An eIF4E allele confers resistance to an uncapped and non-polyadenylated RNA virus in melon

The characterization of natural recessive resistance genes and virus-resistant mutants of Arabidopsis have implicated translation initiation factors of the 4E family [eIF4E and eIF(iso)4E] as susceptibility factors required for virus multiplication and resistance expression. To date, viruses controlled by these genes mainly belong to the family Potyviridae. Melon necrotic spot virus (MNSV) belongs to the family Tombusviridae (genus Carmovirus) and is an uncapped and non-polyadenylated RNA virus. In melon, nsv-mediated resistance is a natural source of recessive resistance against all strains of MNSV except MNSV-264. Analyses of chimeras between non-resistance-breaking and resistance-breaking strains have shown that the avirulence determinant maps to the 3'-untranslated region (3'-UTR) of the viral genome. Using a combination of positional cloning and microsynteny analysis between Arabidopsis thaliana and melon, we genetically and physically delimited the nsv locus to a single bacterial artificial chromosome clone and identified the melon eukaryotic translation initiation factor 4E (Cm-eIF4E) as a candidate gene. Complementation analysis using a biolistic transient expression assay, confirmed Cm-eIF4E as the product of nsv. A single amino acid change at position 228 of the protein led to the resistance to MNSV. Protein expression and cap-binding analysis showed that Cm-eIF4E encoded by a resistant plant was not affected in it's cap-binding activity. The Agrobacterium-mediated transient expression of the susceptibility allele of Cm-eIF4E in Nicotiana benthamiana enhanced MNSV-264 accumulation. Based on these results, a model to explain melon resistance to MNSV is proposed. These data, and data from other authors, suggest that translation initiation factors of the eIF4E family are universal determinants of plant susceptibility to RNA viruses.     

Nicotiana benthamiana resistance to non-adapted Melon necrotic spot virus results from an incompatible interaction between virus RNA and translation initiation factor 4E

Nicotiana benthamiana has been described as non-host for Melon necrotic spot virus (MNSV). We investigated the basis of this resistance using the unique opportunity provided by strain MNSV-264, a recombinant virus that is able to overcome the resistance. Analysis of chimeric MNSV mutants showed that virulence in N. benthamiana is conferred by a 49 nucleotide section of the MNSV-264 3'-UTR, which acts in this host as a cap-independent translational enhancer (3'-CITE). Although the 3'-CITE of non-adapted MNSV-Mα5 is active in susceptible melon, it does not promote efficient translation in N. benthamiana, thus preventing expression of proteins required for virus replication. However, MNSV-Mα5 gains the ability to multiply in N. benthamiana cells if eIF4E from a susceptible melon variety (Cm-eIF4E-S) is supplied in trans. These data show that N. benthamiana resistance to MNSV-Mα5 results from incompatibility between the MNSV-Mα5 3'-CITE and N. benthamiana eIF4E in initiating efficient translation of the viral genome. Therefore, non-host resistance conferred by the inability of a host susceptibility factor to support viral multiplication may be a possible mechanism for this type of resistance to viruses.     

Soybean RNA interference lines silenced for eIF4E show broad potyvirus resistance

Soybean mosaic virus (SMV), a potyvirus, is the most prevalent and destructive viral pathogen in soybean-planting regions of China. Moreover, other potyviruses, including bean common mosaic virus (BCMV) and watermelon mosaic virus (WMV), also threaten soybean farming. The eukaryotic translation initiation factor 4E (eIF4E) plays a critical role in controlling resistance/susceptibility to potyviruses in plants. In the present study, much higher SMV-induced eIF4E1 expression levels were detected in a susceptible soybean cultivar when compared with a resistant cultivar, suggesting the involvement of eIF4E1 in the response to SMV by the susceptible cultivar. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that soybean eIF4E1 interacted with SMV VPg in the nucleus and with SMV NIa-Pro/NIb in the cytoplasm, revealing the involvement of VPg, NIa-Pro, and NIb in SMV infection and multiplication. Furthermore, transgenic soybeans silenced for eIF4E were produced using an RNA interference approach. Through monitoring for viral symptoms and viral titers, robust and broad-spectrum resistance was confirmed against five SMV strains (SC3/7/15/18 and SMV-R), BCMV, and WMV in the transgenic plants. Our findings represent fresh insights for investigating the mechanism underlying eIF4E-mediated resistance in soybean and also suggest an effective alternative for breeding soybean with broad-spectrum viral resistance.     

