To Tobacco Mosaic Virus Resistance Research Articles

Glutathione Can Compensate for Salicylic Acid Deficiency in Tobacco to Maintain Resistance to Tobacco Mosaic Virus.

Earlier studies showed that the artificial elevation of endogenous glutathione (GSH) contents can markedly increase the resistance of plants against different viruses. On the other hand, salicylic acid (SA)-deficient NahG plants display enhanced susceptibility to viral infections. In the present study, the biochemical mechanisms underlying GSH-induced resistance were investigated in various tobacco biotypes displaying markedly different GSH and SA levels. The endogenous GSH levels of Nicotiana tabacum cv. Xanthi NN and N. tabacum cv. Xanthi NN NahG tobacco leaves were increased by infiltration of exogenous GSH or its synthetic precursor R-2-oxo-4-thiazolidine-carboxylic acid (OTC). Alternatively, we also used tobacco lines containing high GSH levels due to transgenes encoding critical enzymes for cysteine and GSH biosynthesis. We crossed Xanthi NN and NahG tobaccos with the GSH overproducer transgenic tobacco lines in order to obtain F1 progenies with increased levels of GSH and decreased levels of SA. We demonstrated that in SA-deficient NahG tobacco the elevation of in planta GSH and GSSG levels either by exogenous GSH or by crossing with glutathione overproducing plants confers enhanced resistance to Tobacco mosaic virus (TMV) manifested as both reduced symptoms (i.e. suppression of hypersensitive-type localized necrosis) and lower virus titers. The beneficial effects of elevated GSH on TMV resistance was markedly stronger in NahG than in Xanthi NN leaves. Infiltration of exogenous GSH and OTC or crossing with GSH overproducer tobacco lines resulted in a substantial rise of bound SA and to a lesser extent of free SA levels in tobacco, especially following TMV infection. Significant increases in expression of pathogenesis related (NtPR-1a, and NtPRB-1b), and glutathione S-transferase (NtGSTtau, and NtGSTphi) genes were evident in TMV-inoculated leaves in later stages of pathogenesis. However, the highest levels of defense gene expression were associated with SA-deficiency, rather than enhanced TMV resistance. In summary, elevated levels of glutathione in TMV-infected tobacco can compensate for SA deficiency to maintain virus resistance. Our results suggest that glutathione-induced redox changes are important components of antiviral signaling in tobacco.

TurboID-based proximity labeling reveals that UBR7 is a regulator of N NLR immune receptor-mediated immunity

Nucleotide-binding leucine-rich repeat (NLR) immune receptors play a critical role in defence against pathogens in plants and animals. However, we know very little about NLR-interacting proteins and the mechanisms that regulate NLR levels. Here, we used proximity labeling (PL) to identify the proteome proximal to N, which is an NLR that confers resistance to Tobacco mosaic virus (TMV). Evaluation of different PL methods indicated that TurboID-based PL provides more efficient levels of biotinylation than BioID and BioID2 in plants. TurboID-based PL of N followed by quantitative proteomic analysis and genetic screening revealed multiple regulators of N-mediated immunity. Interestingly, a putative E3 ubiquitin ligase, UBR7, directly interacts with the TIR domain of N. UBR7 downregulation leads to an increased amount of N protein and enhanced TMV resistance. TMV-p50 effector disrupts the N-UBR7 interaction and relieves negative regulation of N. These findings demonstrate the utility of TurboID-based PL in plants and the N-interacting proteins we identified enhance our understanding of the mechanisms underlying NLR regulation.

Exogenous application of melatonin improves plant resistance to virus infection

In this study, melatonin (MEL)‐mediated plant resistance to tobacco mosaic virus (TMV) was examined to study local infection in Nicotiana glutinosa and systemic infection in Solanum lycopersicum. Exogenous application of 100 µm MEL increased anti‐virus infection activity to 37.4% in virus‐infected N. glutinosa plants. The same treatment significantly reduced relative levels of virus RNA analysed by qRT‐PCR and virus titres measured by dot‐ELISA, and increased the relative expression levels of the PR1 and PR5 genes analysed by qRT‐PCR, in virus‐infected S. lycopersicum. MEL treatment induced considerable accumulations of salicylic acid (SA) and nitric oxide (NO) but did not significantly affect production of hydrogen peroxide (H2O2) in the virus‐infected S. lycopersicum plants. Transgenic nahG N. tabacum was used to determine whether MEL‐induced TMV resistance was dependent on the SA pathway. The results showed that the relative RNA level of the TMV analysed by qRT‐PCR and virus titres analysed by dot‐ELISA were not reduced by the MEL treatment in the nahG transgenic N. tabacum seedlings treated twice with 100 µm MEL. The increased relative expression levels of PR1 and PR5 were greatly reduced when cPTIO, an NO scavenger, was included in the MEL treatment. A working model of MEL‐mediated plant resistance to TMV is proposed. MEL‐mediated plant resistance to viruses provides a new avenue to control plant viral diseases.

