Production of plum pox virus HC-Pro functionally active for aphid transmission in a transient-expression system

The grapevine ABC transporter B family member 15 is a trans-resveratrol transporter out of grapevine cells

Stilbenes, particularly trans-resveratrol, play a highly relevant defense role in grapevines as phytoalexin is induced in response to stress. Metabolism and transport of stilbenes can be conveniently investigated in grapevine cell culture since large amounts of trans-resveratrol are accumulated in the extracellular medium upon treatment with the elicitor methylated cyclodextrin, either alone or combined with methyl jasmonate. A proteomic approach on grapevine cell membrane fractions was performed to find trans-resveratrol transporter candidates. The candidate VvABCB15 was functionally characterized. Its stable expression in both yeast and Silybum marianum cells' heterologous systems led to increased trans-resveratrol transport in these hosts. Transient expression in Vitis cells showed an enhanced absorbent- or elicitor-assisted accumulation of extracellular trans-resveratrol in VvABCB15-expressing or VvGSTU10/VvABCB15-co-expressing cell suspension cultures. Experiments of transient expression in Vitis cell suspensions using light-switchable stilbene synthase (pHYH::VvSTS3) and VvABCB15 further confirmed the candidate's role as a trans-resveratrol transporter. VvABCB15-YFP fusion proteins in Nicotiana leaf showed localization in the plasma membrane, consistent with a functional role in trans-resveratrol transport. This is the first report to provide evidence for the involvement of an ABC transporter B type, VvABCB15, in trans-resveratrol transport to the extracellular medium of grapevine cells.

Development of an attenuated potato virus Y mutant carrying multiple mutations in helper-component protease for cross-protection

Tobacco (Nicotiana tabacum) is one of the major cash crops in China. Potato virus Y (PVY), a representative member of the genus Potyvirus, greatly reduces the quality and yield of tobacco leaves by inducing veinal necrosis. Mild strain-mediated cross-protection is an attractive method of controlling diseases caused by PVY. Currently, there is a lack of effective and stable attenuated PVY mutants. Potyviral helper component-protease (HC-Pro) is a likely target for the development of mild strains. Our previous studies showed that the residues lysine at positions 124 and 182 (K124 and K182) in HC-Pro were involved in PVY virulence, and the conserved KITC motif in HC-Pro was involved in aphid transmission. In this study, to improve the stability of PVY mild strains, K at position 50 (K50) in KITC motif, K124, and K182 were separately substituted with glutamic acid (E), leucine (L), and arginine (R), resulting in a triple-mutant PVY-HCELR. The mutant PVY-HCELR had attenuated virulence and did not induce leaf veinal necrosis symptoms in tobacco plants and could not be transmitted by Myzus persicae. Furthermore, PVY-HCELR mutant was genetically stable after six serial passages, and only caused mild mosaic symptoms in tobacco plants even at 90 days post inoculation. The tobacco plants cross-protected by PVY-HCELR mutant showed high resistance to the wild-type PVY. This study showed that PVY-HCELR mutant was a promising mild mutant for cross-protection to control PVY.

Experimental Transmission of Plant Viruses by Aphids or Whiteflies

Numerous species of plant viruses are naturally transmitted by insect vectors, mainly homopterans like aphids and whiteflies. Depending on the vector specificity and the mode of transmission, different durations of the periods for acquisition, retention, and inoculation are required for a successful transmission. Therefore, the experimental setup to perform controlled transmission experiments under laboratory conditions involves handling the vector organisms and managing the times for the different steps of the process to optimize and standardize the results. This chapter describes some basic procedures that can be applied to vector-mediated transmission experiments with selected viruses using aphids or whiteflies and different host plants.

Several geranylgeranyl diphosphate synthase isoforms supply metabolic substrates for carotenoid biosynthesis in tomato

Geranylgeranyl diphosphate (GGPP) produced by GGPP synthase (GGPPS) serves as a precursor for many plastidial isoprenoids, including carotenoids. Phytoene synthase (PSY) converts GGPP into phytoene, the first committed intermediate of the carotenoid pathway. Here we used biochemical, molecular, and genetic tools to characterise the plastidial members of the GGPPS family in tomato (Solanum lycopersicum) and their interaction with PSY isoforms. The three tomato GGPPS isoforms found to localise in plastids (SlG1, 2 and 3) exhibit similar kinetic parameters. Gene expression analyses showed a preferential association of individual GGPPS and PSY isoforms when carotenoid biosynthesis was induced during root mycorrhization, seedling de-etiolation and fruit ripening. SlG2, but not SlG3, physically interacts with PSY proteins. By contrast, CRISPR-Cas9 mutants defective in SlG3 showed a stronger impact on carotenoid levels and derived metabolic, physiological and developmental phenotypes compared with those impaired in SlG2. Double mutants defective in both genes could not be rescued. Our work demonstrates that the bulk of GGPP production in tomato chloroplasts and chromoplasts relies on two cooperating GGPPS paralogues, unlike other plant species such as Arabidopsis thaliana, rice or pepper, which produce their essential plastidial isoprenoids using a single GGPPS isoform.