Mechanism of plant eIF4E-mediated resistance against a Carmovirus (Tombusviridae): cap-independent translation of a viral RNA controlled in cis by an (a)virulence determinant

Translation initiation factors are universal determinants of plant susceptibility to RNA viruses, but the underlying mechanisms are poorly understood. Here, we show that a sequence in the 3' untranslated region (3'-UTR) of a viral genome that is responsible for overcoming plant eIF4E-mediated resistance (virulence determinant) functions as a 3' cap-independent translational enhancer (3'-CITE). The virus/plant pair studied here is Melon necrotic spot virus (MNSV) and melon, for which a recessive resistance controlled by melon eIF4E was previously described. Chimeric viruses between virulent and avirulent isolates enabled us to map the virulence and avirulence determinants to 49 and 26 nucleotides, respectively. The translational efficiency of a luc reporter gene flanked by 5'- and 3'-UTRs from virulent, avirulent and chimeric viruses was analysed in vitro, in wheatgerm extract, and in vivo, in melon protoplasts, showing that: (i) the virulence determinant mediates the efficient cap-independent translation in vitro and in vivo; (ii) the avirulence determinant was able to promote efficient cap-independent translation in vitro, but only when eIF4E from susceptible melon was added in trans, and, coherently, only in protoplasts of susceptible melon, but not in the protoplasts of resistant melon; (iii) these activities required the 5'-UTR of MNSV in cis. Thus, the virulence and avirulence determinants function as 3'-CITEs. The activity of these 3'-CITEs was host specific, suggesting that an inefficient interaction between the viral 3'-CITE of the avirulent isolate and eIF4E of resistant melon impedes the correct formation of the translation initiation complex at the viral RNA ends, thereby leading to resistance.     

The eukaryotic translation initiation factor 4E confers multiallelic recessive Bymovirus resistance in Hordeum vulgare (L.)

Virus diseases are widespread threats for crop production, which can, in many cases, be controlled efficiently by exploiting naturally occurring resistance. Barley, an important cereal species of the Triticeae, carries two genes, rym4 and rym5 , which are located in the telomeric region of chromosome 3HL and confer recessive resistance to various strains of the Barley yellow mosaic virus complex. The barley 'eukaryotic translation initiation factor 4E' ( Hv-eIF4E ) was identified as a candidate for resistance gene function by physical mapping on a 650 kb contig. It is located in a chromosomal region characterized by suppressed recombination, in a position collinear to its homologue on rice chromosome 1L. Sequence diversity in the coding region of Hv-eIF4E , as calculated from a collection of unrelated barley accessions, revealed non-silent single nucleotide polymorphisms (SNPs) in four of its five exons. Stable transformation of a resistant barley genotype with a genomic fragment or a full-length cDNA of Hv-eIF4E derived from susceptible cultivars induced susceptibility to Barley mild mosaic virus . Moreover, the identification of SNPs diagnostic for rym4 and rym5 provides evidence that these are two alleles, which confer different resistance specificities. These findings demonstrate that variants of Hv-eIF4E confer multiallelic recessive virus resistance in a monocot species. The identification of eIF4E as the causal host factor for bymovirus resistance illustrates that mutations in this basic component of the eukaryotic translation complex form a seminal mechanism for recessive virus resistance in both dicot and monocot plants.     

The same allele of translation initiation factor 4E mediates resistance against two Potyvirus spp. in Pisum sativum

Pathogenicity of two sequenced isolates of Bean yellow mosaic virus (BYMV) was established on genotypes of Pisum sativum L. reported to carry resistance genes to BYMV and other potyviruses. Resistance to the white lupin strain of BYMV (BYMV-W) is inherited as a recessive gene named wlv that maps to linkage group VI together with other Potyvirus resistances. One of these, sbm1, confers resistance to strains of Pea seedborne mosaic virus and previously has been identified as a mutant allele of the eukaryotic translation initiation factor 4E gene (eIF4E). Sequence comparison of eIF4E from BYMV-W-susceptible and -resistant P. sativum genotypes revealed a polymorphism correlating with the resistance profile. Expression of eIF4E from susceptible plants in resistant plants facilitated BYMV-W infection in inoculated leaves. When cDNA of BYMV-W was agroinoculated, resistance mediated by the wlv gene frequently was overcome, and virus from these plants had a codon change causing an Arg to His change at position 116 of the predicted viral genome-linked protein (VPg). Accordingly, plants carrying the wlv resistance gene were infected upon inoculation with BYMV-W derived from cDNA with a His codon at position 116 of the VPg coding region. These results suggested that VPg determined pathogenicity on plants carrying the wlv resistance gene and that wlv corresponded to the sbm1 allele of eIF4E.     