Hsp90 Interacts With Tm-22 and Is Essential for Tm-22-Mediated Resistance to Tobacco mosaic virus

The tomato resistance gene Tm-22 encodes a coiled coil-nucleotide binding site-leucine rich repeat type resistance protein and confers effective immune response against tobamoviruses by detecting the presence of viral movement proteins (MPs). In this study, we show that the Nicotiana benthamiana Heat shock protein 90-kD (Hsp90) interacts with Tm-22. Silencing of Hsp90 reduced Tm-22-mediated resistance to Tobacco mosaic virus (TMV) and the steady-state levels of Tm-22 protein. Further, Hsp90 associates with SGT1 in yeast and in plant cells. These results suggest that Hsp90-SGT1 complex takes part in Tm-22-mediated TMV resistance by functioning as chaperone to regulate Tm-22 stability.

Chitosan oligosaccharide induces resistance to Tobacco mosaic virus in Arabidopsis via the salicylic acid-mediated signalling pathway

Chitosan is one of the most abundant carbohydrate biopolymers in the world, and chitosan oligosaccharide (COS), which is prepared from chitosan, is a plant immunity regulator. The present study aimed to validate the effect of COS on inducing resistance to tobacco mosaic virus (TMV) in Arabidopsis and to investigate the potential defence-related signalling pathways involved. Optimal conditions for the induction of TMV resistance in Arabidopsis were COS pretreatment at 50 mg/L for 1 day prior to inoculation with TMV. Multilevel indices, including phenotype data, and TMV coat protein expression, revealed that COS induced TMV resistance in wild-type and jasmonic acid pathway- deficient (jar1) Arabidopsis plants, but not in salicylic acid pathway deficient (NahG) Arabidopsis plants. Quantitative-PCR and analysis of phytohormone levels confirmed that COS pretreatment enhanced the expression of the defence-related gene PR1, which is a marker of salicylic acid signalling pathway, and increased the amount of salicylic acid in WT and jar1, but not in NahG plants. Taken together, these results confirm that COS induces TMV resistance in Arabidopsis via activation of the salicylic acid signalling pathway.

Jasmonic acid negatively regulates resistance to Tobacco mosaic virus in tobacco

Nicotiana tabacum (tobacco) cultivars possessing the N resistance gene to Tobacco mosaic virus (TMV) induce a hypersensitive response, which is accompanied by the production of phytohormones such as salicylic acid (SA) and jasmonic acid (JA), to enclose the invaded virus at the initial site of infection, which inhibits viral multiplication and spread. SA functions as a positive regulator of TMV resistance. However, the role of JA in TMV resistance has not been fully elucidated. Exogenously applied methyl jasmonate, a methyl ester of JA, reduced local resistance to TMV and permitted systemic viral movement. Furthermore, in contrast to a previous finding, we demonstrated that silencing of CORONATINE-INSENSITIVE 1 (COI1), a JA receptor, reduced viral accumulation in a tobacco cultivar possessing the N gene, as did that of allene oxide synthase, a JA biosynthetic enzyme. The reduction in viral accumulation in COI1-silenced tobacco plants was correlated with an increase in SA, and lowering SA levels by introducing an SA hydroxylase gene attenuated this reduction. Viral susceptibility did not change in a COI1-silenced tobacco cultivar lacking the N gene. These results suggest that JA signaling is not directly responsible for susceptibility to TMV, but is indirectly responsible for viral resistance through the partial inhibition of SA-mediated resistance conferred by the N gene, and that a balance between endogenous JA and SA levels is important for determining the degree of resistance.