Virus and helper component interactions favour the transmission of recombinant potato virus Y strains

In recent years, several recombinant strains of potato virus Y, notably PVYNTN and PVYN:O have displaced the ordinary strain, PVYO, and emerged as the predominant strains affecting the USA potato crop. Previously we reported that recombinant strains were transmitted more efficiently than PVYO when they were acquired sequentially, regardless of acquisition order. In another recent study, we showed that PVYNTN binds preferentially to the aphid stylet over PVYO when aphids feed on a mixture of PVYO and PVYNTN. To understand the mechanism of this transmission bias as well as preferential virus binding, we separated virus and active helper component proteins (HC), mixed them in homologous and heterologous combinations, and then fed them to aphids using Parafilm sachets. Mixtures of PVYO HC with either PVYN:O or PVYNTN resulted in efficient transmission. PVYN:O HC also facilitated the transmission of PVYO and PVYNTN, albeit with reduced efficiency. PVYNTN HC failed to facilitate transmission of either PVYO or PVYN:O. When PVYO HC or PVYN:O HC was mixed with equal amounts of the two viruses, both viruses in all combinations were transmitted at high efficiencies. In contrast, no transmission occurred when combinations of viruses were mixed with PVYNTN HC. Further study evaluated transmission using serial dilutions of purified virus mixed with HCs. While PVYNTN HC only facilitated the transmission of the homologous virus, the HCs of PVYO and PVYN:O facilitated the transmission of all strains tested. This phenomenon has likely contributed to the increase in the recombinant strains affecting the USA potato crop.

Structural and functional analysis of tomato sterol C22 desaturase

BACKGROUND

Sterols are structural and functional components of eukaryotic cell membranes. Plants produce a complex mixture of sterols, among which β-sitosterol, stigmasterol, campesterol, and cholesterol in some Solanaceae, are the most abundant species. Many reports have shown that the stigmasterol to β-sitosterol ratio changes during plant development and in response to stresses, suggesting that it may play a role in the regulation of these processes. In tomato (Solanum lycopersicum), changes in the stigmasterol to β-sitosterol ratio correlate with the induction of the only gene encoding sterol C22-desaturase (C22DES), the enzyme specifically involved in the conversion of β-sitosterol to stigmasterol. However, despite the biological interest of this enzyme, there is still a lack of knowledge about several relevant aspects related to its structure and function.

RESULTS

In this study we report the subcellular localization of tomato C22DES in the endoplasmic reticulum (ER) based on confocal fluorescence microscopy and cell fractionation analyses. Modeling studies have also revealed that C22DES consists of two well-differentiated domains: a single N-terminal transmembrane-helix domain (TMH) anchored in the ER-membrane and a globular (or catalytic) domain that is oriented towards the cytosol. Although TMH is sufficient for the targeting and retention of the enzyme in the ER, the globular domain may also interact and be retained in the ER in the absence of the N-terminal transmembrane domain. The observation that a truncated version of C22DES lacking the TMH is enzymatically inactive revealed that the N-terminal membrane domain is essential for enzyme activity. The in silico analysis of the TMH region of plant C22DES revealed several structural features that could be involved in substrate recognition and binding.

CONCLUSIONS

Overall, this study contributes to expand the current knowledge on the structure and function of plant C22DES and to unveil novel aspects related to plant sterol metabolism.

Characterization of Soybean yellow shoot virus, a New Member of the Family Potyviridae Infecting Soybean Plants in Brazil

A new virus species, belonging to the family Potyviridae and capable of infecting most of the soybean cultivars grown in Brazil, was collected in Lavras, Minas Gerais, Brazil, and named Soybean yellow shoot virus (SoyYSV). In this study, the complete 9,052-nucleotide genome of SoyYSV was determined and the structural, biological, and molecular properties of the virus were investigated. The SoyYSV genome encoded a single polyprotein that could be subsequently cleaved, generating 11 proteins. The SoyYSV genome shared 49% nucleotide and 36% amino acid sequence identity with Blackberry virus Y. However, the P1 protein of SoyYSV was much smaller and lacked the ALK1 domain characteristic of the genus Brambyvirus. Electron microscopy revealed flexuous filamentous virus particles, 760 to 780 nm in length, and cytoplasmic inclusions typical of those found in plant cells infected with Potyviridae species. In addition to soybean, SoyYSV infected species in the Amaranthaceae, Caricaceae, Fabaceae, and Solanaceae families. Among the most common potyviruses present in Brazil, only SoyYSV induced local necrotic lesions in Carica papaya L. SoyYSV was transmissible by Myzus persicae and Aphis gossypii but lacked the HC-Pro domain required for aphid transmission in other potyviruses. No seed transmission in soybean was observed.

HCPro-mediated transmission by aphids of purified virions does not require its silencing suppression function and correlates with its ability to coat cell microtubules in loss-of-function mutant studies

Native and amino acid (aa) substitution mutants of HCPro from potato virus Y (PVY) were transiently expressed in Nicotiana benthamiana leaves. Properties of those HCPro variants with regard to silencing suppression activities, mediation of viral transmission by aphids, and subcellular localization dynamics, were determined. One mutant failed to suppress silencing in agropatch assays, but could efficiently mediate the transmission by aphids of purified virions. This mutant also retained the ability to translocate to microtubules (MTs) in stressed cells. By contrast, another single aa substitution mutant displayed native-like silencing suppression activity in agropatch assays, but could not mediate transmission of PVY virions by aphids, and could not relocate to MTs. Our data show that silencing suppression by HCPro is not required in the aphid-mediated transmission of purified virions. In addition, since the same single aa alteration compromised both, viral transmission and coating of MTs, those two properties could be functionally related.