Potyviral resistance derived from cultivars of Phaseolus vulgaris carrying bc-3 is associated with the homozygotic presence of a mutated eIF4E allele

Eukaryotic translation initiation factors (eIFs) play a central role in potyviral infection. Accordingly, mutations in the gene encoding eIF4E have been identified as a source of recessive resistance in several plant species. In common bean, Phaseolus vulgaris , four recessive genes, bc-1 , bc-2 , bc-3 and bc-u , have been proposed to control resistance to the potyviruses Bean common mosaic virus (BCMV) and Bean common mosaic necrosis virus . In order to identify molecular entities for these genes, we cloned and sequenced P. vulgaris homologues of genes encoding the eIF proteins eIF4E, eIF(iso)4E and nCBP. Bean genotypes reported to carry bc-3 resistance were found specifically to carry non-silent mutations at codons 53, 65, 76 and 111 in eIF4E . This set of mutations closely resembled a pattern of eIF4E mutations determining potyvirus resistance in other plant species. The segregation of BCMV resistance and eIF4E genotype was subsequently analysed in an F₂ population derived from the P. vulgaris all-susceptible genotype and a genotype carrying bc-3 . F₂ plants homozygous for the eIF4E mutant allele were found to display at least the same level of resistance to BCMV as the parental resistant genotype. At 6 weeks after inoculation, all F₂ plants found to be BCMV negative by enzyme-linked immunosorbent assay were found to be homozygous for the mutant eIF4E allele. In F₃ plants homozygous for the mutated allele, virus resistance was subsequently found to be stably maintained. In conclusion, allelic eIF4E appears to be associated with a major component of potyvirus resistance present in bc-3 genotypes of bean.     

Engineering virus resistance using a modified potato gene

Natural mutations in translation initiation factor eIF4E confer resistance to potyviruses in many plant species. Potato is a staple food crop plagued by several potyviruses, yet to date no known eIF4E-mediated resistance genes have been identified. In this study, we demonstrate that transgenic expression of the pvr1(2) gene from pepper confers resistance to Potato virus Y (PVY) in potato. We then use this information to convert the susceptible potato ortholog of this allele into a de novo allele for resistance to PVY using site-directed mutagenesis. Potato plants overexpressing the mutated potato allele are resistant to virus infection. Resistant lines expressed high levels of eIF4E mRNA and protein. The resistant plants showed growth similar to untransformed controls and produced phenotypically similar tubers. This technique disrupts a key step in the viral infection process and may potentially be used to engineer virus resistance in a number of economically important plant-viral pathosystems. Furthermore, the general public may be more amenable to the 'intragenic' nature of this approach because the transferred coding region is modified from a gene in the target crop rather than from a distant species.     

Selective involvement of members of the eukaryotic initiation factor 4E family in the infection of Arabidopsis thaliana by potyviruses

Arabidopsis thaliana plants with mutations in the genes encoding eukaryotic initiation factor (eIF4E) or isoform of eIF4E (eIF(iso)4E) were tested for susceptibility to Clover yellow vein virus (ClYVV), a member of the genus Potyvirus. ClYVV accumulated in both inoculated and upper uninoculated leaves of mutant plants lacking eIF(iso)4E, but not in mutant plants lacking eIF4E. In contrast, Turnip mosaic virus (TuMV), another member of the genus Potyvirus, multiplied in mutant plants lacking eIF4E but not in mutant plants lacking eIF(iso)4E. These results suggest the selective involvement of members of the eIF4E family in infection by potyviruses.     