CDNA-AFLP Analysis of Differentially Expressed Genes in Tobacco Infected by Tobacco Mosaic Virus

Longjiang 925 is a flue-cured tobacco variety with high resistance to Tobacco mosaic virus (TMV). In this study, the cDNA extracted from the TMV-inoculated leaves of Longjiang 925 was amplified with 240 pairs of primers. Approximately 9500 gene transcript fragments were obtained, in which 12 inducible expressed gene fragments were selected by cloning and sequencing. The inducible fragments functions involved in the nucleic acid metabolism, protein synthesis and modulation, energy metabolism, stress responding, intracellular transport, and metabolism of carbohydrates. The function of a differentially expressed gene sequence, TIF2, was validated using real-time PCR with the samples collected at 0, 12, 24, 48, and 72 h post inoculation. The result indicated that TIF2 was related to TMV-resistance. Both 5′- and 3′-rapid amplification of cDNA ends (RACE) were preformed using TIF2 sequence, and the full-length cDNA sequence was 875 bp in length, containing a conjecturable coding region from 101 bp to 613 bp and encoding 170 amino acids. Analyses of Blastn and Blastp showed that this gene was probably a novel gene related to TMV resistance in tobacco.

Systemic Induction and Role of Mitochondrial Alternative Oxidase and Nitric Oxide in a Compatible Tomato–Tobacco mosaic virus Interaction

The role of mitochondrial alternative oxidase (AOX) and the relationship between AOX and nitric oxide (NO) in virus-induced systemic defense to Tobacco mosaic virus (TMV) were investigated in susceptible tomato (Solanum lycopersicum) plants. TMV inoculation to the lower leaves induced a rapid NO synthesis and AOX activation in upper uninoculated leaves as early as 0.5 day postinoculation. Application of exogenous potassium cyanide (KCN, a cytochrome pathway inhibitor) at nonlethal concentrations and NO donor diethylamine NONOate (DEA/NO) to the upper uninoculated leaves greatly induced accumulation of AOX transcript, reduced TMV viral RNA accumulation, and increased the leaf photochemical quantum yield at photosystem II. Pretreatment with NO scavenger almost completely blocked TMV-induced AOX induction and substantially increased TMV susceptibility. Salicylhydroxamic acid (SHAM, an AOX inhibitor) pretreatment reduced the DEA/NO-induced cyanide-resistant respiration and partially compromised induced resistance to TMV. Conversely, KCN and SHAM pretreatment had very little effect on generation of NO, and pretreatment with NO scavenger did not affect KCN-induced AOX induction and TMV resistance. These results suggest that TMV-induced NO generation acts upstream and mediates AOX induction which, in turn, induces mitochondrial alternative electron transport and triggers systemic basal defense against the viral pathogen.

3‐Acetonyl‐3‐hydroxyoxindole: a new inducer of systemic acquired resistance in plants

Systemic acquired resistance (SAR) is an inducible defence mechanism which plays a central role in protecting plants from microbial pathogen attack. Guided by bioassays, a new chemical inducer of SAR was isolated from the extracts of Strobilanthes cusia and identified to be 3-acetonyl-3-hydroxyoxindole (AHO), a derivative of isatin. Tobacco plants treated with AHO exhibited enhanced resistance to tobacco mosaic virus (TMV) and to the fungal pathogen Erysiphe cichoracearum (powdery mildew), accompanied by increased levels of pathogenesis-related gene 1 (PR-1) expression, salicylic acid (SA) accumulation and phenylalanine ammonia-lyase activity. To study the mode of action of AHO, its ability to induce PR-1 expression and TMV resistance in nahG transgenic plants expressing salicylate hydroxylase, which prevents the accumulation of SA, was analysed. AHO treatment did not induce TMV resistance or PR-1 expression in nahG transgenic plants, suggesting that AHO acts upstream of SA in the SAR signalling pathway. In addition, using two-dimensional gel electrophoresis combined with mass spectrometry, five AHO-induced plant proteins were identified which were homologous to the effector proteins with which SA interacts. Our data suggest that AHO may represent a novel class of inducer that stimulates SA-mediated defence responses.