Rosa hybrida orcinol O-methyl transferase-mediated production of pterostilbene in metabolically engineered grapevine cell cultures

Stilbenes are naturally scarce high-added-value plant compounds with chemopreventive, pharmacological and cosmetic properties. Bioproduction strategies include engineering the metabolisms of bacterial, fungal and plant cell systems. Strikingly, one of the most effective strategies consists in the elicitation of wild grapevine cell cultures, which leads to vast stilbene resveratrol accumulation in the extracellular medium. The combination of both cell culture elicitation and metabolic engineering strategies to produce resveratrol analogs proved more efficient for the hydroxylated derivative piceatannol than for the dimethylated derivative pterostilbene, for which human hydroxylase HsCYP1B1- and grapevine O-methyltransferase VvROMT-transformed cell cultures were respectively used. Rose orcinol O-methyltransferase (OOMT) displays enzymatic properties, which makes it an appealing candidate to substitute VvROMT in the combined strategy to enhance the pterostilbene production level by engineered grapevine cells upon elicitation. Here we cloned a Rosa hybrida OOMT gene, and created a genetic construction suitable for Agrobacterium-mediated plant transformation. OOMT's ability to catalyze the conversion of resveratrol into pterostilbene was first assessed in vitro using protein extracts of agroinfiltrated N. benthamiana leaves and transformed grapevine callus. The grapevine cell cultures transformed with RhOOMT produced about 16 mg/L culture of pterostilbene and reached an extracellular distribution of up to 34% of total production at the best, which is by far the highest production reported to date in a plant system. A bonus large resveratrol production of ca. 1500-3000 mg/L was simultaneously obtained. Our results demonstrate a viable successful metabolic engineering strategy to produce pterostilbene, a resveratrol analog with enhanced pharmacological properties.

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

RNA viruses have very compact genomes and so provide a unique opportunity to study how evolution works to optimize the use of very limited genomic information. A widespread viral strategy to solve this issue concerning the coding space relies on the expression of proteins with multiple functions. Members of the family Potyviridae, the most abundant group of RNA viruses in plants, offer several attractive examples of viral factors which play roles in diverse infection-related pathways. The Helper Component Proteinase (HCPro) is an essential and well-characterized multitasking protein for which at least three independent functions have been described: (i) viral plant-to-plant transmission; (ii) polyprotein maturation; and (iii) RNA silencing suppression. Moreover, multitudes of host factors have been found to interact with HCPro. Intriguingly, most of these partners have not been ascribed to any of the HCPro roles during the infectious cycle, supporting the idea that this protein might play even more roles than those already established. In this comprehensive review, we attempt to summarize our current knowledge about HCPro and its already attributed and putative novel roles, and to discuss the similarities and differences regarding this factor in members of this important viral family.

Identification and Characterization of Sterol Acyltransferases Responsible for Steryl Ester Biosynthesis in Tomato

Steryl esters (SEs) serve as a storage pool of sterols that helps to maintain proper levels of free sterols (FSs) in cell membranes throughout plant growth and development, and participates in the recycling of FSs and fatty acids released from cell membranes in aging tissues. SEs are synthesized by sterol acyltransferases, a family of enzymes that catalyze the transfer of fatty acil groups to the hydroxyl group at C-3 position of the sterol backbone. Sterol acyltransferases are categorized into acyl-CoA:sterol acyltransferases (ASAT) and phospholipid:sterol acyltransferases (PSAT) depending on whether the fatty acyl donor substrate is a long-chain acyl-CoA or a phospolipid. Until now, only Arabidopsis ASAT and PSAT enzymes (AtASAT1 and AtPSAT1) have been cloned and characterized in plants. Here we report the identification, cloning, and functional characterization of the tomato (Solanum lycopersicum cv. Micro-Tom) orthologs. SlPSAT1 and SlASAT1 were able to restore SE to wild type levels in the Arabidopsis psat1-2 and asat1-1 knock-out mutants, respectively. Expression of SlPSAT1 in the psat1-2 background also prevented the toxicity caused by an external supply of mevalonate and the early senescence phenotype observed in detached leaves of this mutant, whereas expression of SlASAT1 in the asat1-1 mutant revealed a clear substrate preference of the tomato enzyme for the sterol precursors cycloartenol and 24-methylene cycloartanol. Subcellular localization studies using fluorescently tagged SlPSAT1 and SlASAT1 proteins revealed that SlPSAT1 localize in cytoplasmic lipid droplets (LDs) while, in contrast to the endoplasmic reticulum (ER) localization of AtASAT1, SlASAT1 resides in the plasma membrane (PM). The possibility that PM-localized SlASAT1 may act catalytically in trans on their sterol substrates, which are presumably embedded in the ER membrane, is discussed. The widespread expression of SlPSAT1 and SlASAT1 genes in different tomato organs together with their moderate transcriptional response to several stresses suggests a dual role of SlPSAT1 and SlASAT1 in tomato plant and fruit development and the adaptive responses to stress. Overall, this study contributes to enlarge the current knowledge on plant sterol acyltransferases and set the basis for further studies aimed at understanding the role of SE metabolism in tomato plant growth and development.

Unusual occurrence of a DAG motif in the Ipomovirus Cassava brown streak virus and implications for its vector transmission

Cassava is the main staple food for over 800 million people globally. Its production in eastern Africa is being constrained by two devastating Ipomoviruses that cause cassava brown streak disease (CBSD); Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), with up to 100% yield loss for smallholder farmers in the region. To date, vector studies have not resulted in reproducible and highly efficient transmission of CBSV and UCBSV. Most virus transmission studies have used Bemisia tabaci (whitefly), but a maximum of 41% U/CBSV transmission efficiency has been documented for this vector. With the advent of next generation sequencing, researchers are generating whole genome sequences for both CBSV and UCBSV from throughout eastern Africa. Our initial goal for this study was to characterize U/CBSV whole genomes from CBSD symptomatic cassava plants sampled in Kenya. We have generated 8 new whole genomes (3 CBSV and 5 UCBSV) from Kenya, and in the process of analyzing these genomes together with 26 previously published sequences, we uncovered the aphid transmission associated DAG motif within coat protein genes of all CBSV whole genomes at amino acid positions 52-54, but not in UCBSV. Upon further investigation, the DAG motif was also found at the same positions in two other Ipomoviruses: Squash vein yellowing virus (SqVYV), Coccinia mottle virus (CocMoV). Until this study, the highly-conserved DAG motif, which is associated with aphid transmission was only noticed once, in SqVYV but discounted as being of minimal importance. This study represents the first comprehensive look at Ipomovirus genomes to determine the extent of DAG motif presence and significance for vector relations. The presence of this motif suggests that aphids could potentially be a vector of CBSV, SqVYV and CocMov. Further transmission and ipomoviral protein evolutionary studies are needed to confirm this hypothesis.

