein.

The Interactions of Aquaporins and Mineral Nutrients in Higher Plants

Aquaporins, major intrinsic proteins (MIPs) present in the plasma and intracellular membranes, facilitate the transport of small neutral molecules across cell membranes in higher plants. Recently, progress has been made in understanding the mechanisms of aquaporin subcellular localization, transport selectivity, and gating properties. Although the role of aquaporins in maintaining the plant water status has been addressed, the interactions between plant aquaporins and mineral nutrients remain largely unknown. This review highlights the roles of various aquaporin orthologues in mineral nutrient uptake and transport, as well as the regulatory effects of mineral nutrients on aquaporin expression and activity, and an integrated link between aquaporins and mineral nutrient metabolism was identified.

Phosphatidylserine Synthase Controls Cell Elongation Especially in the Uppermost Internode in Rice by Regulation of Exocytosis

The uppermost internode is one of the fastest elongating organs in rice, and is expected to require an adequate supply of cell-wall materials and enzymes to the cell surface to enhance mechanical strength. Although it has been reported that the phenotype of shortened uppermost internode 1 (sui1) is caused by mutations in PHOSPHATIDYLSERINE SYNTHASE (OsPSS), the underlying mechanism remains unclear. Here we show that the OsPSS-1, as a gene expressed predominantly in elongating cells, regulates post-Golgi vesicle secretion to intercellular spaces. Mutation of OsPSS-1 leads to compromised delivery of CESA4 and secGFP towards the cell surface, resulting in weakened intercellular adhesion and disorganized cell arrangement in parenchyma. The phenotype of sui1-4 is caused largely by the reduction in cellulose contents in the whole plant and detrimental delivery of pectins in the uppermost internode. We found that OsPSS-1 and its potential product PS (phosphatidylserine) localized to organelles associated with exocytosis. These results together suggest that OsPSS-1 plays a potential role in mediating cell expansion by regulating secretion of cell wall components.

The Roles of Aquaporins in Plant Stress Responses

Aquaporins are membrane channel proteins ubiquitously present in all kingdoms of life. Although aquaporins were originally discovered as water channels, their roles in the transport of small neutral solutes, gasses, and metal ions are now well established. Plants contain the largest number and greatest diversity of aquaporin homologs with diverse subcellular localization patterns, gating properties, and solute specificity. The roles of aquaporins in physiological functions throughout plant growth and development are well known. As an integral regulator of plant–water relations, they are presumed to play an important role in plant defense responses against biotic and abiotic stressors. This review highlights involvement of various aquaporin homologs in plant stress responses against a variety of environmental stresses that disturb plant cell osmotic balance and nutrient homeostasis.

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.

The group IV-A cyclic nucleotide-gated channels, CNGC19 and CNGC20, localize to the vacuole membrane in Arabidopsis thaliana

The cyclic nucleotide-gated channels, CNGC19 and CNGC20, are the sole members of the highly isolated evolutionary group IV-A in Arabidopsis plants. Prior studies have shown that the expression of both CNGC19 and CNGC20 genes are induced by salinity and biotic stress. In this report, CNGC19 and CNGC20 were determined to localize to the vacuolar membrane. Co-expression of CNGC19 and CNGC20 increased the efficiency of vacuolar localization. CNGC19 and CNGC20 are, therefore, vacuolar membrane channels that are hypothesized to mediate plant response to salinity and biotic stress.