The negative influence of N-mediated TMV resistance on yield in tobacco: linkage drag versus pleiotropy

Resistance to tobacco mosaic virus (TMV) is controlled by the single dominant gene N in Nicotiana glutinosa L. This gene has been transferred to cultivated tobacco (N. tabacum L.) by interspecific hybridization and backcrossing, but has historically been associated with reduced yields and/or quality in flue-cured tobacco breeding materials. Past researchers have suggested the role of pleiotropy and/or linkage drag effects in this unfavorable relationship. Introduction of the cloned N gene into a TMV-susceptible tobacco genotype (cultivar 'K326') via plant transformation permitted investigation of the relative importance of these possibilities. On average, yield and cash return ($ ha(-1)) of 14 transgenic NN lines of K326 were significantly higher relative to an isoline of K326 carrying N introduced via interspecific hybridization and backcrossing. The negative effects of tissue culture-induced genetic variation confounded comparisons with the TMV-susceptible cultivar, K326, however. Backcrossing the original transgenic lines to non-tissue cultured K326 removed many of these unfavorable effects, and significantly improved their performance for yield and cash return. Comparisons of the 14 corresponding transgenic NN backcross-derived lines with K326 indicated that linkage drag is the main factor contributing to reduced yields in TMV-resistant flue-cured tobacco germplasm. On average, these transgenic lines outyielded the conventionally-developed TMV-resistant K326 isoline by 427 kg ha(-1) (P < 0.05) and generated $1,365 ha(-1) more (P < 0.05). Although transgenic tobacco cultivars are currently not commercially acceptable, breeding strategies designed to reduce the amount of N. glutinosa chromatin linked to N may increase the likelihood of developing high-yielding TMV-resistant flue-cured tobacco cultivars.

Salicylate Activity. 3. Structure Relationship to Systemic Acquired Resistance

Salicylic acid (2-hydroxybenzoic acid; SA) is a primary signal inducing plant defenses against pathogens. This plant disease resistance, known as systemic acquired resistance (SAR), is an attractive target for the development of new plant protection agents. SAR induction is a multistep process that includes accumulation of pathogenesis-related (PR) proteins. The structure-activity profile of salicylates and related compounds has been evaluated using an inducible PR protein (PR-1a) and plant resistance to tobacco mosaic virus (TMV) as markers. Among the 47 selected monosubstituted and multiple-substituted salicylate derivatives tested, all 8 derivatives that induced more PR-1a protein than SA were fluorinated or chlorinated in the 3- and/or 5-position (3,5-difluorosalicylate > 3-chlorosalicylate > 5-chlorosalicylate > 3,5-dichlorosalicylate > 3-chloro-5-fluorosalicylate > 3-fluorosalicylate > 3-fluoro-5-chlorosalicylate > 3,5-dichloro-6-hydroxysalicylate > SA). In general, substitutions for or on the 2-hydroxyl group or at the 4-position of the ring reduced or eliminated PR-1a protein induction. In contrast, substitutions in positions ortho (3-position) or para (5-position) to the hydroxyl group with electron-withdrawing groups other than chlorine or fluorine decreased induction, and electron-donating groups in these positions also had a deleterious effect on PR-1a induction. PR-1a protein accumulation and reduction in TMV lesion diameter exhibited a log-linear relationship. The seven salicylate derivatives that were the most active TMV resistance inducers were all halogenated in the 3- and/or 5-position (3-chlorosalicylate > 3,5-difluorosalicylate > 3,5-dichloro-6-hydroxysalicylate > 3,5,6-trichlorosalicylate > 5-chlorosalicylate > 5-fluorosalicylate > 3,5-dichlorosalicylate > 4-fluorosalicylate > 3-fluorosalicylate > 3-chloro-5-fluorosalicylate > 4-chlorosalicylate > SA). The correlation between PR-1a protein induction and resistance to TMV confirms the value of using PR-1a induction as a screening tool for developing new plant disease control agents.