Tomato UDP-Glucose Sterol Glycosyltransferases: A Family of Developmental and Stress Regulated Genes that Encode Cytosolic and Membrane-Associated Forms of the Enzyme

Sterol glycosyltransferases (SGTs) catalyze the glycosylation of the free hydroxyl group at C-3 position of sterols to produce sterol glycosides. Glycosylated sterols and free sterols are primarily located in cell membranes where in combination with other membrane-bound lipids play a key role in modulating their properties and functioning. In contrast to most plant species, those of the genus Solanum contain very high levels of glycosylated sterols, which in the case of tomato may account for more than 85% of the total sterol content. In this study, we report the identification and functional characterization of the four members of the tomato (Solanum lycopersicum cv. Micro-Tom) SGT gene family. Expression of recombinant SlSGT proteins in E. coli cells and N. benthamiana leaves demonstrated the ability of the four enzymes to glycosylate different sterol species including cholesterol, brassicasterol, campesterol, stigmasterol, and β-sitosterol, which is consistent with the occurrence in their primary structure of the putative steroid-binding domain found in steroid UDP-glucuronosyltransferases and the UDP-sugar binding domain characteristic for a superfamily of nucleoside diphosphosugar glycosyltransferases. Subcellular localization studies based on fluorescence recovery after photobleaching and cell fractionation analyses revealed that the four tomato SGTs, like the Arabidopsis SGTs UGT80A2 and UGT80B1, localize into the cytosol and the PM, although there are clear differences in their relative distribution between these two cell fractions. The SlSGT genes have specialized but still largely overlapping expression patterns in different organs of tomato plants and throughout the different stages of fruit development and ripening. Moreover, they are differentially regulated in response to biotic and abiotic stress conditions. SlSGT4 expression increases markedly in response to osmotic, salt, and cold stress, as well as upon treatment with abscisic acid and methyl jasmonate. Stress-induced SlSGT2 expression largely parallels that of SlSGT4. On the contrary, SlSGT1 and SlSGT3 expression remains almost unaltered under the tested stress conditions. Overall, this study contributes to broaden the current knowledge on plant SGTs and provides support to the view that tomato SGTs play overlapping but not completely redundant biological functions involved in mediating developmental and stress responses.

Production of highly bioactive resveratrol analogues pterostilbene and piceatannol in metabolically engineered grapevine cell cultures

Grapevine stilbenes, particularly trans-resveratrol, have a demonstrated pharmacological activity. Other natural stilbenes derived from resveratrol such as pterostilbene or piceatannol, display higher oral bioavailability and bioactivity than the parent compound, but are far less abundant in natural sources. Thus, to efficiently obtain these bioactive resveratrol derivatives, there is a need to develop new bioproduction systems. Grapevine cell cultures are able to produce large amounts of easily recoverable extracellular resveratrol when elicited with methylated cyclodextrins and methyl jasmonate. We devised this system as an interesting starting point of a metabolic engineering-based strategy to produce resveratrol derivatives using resveratrol-converting enzymes. Constitutive expression of either Vitis vinifera resveratrol O-methyltransferase (VvROMT) or human cytochrome P450 hydroxylase 1B1 (HsCYP1B1) led to pterostilbene or piceatannol, respectively, after the engineered cell cultures were treated with the aforementioned elicitors. Functionality of both gene products was first assessed in planta by Nicotiana benthamiana agroinfiltration assays, in which tobacco cells transiently expressed stilbene synthase and VvROMT or HsCYP1B1. Grapevine cell cultures transformed with VvROMT produced pterostilbene, which was detected in both intra- and extracellular compartments, at a level of micrograms per litre. Grapevine cell cultures transformed with HsCYP1B1 produced about 20 mg/L culture of piceatannol, displaying a sevenfold increase in relation to wild-type cultures, and reaching an extracellular distribution of up to 45% of total production. The results obtained demonstrate the feasibility of this novel system for the bioproduction of natural and more bioactive resveratrol derivatives and suggest new ways for the improvement of production yields.

Surveys of Viruliferous Alate Aphid of Plum pox virus in Prunus mume Orchards in Japan

Plum pox virus (PPV) is transmitted by infected buds and aphids. It is important to analyze the outbreak trends and viruliferous rate of aphids in areas where the occurrence of PPV is reported, so as to develop strategies for disease control. Between April 2011 and December 2012, yellow insect-trapping adhesive plates were placed for 2 days at a time each week in an area where PPV is occurring in Japan. Outbreak trends were analyzed based on the trapped alate aphid samples, and up to 50 of them were tested per week to identify species and determine the rate of viruliferous specimens. Although the number of aphids varied according to survey year, three peaks were noticeable in each year. Based on the sequence data for the mitochondrial cytochrome c oxidase I region, approximately 40 different species of aphid were trapped in both years. Of the five dominant species of aphids identified during the 2 years, Aphis spiraecola was trapped in large numbers. PPV-positive aphids were higher in fall onward, when the total number of trapped aphids decreased, than in spring and summer, when a larger number of aphids was caught. PPV transmission tests using the most abundant species revealed that A. spiraecola, A. craccivora, A. gossypii, and Rhopalosiphum maidis were transmitters, although A. spiraecola is likely of epidemiological significance.