Plant cyclic nucleotide-gated channels (CNGCs) are implicated in the uptake of both essential and toxic cations, Ca²⁺ signalling, and responses to biotic and abiotic stress. The 20 CNGC paralogues of Arabidopsis are divided into five evolutionary groups. Group IV-A is highly isolated and consists only of two closely spaced genes, CNGC19 and CNGC20. Prior studies have shown that both genes are induced by salinity and biotic stress. A unique feature of CNGC19 and CNGC20 is their long hydrophilic N-termini. To determine the subcellular locations of CNGC19 and CNGC20, partial and full-length fusions to GFP(S65T) were generated. Translational fusions of the N-termini of CNGC19 (residues 1–171) and CNGC20 (residues 1–200) to GFP(S65T) were targeted to punctate structures when transiently expressed in leaf protoplasts. In the case of CNGC20, but not CNGC19, the punctate structures were co-labelled with a marker for the Golgi. The full-length CNGC19-GFP fusion co-localized with markers for the vacuole membrane (αTIP- and γTIP-mCherry). Vacuole membrane labelling by the full-length CNGC20-GFP fusion was also observed, but the signal was weak and accompanied by numerous punctate signals that did not co-localize with αTIP- or γTIP-mCherry. These punctate structures diminished, and localization of full-length CNGC20-GFP to the vacuole increased, when it was co-expressed with the full-length CNGC19-mCherry. Vacuole membrane labelling was also detected in planta via immunoelectron microscopy using a CNGC20-antiserum on cryopreserved ultrathin sections of roots. We hypothesize that the role of group IV-A CNGCs is to mediate the movement of cations between the central vacuole and the cytosol in response to certain types of abiotic and biotic stress.

Arabidopsis Pumilio protein APUM5 suppresses Cucumber mosaic virus infection via direct binding of viral RNAs

Posttranscriptional/translational regulation of gene expression is mediated by diverse RNA binding proteins and plays an important role in development and defense processes. Among the RNA-binding proteins, the mammalian Pumilio RNA-binding family (Puf) acts as posttranscriptional and translational repressors. An Arabidopsis Puf mutant, apum5-D, was isolated during a T-DNA insertional mutant screen for mutants with reduced susceptibility to Cucumber mosaic virus (CMV) infection. Interestingly, CMV RNA contained putative Pumilio-homology domain binding motifs in its 3' untranslated region (UTR) and internal places in its genome. APUM5 directly bound to the 3' UTR motifs and some internal binding motifs in CMV RNAs in vitro and in vivo. We showed that APUM5 acts as a translational repressor that regulates the 3' UTR of CMV and affects CMV replication. This study uncovered a unique defense system that Arabidopsis APUM5 specifically regulates CMV infection by the direct binding of CMV RNAs.

Alfalfa mosaic virus replicase proteins, P1 and P2, localize to the tonoplast in the presence of virus RNA

To identify the virus components important for assembly of the Alfalfa mosaic virus replicase complex, we used live cell imaging of Arabidopsis thaliana protoplasts that expressed various virus cDNAs encoding native and GFP-fusion proteins of P1 and P2 replicase proteins and full-length virus RNAs. Expression of P1-GFP alone resulted in fluorescent vesicle-like bodies in the cytoplasm that colocalized with FM4-64, an endocytic marker, and RFP-AtVSR2, RabF2a/Rha1-mCherry, and RabF2b/Ara7-mCherry, all of which localize to multivesicular bodies (MVBs), which are also called prevacuolar compartments, that mediate traffic to the lytic vacuole. GFP-P2 was driven from the cytosol to MVBs when expressed with P1 indicating that P1 recruited GFP-P2. P1-GFP localized on the tonoplast, which surrounds the vacuole, in the presence of infectious virus RNA, replication competent RNA2, or P2 and replication competent RNA1 or RNA3. This suggests that a functional replication complex containing P1, P2, and a full-length AMV RNA assembles on MVBs to traffic to the tonoplast.

Helicase domain encoded by Cucumber mosaic virus RNA1 determines systemic infection of Cmr1 in pepper