Evidence that the recessive bymovirus resistance locus rym4 in barley corresponds to the eukaryotic translation initiation factor 4E gene

Recent studies have shown that resistance in several dicotyledonous plants to viruses in the genus Potyvirus is controlled by recessive alleles of the plant translation initiation factor eIF4E or eIF ( iso ) 4E genes. Here we provide evidence that the barley rym 4 gene locus, controlling immunity to viruses in the genus Bymovirus , corresponds to eIF4E . A molecular marker based on the sequence of eIF4E was developed and used to demonstrate that eIF4E and rym 4 map to the same genetic interval on chromosome 3HL in barley . Another genetic marker was developed that detects a polymorphism in the coding sequence of eIF4E and consistently distinguishes between rym 4 and susceptible barley cultivars of diverse parentage. The eIF4E gene product from barley genotypes carrying rym 4 and allelic rym 5 and rym 6 genes, originating from separate exotic germplasm, and a novel resistant allele that we identified through a reverse genetics approach all contained unique amino acid substitutions compared with the wild-type protein. Three-dimensional models of the barley eIF4E protein revealed that the polymorphic residues identified are all located at or near the mRNA cap-binding pocket, similarly to recent findings from studies on recessive potyvirus resistance in dicotyledonous plants. These new data complement our earlier observations that specific mutations in bymovirus VPg are responsible for overcoming rym 4/5-controlled resistance. Because the potyviral VPg is known to interact with eIF4E in dicotyledonous plants, it appears that monocotyledonous and dicotyledonous plants have evolved a similar strategy to combat VPg-encoding viruses in the family Potyviridae .     

The Arabidopsis eukaryotic initiation factor (iso)4E is dispensable for plant growth but required for susceptibility to potyviruses

An Arabidopsis thaliana line bearing a transposon insertion in the gene coding for the isozyme form of the plant-specific cap-binding protein, eukaryotic initiation factor (iso) 4E (eIF (iso) 4E), has been isolated. This mutant line completely lacks both eIF(iso)4E mRNA and protein, but was found to have a phenotype and fertility indistinguishable from wild-type plants under standard laboratory conditions. In contrast, the amount of the related eIF4E protein was found to increase in seedling extracts. Furthermore, polysome analysis shows that the mRNA encoding eIF4E was being translated at increased levels. Given the known interaction between cap-binding proteins and potyviral genome-linked proteins (VPg), this plant line was challenged with two potyviruses, Turnip mosaic virus (TuMV) and Lettuce mosaic virus (LMV) and was found resistant to both, but not to the Nepovirus, Tomato black ring virus (TBRV) and the Cucumovirus, Cucumber mosaic virus (CMV). Together with previous data showing that the VPg-eIF4E interaction is necessary for virus infectivity and upregulates genome amplification, this shows that the eIF4E proteins are specifically recruited for the replication cycle of potyviruses.     

Mutational analysis of plant cap-binding protein eIF4E reveals key amino acids involved in biochemical functions and potyvirus infection

The eukaryotic translation initiation factor 4E (eIF4E) (the cap-binding protein) is involved in natural resistance against several potyviruses in plants. In lettuce, the recessive resistance genes mo1¹ and mo1² against Lettuce mosaic virus (LMV) are alleles coding for forms of eIF4E unable, or less effective, to support virus accumulation. A recombinant LMV expressing the eIF4E of a susceptible lettuce variety from its genome was able to produce symptoms in mo1¹ or mo1² varieties. In order to identify the eIF4E amino acid residues necessary for viral infection, we constructed recombinant LMV expressing eIF4E with point mutations affecting various amino acids and compared the abilities of these eIF4E mutants to complement LMV infection in resistant plants. Three types of mutations were produced in order to affect different biochemical functions of eIF4E: cap binding, eIF4G binding, and putative interaction with other virus or host proteins. Several mutations severely reduced the ability of eIF4E to complement LMV accumulation in a resistant host and impeded essential eIF4E functions in yeast. However, the ability of eIF4E to bind a cap analogue or to fully interact with eIF4G appeared unlinked to LMV infection. In addition to providing a functional mutational map of a plant eIF4E, this suggests that the role of eIF4E in the LMV cycle might be distinct from its physiological function in cellular mRNA translation.     