Transcript profiling of jasmonate-elicited Taxus cells reveals a β-phenylalanine-CoA ligase

Plant cell cultures constitute eco-friendly biotechnological platforms for the production of plant secondary metabolites with pharmacological activities, as well as a suitable system for extending our knowledge of secondary metabolism. Despite the high added value of taxol and the importance of taxanes as anticancer compounds, several aspects of their biosynthesis remain unknown. In this work, a genomewide expression analysis of jasmonate-elicited Taxus baccata cell cultures by complementary DNA-amplified fragment length polymorphism (cDNA-AFLP) indicated a correlation between an extensive elicitor-induced genetic reprogramming and increased taxane production in the targeted cultures. Subsequent in silico analysis allowed us to identify 15 genes with a jasmonate-induced differential expression as putative candidates for genes encoding enzymes involved in five unknown steps of taxane biosynthesis. Among them, the TB768 gene showed a strong homology, including a very similar predicted 3D structure, with other genes previously reported to encode acyl-CoA ligases, thus suggesting a role in the formation of the taxol lateral chain. Functional analysis confirmed that the TB768 gene encodes an acyl-CoA ligase that localizes to the cytoplasm and is able to convert β-phenylalanine, as well as coumaric acid, into their respective derivative CoA esters. β-phenylalanyl-CoA is attached to baccatin III in one of the last steps of the taxol biosynthetic pathway. The identification of this gene will contribute to the establishment of sustainable taxol production systems through metabolic engineering or synthetic biology approaches.

A hydrophobic proline-rich motif is involved in the intracellular targeting of temperature-induced lipocalin

Temperature-induced lipocalins (TILs) play an essential role in the response of plants to different abiotic stresses. In agreement with their proposed role in protecting membrane lipids, TILs have been reported to be associated to cell membranes. However, TILs show an overall hydrophilic character and do not contain any signal for membrane targeting nor hydrophobic sequences that could represent transmembrane domains. Arabidopsis TIL (AtTIL) is considered the ortholog of human ApoD, a protein known to associate to membranes through a short hydrophobic loop protruding from strands 5 and 6 of the lipocalin β-barrel. An equivalent loop (referred to as HPR motif) is also present between β-strands 5 and 6 of TILs. The HPR motif, which is highly conserved among TIL proteins, extends over as short stretch of eight amino acids and contains four invariant proline residues. Subcellular localization studies have shown that TILs are targeted to a variety of cell membranes and organelles. We have also found that the HPR motif is necessary and sufficient for the intracellular targeting of TILs. Modeling studies suggest that the HPR motif may directly anchor TILs to cell membranes, favoring in this way further contact with the polar group of membrane lipids. However, some particular features of the HPR motif open the possibility that targeting of TILs to cell membranes could be mediated by interaction with other proteins. The functional analysis of the HPR motif unveils the existence of novel mechanisms involved in the intracellular targeting of proteins in plants.

The P25 protein of potato virus X (PVX) is the main pathogenicity determinant responsible for systemic necrosis in PVX-associated synergisms

Most plant viruses counter the RNA silencing-based antiviral defense by expressing viral suppressors of RNA silencing (VSRs). In this sense, VSRs may be regarded as virulence effectors that can be recognized by the host as avirulence (avr) factors to induce R-mediated resistance. We made use of Agrobacterium-mediated transient coexpression of VSRs in combination with Potato virus X (PVX) to recapitulate in local tissues the systemic necrosis (SN) caused by PVX-potyvirus synergistic infections in Nicotiana benthamiana. The hypersensitive response (HR)-like response was associated with an enhanced accumulation of PVX subgenomic RNAs. We further show that expression of P25, the VSR of PVX, in the presence of VSR from different viruses elicited an HR-like response in Nicotiana spp. Furthermore, the expression of P25 by a Plum pox virus (PPV) vector was sufficient to induce an increase of PPV pathogenicity that led to necrotic mottling. A frameshift mutation in the P25 open reading frame (ORF) of PVX did not lead to necrosis when coexpressed with VSRs. These findings indicate that P25 is the main PVX determinant involved in eliciting a systemic HR-like response in PVX-associated synergisms. Moreover, we show that silencing of SGT1 and RAR1 attenuated cell death in both PVX-potyvirus synergistic infection and the HR-like response elicited by P25. Our study underscores that P25 variants that have impaired ability to suppress RNA silencing cannot act as elicitors when synergized by the presence of other VSRs. These findings highlight the importance of RNA silencing suppression activity in the HR-like response elicited by VSRs in certain hosts.

IMPORTANCE

The work presented here describes how the activity of the PVX suppressor P25 elicits an HR-like response in Nicotiana spp. when overexpressed with other VSR proteins. This finding suggests that the SN response caused by PVX-associated synergisms is a delayed immune response triggered by P25, once it reaches a threshold level by the action of other VSRs. Moreover, this work supports the contention that the silencing suppressor activity of PVX P25 protein is a prerequisite for HR elicitation. We propose that unidentified avr determinants could be involved in other cases of viral synergisms in which heterologous "helper" viruses encoding strong VSRs exacerbate the accumulation of the avr-encoding virus.

Artificially-induced organelles are optimal targets for optical trapping experiments in living cells

Optical trapping supplies information on the structural, kinetic or rheological properties of inner constituents of the cell. However, the application of significant forces to intracellular objects is notoriously difficult due to a combination of factors, such as the small difference between the refractive indices of the target structures and the cytoplasm. Here we discuss the possibility of artificially inducing the formation of spherical organelles in the endoplasmic reticulum, which would contain densely packed engineered proteins, to be used as optimized targets for optical trapping experiments. The high index of refraction and large size of our organelles provide a firm grip for optical trapping and thereby allow us to exert large forces easily within safe irradiation limits. This has clear advantages over alternative probes, such as subcellular organelles or internalized synthetic beads.