The Cmr1 gene in peppers confers resistance to Cucumber mosaic virus isolate-P0 (CMV-P0). Cmr1 restricts the systemic spread of CMV strain-Fny (CMV-Fny), whereas this gene cannot block the spread of CMV isolate-P1 (CMV-P1) to the upper leaves, resulting in systemic infection. To identify the virulence determinant of CMV-P1, six reassortant viruses and six chimeric viruses derived from CMV-Fny and CMV-P1 cDNA clones were used. Our results demonstrate that the C-terminus of the helicase domain encoded by CMV-P1 RNA1 determines susceptibility to systemic infection, and that the helicase domain contains six different amino acid substitutions between CMV-Fny and CMV-P1(.) To identify the key amino acids of the helicase domain determining systemic infection with CMV-P1, we then constructed amino acid substitution mutants. Of the mutants tested, amino acid residues at positions 865, 896, 957, and 980 in the 1a protein sequence of CMV-P1 affected the systemic infection. Virus localization studies with GFP-tagged CMV clones and in situ localization of virus RNA revealed that these four amino acid residues together form the movement determinant for CMV-P1 movement from the epidermal cell layer to mesophyll cell layers. Quantitative real-time PCR revealed that CMV-P1 and a chimeric virus with four amino acid residues of CMV-P1 accumulated more genomic RNA in inoculated leaves than did CMV-Fny, indicating that those four amino acids are also involved in virus replication. These results demonstrate that the C-terminal region of the helicase domain is responsible for systemic infection by controlling virus replication and cell-to-cell movement. Whereas four amino acids are responsible for acquiring virulence in CMV-Fny, six amino acid (positions at 865, 896, 901, 957, 980 and 993) substitutions in CMV-P1 were required for complete loss of virulence in 'Bukang'.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) is an important virus because of its agricultural impact in the Mediterranean Basin and worldwide, and also as a model for understanding plant-virus interactions. This review focuses on those areas where most progress has been made over the past decade in our understanding of CMV. Clearly, a deep understanding of the role of the recently described CMV 2b gene in suppression of host RNA silencing and viral virulence is the most important discovery. These findings have had an impact well beyond the virus itself, as the 2b gene is an important tool in the studies of eukaryotic gene regulation. Protein 2b was shown to be involved in most of the steps of the virus cycle and to interfere with several basal host defenses. Progress has also been made concerning the mechanisms of virus replication and movement. However, only a few host proteins that interact with viral proteins have been identified, making this an area of research where major efforts are still needed. Another area where major advances have been made is CMV population genetics, where contrasting results were obtained. On the one hand, CMV was shown to be prone to recombination and to show high genetic diversity based on sequence data of different isolates. On the other hand, populations did not exhibit high genetic variability either within plants, or even in a field and the nearby wild plants. The situation was partially clarified with the finding that severe bottlenecks occur during both virus movement within a plant and transmission between plants. Finally, novel studies were undertaken to elucidate mechanisms leading to selection in virus population, according to the host or its environment, opening a new research area in plant-virus coevolution.

A zinc finger protein Tsip1 controls Cucumber mosaic virus infection by interacting with the replication complex on vacuolar membranes of the tobacco plant

• In Cucumber mosaic virus (CMV) RNA replication, replicase-associated protein CMV 1a and RNA-dependent RNA polymerase protein CMV 2a are essential for formation of an active virus replicase complex on vacuolar membranes. • To identify plant host factors involved in CMV replication, a yeast two-hybrid system was used with CMV 1a protein as bait. One of the candidate genes encoded Tsi1-interacting protein 1 (Tsip1), a zinc (Zn) finger protein. Tsip1 strongly interacted with CMV 2a protein, too. • Formation of a Tsip1 complex involving CMV 1a or CMV 2a was confirmed in vitro and in planta. When 35S::Tsip1 tobacco (Nicotiana tabacum) plants were inoculated with CMV-Kor, disease symptom development was delayed and the accumulation of CMV RNAs and coat protein was decreased in both the infected local leaves and the uninfected upper leaves, compared with the wild type, whereas Tsip1-RNAi plants showed modestly but consistently increased CMV susceptibility. In a CMV replication assay, CMV RNA concentrations were reduced in the 35S::Tsip1 transgenic protoplasts compared with wild-type (WT) protoplasts. • These results indicate that Tsip1 might directly control CMV multiplication in tobacco plants by formation of a complex with CMV 1a and CMV 2a.