Evaluation of reference genes for real-time RT-PCR expression studies in the plant pathogen Pectobacterium atrosepticum

Background

Translation initiation factors of the 4E and 4G protein families mediate resistance to several RNA plant viruses in the natural diversity of crops. Particularly, a single point mutation in melon eukaryotic translation initiation factor 4E (eIF4E) controls resistance to Melon necrotic spot virus (MNSV) in melon. Identification of allelic variants within natural populations by EcoTILLING has become a rapid genotype discovery method.

Results

A collection of Cucumis spp. was characterised for susceptibility to MNSV and Cucumber vein yellowing virus (CVYV) and used for the implementation of EcoTILLING to identify new allelic variants of eIF4E. A high conservation of eIF4E exonic regions was found, with six polymorphic sites identified out of EcoTILLING 113 accessions. Sequencing of regions surrounding polymorphisms revealed that all of them corresponded to silent nucleotide changes and just one to a non-silent change correlating with MNSV resistance. Except for the MNSV case, no correlation was found between variation of eIF4E and virus resistance, suggesting the implication of different and/or additional genes in previously identified resistance phenotypes. We have also characterized a new allele of eIF4E from Cucumis zeyheri, a wild relative of melon. Functional analyses suggested that this new eIF4E allele might be responsible for resistance to MNSV.

Conclusion

This study shows the applicability of EcoTILLING in Cucumis spp., but given the conservation of eIF4E, new candidate genes should probably be considered to identify new sources of resistance to plant viruses. Part of the methodology described here could alternatively be used in TILLING experiments that serve to generate new eIF4E alleles.     

When a knockout is an Achilles’ heel: Resistance to one potyvirus species triggers hypersusceptibility to another one in Arabidopsis thaliana

The translation initiation factors 4E are a small family of major susceptibility factors to potyviruses. It has been suggested that knocking out these genes could provide genetic resistance in crops when natural resistance alleles, which encode functional eIF4E proteins, are not available. Here, using the well-characterized Arabidopsis thaliana–potyvirus pathosystem, we evaluate the resistance spectrum of plants knocked out for eIF4E1, the susceptibility factor to clover yellow vein virus (ClYVV). We show that besides resistance to ClYVV, the eIF4E1 loss of function is associated with hypersusceptibility to turnip mosaic virus (TuMV), a potyvirus known to rely on the paralog host factor eIFiso4E. On TuMV infection, plants knocked out for eIF4E1 display striking developmental defects such as early senescence and primordia development stoppage. This phenotype is coupled with a strong TuMV overaccumulation throughout the plant, while remarkably the levels of the viral target eIFiso4E remain uninfluenced. Our data suggest that this hypersusceptibility cannot be explained by virus evolution leading to a gain of TuMV aggressiveness. Furthermore, we report that a functional eIF4E1 resistance allele engineered by CRISPR/Cas9 base-editing technology successfully circumvents the increase of TuMV susceptibility conditioned by eIF4E1 disruption. These findings in Arabidopsis add to several previous findings in crops suggesting that resistance based on knocking out eIF4E factors should be avoided in plant breeding, as it could also expose the plant to the severe threat of potyviruses able to recruit alternative eIF4E copies. At the same time, it provides a simple model that can help understanding of the homeostasis among eIF4E proteins in the plant cell and what makes them available to potyviruses.     

High overexpression of CERES,a plant regulator of translation,induces different phenotypical defence responses during TuMV infection

Mutations in the eukaryotic translation initiation factors eIF4E and eIF(iso)4E confer potyvirus resistance in a range of plant hosts. This supports the notion that, in addition to their role in translation of cellular mRNAs, eIF4E isoforms are also essential for the potyvirus cycle. CERES is a plant eIF4E- and eIF(iso)4E-binding protein that, through its binding to the eIF4Es, modulates translation initiation; however, its possible role in potyvirus resistance is unknown. In this article, we analyse if the ectopic expression of AtCERES is able to interfere with turnip mosaic virus replication in plants. Our results demonstrate that, during infection, the ectopic expression of CERES in Nicotiana benthamiana promotes the development of a mosaic phenotype when it is accumulated to moderate levels, but induces veinal necrosis when it is accumulated to higher levels. This necrotic process resembles a hypersensitive response (HR)-like response that occurs with different HR hallmarks. Remarkably, Arabidopsis plants inoculated with a virus clone that promotes high expression of CERES do not show signs of infection. These final phenotypical outcomes are independent of the capacity of CERES to bind to eIF4E. All these data suggest that CERES, most likely due to its leucine-rich repeat nature, could act as a resistance protein, able to promote a range of different defence responses when it is highly overexpressed from viral constructs.     