Resistance to Tobacco mosaic virus and Botrytis cinerea in tobacco transformed with complementary DNA encoding an inhibitor of viral replication‐like protein

AbstractWe have previously cloned a 1016‐bp‐long complementary DNA (cDNA) clone (named NC330) encoding an inhibitor of virus replication (IVR)‐like protein from induced‐resistant leaves of Nicotiana tabacum cv. Samsun NN. In the present work, NC330 cDNA was cloned in an Agrobacterium binary vector and introduced into the genome of N. tabacum cv. Samsun nn that is susceptible to Tobacco mosaic virus (TMV). Eight R0 were highly resistant, with TMV titres less than 2% of the controls. The progeny of these primary transformants resulted in four TMV‐resistant and four susceptible lines. The progeny of the plant with highest resistance for TMV was selfed during four generations. In the R2 generation, out of 112 plants tested, 3 plants showed excellent resistance and 5 a lesser degree of resistance. NC330 transcripts were detected in these R2 plants as well as in their R1 and R0 parents. Two R2 plants were self‐pollinated to obtain the R3 generation. All the 25 R3 progeny plants of one line contained PCR‐amplifiable NC330, compared with only 6 of the 23 progeny plants from the other line. Following TMV inoculation, virus was undetectable in seven of the eight tested R3 progeny of the first line and five of the six R3 progeny plants of the second line. NC330 transcripts were detected in the TMV‐resistant transgenic plants but also in some susceptible plants. TMV resistance was suppressed when the plants were kept at higher temperatures (32–34°C), whereas returning the plants to lower temperatures (24–26°C) restored resistance. Transgenic plants resistant to TMV were also resistant to B. cinerea following inoculation of seedlings or of whole plants. The presence of the NC330 DNA was correlated with changes in the physiology of the transgenic plant. Seed germination was inhibited at high temperatures and in the dark. Rootlets of germinating seeds of the transgenic plants grew significantly faster than those of control seeds. These observations were correlated with a significantly higher concentration of abscisic acid in the seeds and seedlings of the transgenic plants. In silico analysis suggests that the NC330 protein translated from the NC330 cDNA is a tetratricopeptide repeats protein and that it may be involved in protein–protein interaction as part of the pathogen resistance mechanism.

Partial Resistance of Pepper to Bacterial Wilt Is Oligogenic and Stable Under Tropical Conditions.

Genetic analysis of resistance of pepper to bacterial wilt was performed in the doubled haploid progeny from a cross between a resistant parental line PM 687 and a susceptible cultivar Yolo Wonder. After artificial inoculation with a local isolate of Ralstonia solanacearum, the progeny consisting of 90 lines was transplanted into a naturally infested field in Guadeloupe, Lesser Antilles. The 2 years of experimentation resulted in repeatable results, with a high heritability of the resistance, attesting the reliability of the evaluation procedure and the stability of the resistance over years. Two to five genes with additive effects were estimated to control the resistance, indicating an oligogenic control as observed in tomato sources of resistance. Relationships with resistance to other soilborne or tropical diseases were examined. Susceptibility to Tobacco mosaic virus (TMV) and to nematodes (Meloidogyne spp.) were significantly linked with resistance to bacterial wilt, whereas neither resistance to Phytophthora capsici nor to Leveillula taurica were linked. The similarity of the genetics of resistance to bacterial wilt in pepper and tomato and linkage with TMV resistance locus warrant the comparative mapping of the resistance quantitative trait loci in the genomes of the two species.

Effects of homeopathic arsenic on tobacco plant resistance to tobacco mosaic virus.: Theoretical suggestions about system variability, based on a large experimental data set

Context: This research aimed at verifying the efficacy of homeopathic treatments by plant-based bioassays, which may be suitable for basic research, because they lack placebo effects and provide large datasets for statistical analyses. Objective: To evaluate the effects of homeopathic treatments of arsenic trioxide (As 2O 3) on tobacco plants subjected to tobacco mosaic virus (TMV) inoculation as biotic stress. Design: Blind, randomized experiment using tobacco leaf disks. Materials and methods: Tobacco plants ( Nicotiana tabacum L. cultivar Samsun) carrying the TMV resistance gene N. TMV inoculated leaf disks were floated for 3 days in the following: • Distilled water (control) • H 2O 5 and 45 decimal and centesimal potencies • As 2O 3 5 and 45 decimal and centesimal potencies The main outcome measures is the number of hypersensitive lesions observed in a leaf disk. Results: Homeopathic treatments of arsenic induce two effects on the plant: (i) increased resistance to TMV; (ii) decrease variability between experiments (system variability). Conclusions: In this experimental model two actions of homeopathic treatment were detected: decrease in system variability and enhancement of the natural tendency of the system towards an ‘equilibrium point’.

Genetic modification of alternative respiration has differential effects on antimycin A-induced versus salicylic acid-induced resistance to Tobacco mosaic virus.