Detecting protein-protein interactions in vesicular stomatitis virus using a cytoplasmic yeast two hybrid system

Protein-protein interactions play an important role in many virus-encoded functions and in virus-host interactions. While a "classical" yeast two-hybrid system (Y2H) is one of the most common techniques to detect such interactions, it has a number of limitations, including a requirement for the proteins of interest to be relocated to the nucleus. Modified Y2H, such as the Sos recruitment system (SRS), which detect interactions occurring in the cytoplasm rather than the nucleus, allow proteins from viruses replicating in the cytoplasm to be tested in a more natural context. In this study, a SRS was used to detect interactions involving proteins from vesicular stomatitis virus (VSV), a prototypic non-segmented negative strand RNA (NNS) virus. All five full-length VSV proteins, as well as several truncated proteins, were screened against each other. Using the SRS, most interactions demonstrated previously involving VSV phosphoprotein, nucleocapsid (N) and large polymerase proteins were confirmed independently, while difficulties were encountered using the membrane associated matrix and glycoproteins. A human cDNA library was also screened against VSV N protein and one cellular protein, SFRS18, was identified which interacted with N in this context. The system presented can be redesigned easily for studies in other less tractable NNS viruses.

Analysis of potato virus X replicase and TGBp3 subcellular locations

Potato virus X (PVX) infection leads to certain cytopathological modifications of the host endomembrane system. The subcellular location of the PVX replicase was previously unknown while the PVX TGBp3 protein was previously reported to reside in the ER. Using PVX infectious clones expressing the green fluorescent protein reporter, and antisera detecting the PVX replicase and host membrane markers, we examined the subcellular distribution of the PVX replicase in relation to the TGBp3. Confocal and electron microscopic observations revealed that the replicase localizes in membrane bound structures that derive from the ER. A subset of TGBp3 resides in the ER at the same location as the replicase. Sucrose gradient fractionation showed that the PVX replicase and TGBp3 proteins co-fractionate with ER marker proteins. This localization represents a region where both proteins may be synthesized and/or function. There is no evidence to indicate that either PVX protein moves into the Golgi apparatus. Cerulenin, a drug that inhibits de novo membrane synthesis, also inhibited PVX replication. These combined data indicate that PVX replication relies on ER-derived membrane recruitment and membrane proliferation.

A look inside: localization patterns and functions of intracellular plant aquaporins

Aquaporins form a superfamily of intrinsic channel proteins in the plasma and intracellular membranes of plant cells. While a lot of research effort has substantiated the importance of plasma membrane aquaporins for the regulation of plant water homeostasis, comparably little is known about the function of intracellular aquaporins. Yet, various low-molecular-weight compounds, in addition to water, were recently shown to permeate some of these aquaporins. In this review, we examine the diversity of transport properties and localization patterns of intracellular aquaporins. The discussed profiles include, for example, water and ammonia transport across the tonoplast or CO2 transport through the chloroplast envelope. Furthermore, we try to assess to what extent the diverse aquaporin distribution patterns, in relation to the high degree of compartmentation of plant cells, can be linked to a wide range of cellular functions.

The 2b protein and the C-terminus of the 2a protein of cucumber mosaic virus subgroup I strains both play a role in viral RNA accumulation and induction of symptoms

Two chimeras of cucumber mosaic virus (CMV), FCb7(2b)-CMV and FRad35(2b)-CMV, with the 2b genes of strains Cb7-CMV and Rad35-CMV, respectively, in an Fny-CMV background, gave different responses on Nicotiana glutinosa: FCb7(2b)-CMV induced systemic necrosis while FRad35(2b)-CMV caused only mild mosaic. This differential virulence was attributable to the nature of amino acid 55 of their 2b proteins. However, sequence analysis revealed that Leu(55) of the 2b protein was necessary but not sufficient for FCb7(2b)-CMV to induce systemic necrosis. Surprisingly, inhibition of translation of the 2a/2b overlapping region of the 2a protein in FCb7(2b)-CMV led to a loss of systemic necrosis and a reduction in accumulation of viral progeny RNAs. The 2a/2b overlapping region of Fny-CMV had a similar effect on virulence and viral accumulation. Thus, the 2a protein C-terminus of subgroup I strains, as well as the 2b protein, play a role in symptom induction and accumulation of viral RNAs.

Plant aquaporins: membrane channels with multiple integrated functions

Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide). Recent progress was made in understanding the molecular bases of aquaporin transport selectivity and gating. The present review examines how a wide range of selectivity profiles and regulation properties allows aquaporins to be integrated in numerous functions, throughout plant development, and during adaptations to variable living conditions. Although they play a central role in water relations of roots, leaves, seeds, and flowers, aquaporins have also been linked to plant mineral nutrition and carbon and nitrogen fixation.