Regulation of Eukaryotic Initiation Factor 4E and Its Isoform： Implications for Antiviral Strategy in Plants

In recent years, biotechnology has permitted regulation of the expression of endogenous plant genes to improve agronomlcally important traits. Genetic modification of crops has benefited from emerging knowledge of new genes, especially genes that exhibit novel functions, one of which is eukaryotlc initiation factor 4E （eIF4E）. eIF4E Is one of the most important translation initiation factors Involved in eukaryotic initiation. Recent research has demonstrated that virus resistance mediated by eIF4E and Its isoform elf （Iso）4E occurs in several plant-virus interactions, thus indicating a potential new role for eIF4E/elF（Iso）4E In resistance strategies against plant viruses. In this review, we briefly describe eIF4E activity In plant translation, its potential role, and functions of the eIF4E subfamily In plant-virus interactions. Other initiation factors such as elF4G could also play a role In plant resistance against viruses. Finally, the potential for developing eIF4E-mediated resistance to plant viruses in the future Is discussed. Future research should focus on elucidation of the resistance mechanism and spectrum mediated by eIF4E. Knowledge of a particu- lar plant-virus interaction will help to deepen our understanding of eIF4E and other eukaryotic Initiation factors, and their involvement in virus disease control.     

Loss-of-susceptibility mutants of Arabidopsis thaliana reveal an essential role for eIF(iso)4E during potyvirus infection

The Arabidopsis thaliana-potyvirus system was developed to identify compatibility and incompatibility factors involved during infection and disease caused by positive-strand RNA viruses. Several Arabidopsis mutants with increased susceptibility to Tobacco etch potyvirus (TEV) were isolated previously, revealing a virus-specific resistance system in the phloem. In this study, Arabidopsis mutants with decreased susceptibility to Turnip mosaic potyvirus (TuMV) were isolated. Three independent mutants that conferred immunity to TuMV were isolated and assigned to the same complementation group. These mutants were also immune or near-immune to TEV but were susceptible to an unrelated virus. The locus associated with decreased susceptibility was named loss-of-susceptibility to potyviruses 1 (lsp1). The LSP1 locus was isolated by map-based cloning and was identified as the gene encoding translation factor eIF(iso)4E, one of several known Arabidopsis isoforms that has cap binding activity. eIF4E and eIF(iso)4E from different plant species were shown previously to interact with the genome-linked protein (VPg) of TEV and TuMV, respectively. Models to explain the roles of eIF(iso)4E during virus infection are presented.     

Construction of a fosmid library of cucumber (Cucumis sativus) and comparative analyses of the eIF4E and eIF(iso)4E regions from cucumber and melon (Cucumis melo)

A fosmid library of cucumber was synthesized as an unrestricted resource for researchers and used for comparative sequence analyses to assess synteny between the cucumber and melon genomes, both members of the genus Cucumis and the two most economically important plants in the family Cucurbitaceae. End sequencing of random fosmids produced over 680 kilobases of cucumber genomic sequence, of which 25% was similar to ribosomal DNAs, 25% to satellite sequences, 20% to coding regions in other plants, 4% to transposable elements, 13% to mitochondrial and chloroplast sequences, and 13% showed no hits to the databases. The relatively high frequencies of ribosomal and satellite DNAs are consistent with previous analyses of cucumber DNA. Cucumber fosmids were selected and sequenced that carried eukaryotic initiation factors (eIF) 4E and iso(4E), genes associated with recessively inherited resistances to potyviruses in a number of plants. Indels near eIF4E and eIF(iso)4E mapped independently of the zym, a recessive locus conditioning resistance to Zucchini yellow mosaic virus, establishing that these candidate genes are not zym. Cucumber sequences were compared with melon BACs carrying eIF4E and eIF(iso)4E and revealed extensive sequence conservation and synteny between cucumber and melon across these two independent genomic regions. This high degree of microsynteny will aid in the cloning of orthologous genes from both species, as well as allow for genomic resources developed for one Cucumis species to be used for analyses in other species. Names are necessary to report factually on available data; however, the US Department of Agriculture (USDA) neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.