The evolutionary conserved oil body associated protein OBAP1 participates in the regulation of oil body size

A transcriptomic approach has been used to identify genes predominantly expressed in maize (Zea mays) scutellum during maturation. One of the identified genes is oil body associated protein1 (obap1), which is transcribed during seed maturation predominantly in the scutellum, and its expression decreases rapidly after germination. Proteins similar to OBAP1 are present in all plants, including primitive plants and mosses, and in some fungi and bacteria. In plants, obap genes are divided in two subfamilies. Arabidopsis (Arabidopsis thaliana) genome contains five genes coding for OBAP proteins. Arabidopsis OBAP1a protein is accumulated during seed maturation and disappears after germination. Agroinfiltration of tobacco (Nicotiana benthamiana) epidermal leaf cells with fusions of OBAP1 to yellow fluorescent protein and immunogold labeling of embryo transmission electron microscopy sections showed that OBAP1 protein is mainly localized in the surface of the oil bodies. OBAP1 protein was detected in the oil body cellular fraction of Arabidopsis embryos. Deletion analyses demonstrate that the most hydrophilic part of the protein is responsible for the oil body localization, which suggests an indirect interaction of OBAP1 with other proteins in the oil body surface. An Arabidopsis mutant with a transfer DNA inserted in the second exon of the obap1a gene and an RNA interference line against the same gene showed a decrease in the germination rate, a decrease in seed oil content, and changes in fatty acid composition, and their embryos have few, big, and irregular oil bodies compared with the wild type. Taken together, our findings suggest that OBAP1 protein is involved in the stability of oil bodies.

The production of recombinant cationic α-helical antimicrobial peptides in plant cells induces the formation of protein bodies derived from the endoplasmic reticulum

Synthetic linear antimicrobial peptides with cationic α-helical structures, such as BP100, are valuable as novel therapeutics and preservatives. However, they tend to be toxic when expressed at high levels as recombinant peptides in plants, and they can be difficult to detect and isolate from complex plant tissues because they are strongly cationic and display low extinction coefficient and extremely limited immunogenicity. We therefore expressed BP100 with a C-terminal tag which preserved its antimicrobial activity and demonstrated significant accumulation in plant cells. We used a fluorescent tag to trace BP100 following transiently expression in Nicotiana benthamiana leaves and showed that it accumulated in large vesicles derived from the endoplasmic reticulum (ER) along with typical ER luminal proteins. Interestingly, the formation of these vesicles was induced by BP100. Similar vesicles formed in stably transformed Arabidopsis thaliana seedlings, but the recombinant peptide was toxic to the host during latter developmental stages. This was avoided by selecting active BP100 derivatives based on their low haemolytic activity even though the selected peptides remained toxic to plant cells when applied exogenously at high doses. Using this strategy, we generated transgenic rice lines producing active BP100 derivatives with a yield of up to 0.5% total soluble protein.

Copper-containing amine oxidases contribute to terminal polyamine oxidation in peroxisomes and apoplast of Arabidopsis thaliana

BACKGROUND

Polyamines (PAs) are oxidatively deaminated at their primary or secondary amino-groups by copper-containing amine oxidases (CuAOs) or FAD-dependent amine oxidases (PAOs), respectively. Both enzymes have long been considered to be apoplastic proteins. However, three out of five PAO isoforms in Arabidopsis thaliana are localized in peroxisomes, while the other two PAOs are predicted to be cytosolic. Interestingly, most of these PAOs do not contribute to terminal PA oxidation, but instead are involved in the back-conversion pathway, producing spermidine from spermine and putrescine from spermidine, which in turn is inhibited by putrescine. This opens the question as to whether PAs are catabolized in the apoplast of Arabidopsis and if the terminal oxidation occurs in the peroxisomes. The main objective of this study was to know if these catabolic processes are mediated by CuAOs.

RESULTS

A. thaliana contains ten genes annotated as CuAOs, but only one (ATAO1) has been characterized at the protein level. Reported herein is the characterization of three genes encoding putative Arabidopsis CuAOs (AtCuAO1, AtCuAO2 and AtCuAO3). These genes encode functional CuAOs that use putrescine and spermidine as substrates. AtCuAO1, like ATAO1, is an extracellular protein, while AtCuAO2 and AtCuAO3 are localized in peroxisomes. The three genes present a different expression profile in response to exogenous treatments, such as application of abcisic acid, methyl jasmonate, salycilic acid, flagellin 22 and wounding.

CONCLUSIONS

PA catabolism in the Arabidopsis apoplast is mediated predominantly by CuAOs, while in peroxisomes the co-localization of CuAO-dependent terminal catabolism with PAO-back-conversion machineries might contribute to modulating putrescine-mediated inhibition of the back-conversion, suggesting the occurrence of a tight coordination between both catabolic pathways. The expression profile of AtCuAO1-3 in response to different exogenous treatments, together with the different localization of the corresponding proteins, provides evidence for the functional diversification of Arabidopsis CuAO proteins.

Proteomic characterisation of endoplasmic reticulum-derived protein bodies in tobacco leaves

BACKGROUND

The N-terminal proline-rich domain (Zera) of the maize storage protein γ-zein, is able to induce the formation of endoplasmic reticulum (ER)-derived protein bodies (PBs) when fused to proteins of interest. This encapsulation enables a recombinant fused protein to escape from degradation and facilitates its recovery from plant biomass by gradient purification. The aim of the present work was to evaluate if induced PBs encapsulate additional proteins jointly with the recombinant protein. The exhaustive analysis of protein composition of PBs is expected to facilitate a better understanding of PB formation and the optimization of recombinant protein purification approaches from these organelles.

RESULTS

We analysed the proteome of PBs induced in Nicotiana benthamiana leaves by transient transformation with Zera fused to a fluorescent marker protein (DsRed). Intact PBs with their surrounding ER-membrane were isolated on iodixanol based density gradients and their integrity verified by confocal and electron microscopy. SDS-PAGE analysis of isolated PBs showed that Zera-DsRed accounted for around 85% of PB proteins in term of abundance. Differential extraction of PBs was performed for in-depth analysis of their proteome and structure. Besides Zera-DsRed, 195 additional proteins were identified including a broad range of proteins resident or trafficking through the ER and recruited within the Zera-DsRed polymer.

CONCLUSIONS

This study indicates that Zera-protein fusion is still the major protein component of the new formed organelle in tobacco leaves. The analysis also reveals the presence of an unexpected diversity of proteins in PBs derived from both the insoluble Zera-DsRed polymer formation, including ER-resident and secretory proteins, and a secretory stress response induced most likely by the recombinant protein overloading. Knowledge of PBs protein composition is likely to be useful to optimize downstream purification of recombinant proteins in molecular farming applications.

Helper component proteinase of the genus Potyvirus is an interaction partner of translation initiation factors eIF(iso)4E and eIF4E and contains a 4E binding motif

The multifunctional helper component proteinase (HCpro) of potyviruses (genus Potyvirus; Potyviridae) shows self-interaction and interacts with other potyviral and host plant proteins. Host proteins that are pivotal to potyvirus infection include the eukaryotic translation initiation factor eIF4E and the isoform eIF(iso)4E, which interact with viral genome-linked protein (VPg). Here we show that HCpro of Potato virus A (PVA) interacts with both eIF4E and eIF(iso)4E, with interactions with eIF(iso)4E being stronger, as judged by the data of a yeast two-hybrid system assay. A bimolecular fluorescence complementation assay on leaves of Nicotiana benthamiana showed that HCpro from three potyviruses (PVA, Potato virus Y, and Tobacco etch virus) interacted with the eIF(iso)4E and eIF4E of tobacco (Nicotiana tabacum); interactions with eIF(iso)4E and eIF4E of potato (Solanum tuberosum) were weaker. In PVA-infected cells, interactions between HCpro and tobacco eIF(iso)4E were confined to round structures that colocalized with 6K2-induced vesicles. Point mutations introduced to a 4E binding motif identified in the C-terminal region of HCpro debilitated interactions of HCpro with translation initiation factors and were detrimental to the virulence of PVA in plants. The 4E binding motif conserved in HCpro of potyviruses and HCpro-initiation factor interactions suggest new roles for HCpro and/or translation factors in the potyvirus infection cycle.

The expression of a xylanase targeted to ER-protein bodies provides a simple strategy to produce active insoluble enzyme polymers in tobacco plants

BACKGROUND

Xylanases deserve particular attention due to their potential application in the feed, pulp bleaching and paper industries. We have developed here an efficient system for the production of an active xylanase in tobacco plants fused to a proline-rich domain (Zera) of the maize storage protein γ-zein. Zera is a self-assembling domain able to form protein aggregates in vivo packed in newly formed endoplasmic reticulum-derived organelles known as protein bodies (PBs).

METHODOLOGY/PRINCIPAL FINDINGS

Tobacco leaves were transiently transformed with a binary vector containing the Zera-xylanase coding region, which was optimized for plant expression, under the control of the 35S CaMV promoter. The fusion protein was efficiently expressed and stored in dense PBs, resulting in yields of up to 9% of total protein. Zera-xylanase was post-translationally modified with high-mannose-type glycans. Xylanase fused to Zera was biologically active not only when solubilized from PBs but also in its insoluble form. The resistance of insoluble Zera-xylanase to trypsin digestion demonstrated that the correct folding of xylanase in PBs was not impaired by Zera oligomerization. The activity of insoluble Zera-xylanase was enhanced when substrate accessibility was facilitated by physical treatments such as ultrasound. Moreover, we found that the thermostability of the enzyme was improved when Zera was fused to the C-terminus of xylanase.

CONCLUSION/SIGNIFICANCE

In the present work we have successfully produced an active insoluble aggregate of xylanase fused to Zera in plants. Zera-xylanase chimeric protein accumulates within ER-derived protein bodies as active aggregates that can easily be recovered by a simple density-based downstream process. The production of insoluble active Zera-xylanase protein in tobacco outlines the potential of Zera as a fusion partner for producing enzymes of biotechnological relevance. Zera-PBs could thus become efficient and low-cost bioreactors for industrial purposes.

Relevant elements of a maize gamma-zein domain involved in protein body biogenesis

The N-terminal proline-rich domain of γ-zein (Zera) plays an important role in protein body (PB) formation not only in the original host (maize seeds) but in a broad spectrum of eukaryotic cells. However, the elements within the Zera sequence that are involved in the biogenesis of PBs have not been clearly identified. Here, we focused on amino acid sequence motifs that could be involved in Zera oligomerization, leading to PB-like structures in Nicotiana benthamiana leaves. By using fusions of Zera with fluorescent proteins, we found that the lack of the repeat region (PPPVHL)(8) of Zera resulted in the secretion of the fusion protein but that this repeat by itself did not form PBs. Although the repeat region containing eight units was the most efficient for Zera self-assembly, shorter repeats of 4-6 units still formed small multimers. Based on site-directed mutagenesis of Zera cysteine residues and analysis of multimer formation, we conclude that the two N-terminal Cys residues of Zera (Cys(7) and Cys(9)) are critical for oligomerization. Immunoelectron microscopy and confocal studies on PB development over time revealed that early, small, Zera-derived oligomers were sequestered in buds along the rough ER and that the mature size of the PBs could be attained by both cross-linking of preformed multimers and the incorporation of new chains of Zera fusions synthesized by active membrane-bound ribosomes. Based on these results and on the behavior of the Zera structure determined by molecular dynamics simulation studies, we propose a model of Zera-induced PB biogenesis.

The helper-component protease transmission factor of tobacco etch potyvirus binds specifically to an aphid ribosomal protein homologous to the laminin receptor precursor

Potyviruses are plant pathogens transmitted by aphids in a non-persistent manner. During transmission, the virus-encoded factor helper-component protease (HCPro) is presumed to act as a molecular bridge, mediating the reversible retention of virions to uncharacterized binding sites in the vector mouthparts. Whilst the predicted interaction between HCPro and the coat protein (CP) of virions has been confirmed experimentally, the characterization of putative HCPro-specific receptors in aphids has remained elusive, with the exception of a report that described binding of HCPro of zucchini yellow mosaic virus to several cuticle proteins. To identify other aphid components that could play a role during transmission, this study used purified HCPro of tobacco etch virus (TEV) in far-Western blotting assays as bait to select interactors among proteins extracted from aphid heads. With this approach, new HCPro-interacting proteins were found, and several were identified after mass spectrometry analysis and searches in databases dedicated to aphid sequences. Among these interactors, a ribosomal protein S2 (RPS2) was chosen for further investigation due to its homology with the laminin receptor precursor, known to act as the receptor of several viruses. The specific interaction between RPS2 and TEV HCPro was confirmed after cloning and heterologous expression of the corresponding Myzus persicae gene. The possible involvement of RPS2 in the transmission process was further suggested by testing a variant of HCPro that was non-functional for transmission due to a mutation in the conserved KITC motif (EITC variant). This variant retained its ability to bind CP but failed to interact with RPS2.

Eukaryotic protein production in designed storage organelles

BACKGROUND

Protein bodies (PBs) are natural endoplasmic reticulum (ER) or vacuole plant-derived organelles that stably accumulate large amounts of storage proteins in seeds. The proline-rich N-terminal domain derived from the maize storage protein gamma zein (Zera) is sufficient to induce PBs in non-seed tissues of Arabidopsis and tobacco. This Zera property opens up new routes for high-level accumulation of recombinant proteins by fusion of Zera with proteins of interest. In this work we extend the advantageous properties of plant seed PBs to recombinant protein production in useful non-plant eukaryotic hosts including cultured fungal, mammalian and insect cells.

RESULTS

Various Zera fusions with fluorescent and therapeutic proteins accumulate in induced PB-like organelles in all eukaryotic systems tested: tobacco leaves, Trichoderma reesei, several mammalian cultured cells and Sf9 insect cells. This accumulation in membranous organelles insulates both recombinant protein and host from undesirable activities of either. Recombinant protein encapsulation in these PBs facilitates stable accumulation of proteins in a protected sub-cellular compartment which results in an enhancement of protein production without affecting the viability and development of stably transformed hosts. The induced PBs also retain the high-density properties of native seed PBs which facilitate the recovery and purification of the recombinant proteins they contain.

CONCLUSION

The Zera sequence provides an efficient and universal means to produce recombinant proteins by accumulation in ER-derived organelles. The remarkable cross-kingdom conservation of PB formation and their biophysical properties should have broad application in the manufacture of non-secreted recombinant proteins and suggests the existence of universal ER pathways for protein insulation.

Protein body induction: a new tool to produce and recover recombinant proteins in plants

Stable accumulation of storage proteins, lipids and carbohydrates is a hallmark of the plant seed, and is a characteristic that is typically deficient in existing platforms for recombinant protein manufacture. One of the biological sequestration mechanisms that facilitate the folding, assembly and stabilization of plant seed storage proteins involve the de novo formation of unique intracellular organelles, the endoplasmic reticulum (ER)-derived protein bodies (PBs). In cereals, such as maize, PBs are formed directly in the lumen of the ER of endosperm cells and contain zeins, a group of polypeptides, which account for more than half of the total seed protein mass. The 27 kD gamma zein protein localizes to the periphery of the PBs surrounding aggregates of other zeins (including a zein and delta zein). Heterologous expression of gamma zein has been shown to result in the formation of PB-like structures, and the N-terminal proline-rich domain of gamma zein (Zera), containing eight PPPVHL repeats and a Pro-X sequence is by itself capable of directing ER retention and PB formation in non-seed tissues. We present a novel approach to produce recombinant proteins in plants based on the ability of gamma zein-Zera domain to store recombinant proteins inside PBs. Zera domain fused to several proteins, including a enhanced cyan fluorescent protein (ECFP), calcitonin (Ct) and epidermal growth factor (EGF), were cloned into vectors for transient or stable transformation of tobacco plants. In tobacco leaves, we observed the formation of dense, ER-localized structures containing high concentrations of the respective target proteins. The intact synthetic organelles containing Zera fusions were readily isolated from cellular material using density-based separation methods.

Role of the 25-26 nt siRNA in the resistance of transgenic Prunus domestica graft inoculated with plum pox virus

The reaction of a genetically engineered plum clone (C5) resistant to plum pox virus (PPV) by graft inoculation with the virus was evaluated. The resistance in this clone has been demonstrated to be mediated through post-transcriptional gene silencing (PTGS). A single C5 plant out of 30 plants inoculated with PPV M strain by double chip-budding showed mild diffuse mosaic 'Sharka' symptom at the bottom section of the scion. The upper leaves of this PPV-infected C5 plant remained symptomless and the virus was not detected in them by either DAS-ELISA or RT-PCR. An RNA silencing associated small interfering RNA duplex, siRNA (21-26 nt), was detected in non-inoculated C5 plants and in the portions of inoculated C5 plant in which PPV could not be detected. In the PPV-infected portion of the C5 plant and in C6 PPV susceptible plants only the approximately 21-22 nt siRNAs was detected. Cytosine-methylation was confirmed in C5 plants both uninfected and showing PPV symptoms. The 25-26 nt siRNA normally present in C5 was absent in PPV-infected C5 tissues confirming the critical role of this siRNA in the resistance of clone C5 to PPV infection. We also show that this PPV infection was limited and transient. It was only detected in one plant at one of four post-dormancy sampling dates and did not appear to affect the overall PPV resistance of the C5 clone.