Salicylic acid (SA), a natural defensive signal chemical, and antimycin A, a cytochrome pathway inhibitor, induce resistance to Tobacco mosaic virus (TMV). Pharmacological evidence suggested signaling during resistance induction by both chemicals involved alternative oxidase (AOX), sole component of the alternative respiratory pathway (AP). Roles of the AP include regulation of intramitochondrial reactive oxygen species and maintenance of metabolic homeostasis. Transgenic tobacco (Nicotiana tabacum) with modified AP capacities (2- to 3-fold increased or decreased) showed no alteration in phenotype with respect to basal susceptibility to TMV or the ability to display SA-induced resistance to systemic viral disease. However, in directly inoculated tissue, antimycin A-induced TMV resistance was inhibited in plants with increased AP capacities, whereas SA and antimycin A-induced resistance was transiently enhanced in plant lines with decreased AP capacities. We conclude that SA-induced TMV resistance results from activation of multiple mechanisms, a subset of which are inducible by antimycin A and influenced by AOX. Other antiviral factors, potentially including the SA-inducible RNA-dependent RNA polymerase, are regulated by AOX-independent mechanisms.

The tobacco mosaic virus resistance gene, N

Summary In this mini review we discuss recent advances in the understanding of the N gene-mediated resistance to tobacco mosaic virus (TMV). The tobacco N gene belongs to toll-interleukin-1 receptor homology/nucleotide binding/leucine rich repeat (TIR-NB-LRR) class of resistance genes. It encodes two transcripts, N(S) and N(L), by alternative splicing, both of which are required to confer resistance to TMV. The structure-function analysis of the N gene indicates that the TIR, NB and LRR domains are indispensable for its function. The N gene response is elicited by the C-terminal helicase domain of the 126 kDa TMV replicase protein. Tobacco N gene can also confer resistance to TMV in heterologous plants like tomato and Nicotiana benthamiana. Recent studies on N-mediated signalling suggest that EDS1, Rar1 and NPR1 genes play an important role in TMV resistance. Finally, we discuss current status of the N-mediated signal transduction and speculate directions for future work to understand N-TMV interaction.

Structure-function analysis of the tobacco mosaic virus resistance gene N.

The tobacco N gene is a member of the Toll-interleukin-1 receptor/nucleotide-binding site/leucine-rich repeat (TIR-NBS-LRR) class of plant resistance (R) genes and confers resistance to tobacco mosaic virus (TMV). We investigated the importance of specific domains of N in inducing TMV resistance, by examining various N deletion and point mutations that introduce single amino acid substitution mutants in vivo. Our deletion analysis suggests that the TIR, NBS, and LRR domains play an indispensable role in the induction of resistance responses against TMV. We show that amino acids conserved among the Toll/IL-1R/plant R gene TIR domain and NBS-containing proteins play a critical role in N-mediated TMV resistance. Some loss-of-function N alleles such as the TIR deletion and point mutations in the NBS (G216A/E/V/R, G218R, G219D, K222E/N, and T223A/N) interfere with the wild-type N function and behave like dominant negative mutations. These F(1) plants mount a hypersensitive response (HR) that is indistinguishable from that of the wild-type N plants, yet TMV was able to move systemically, causing a systemic hypersensitive response (SHR). Many amino acid substitutions in the TIR, NBS, and LRR domains of N lead to a partial loss-of-function phenotype. These mutant plants mount delayed HR compared with the wild-type N plants and fail to contain the virus to the infection site. In addition, some partial loss-of-function alleles (W82S/A, W141S/A, G218V/S, and G219V) interfere with the wild-type N function, leading to SHR. The partial loss-of-function and dominant negative mutant alleles described in this report will be useful in furthering our understanding of the TIR-NBS-LRR class of R genes.

Effects of Two Introgressed Disease Resistance Factors on Agronomic Characteristics and Certain Chemical Components in Burley Tobacco1

Four burley tobacco (Nicotiana tabacum L.) selections differing for resistance or susceptibility to tobacco mosaic virus (TMV) and wildfire (Pseudomonas tabaci L., Wolf and Foster, Stevens) were developed in each of three genetic backgrounds and later evaluated to determine if introgression of genetic factors for disease resistance affected any of six agronomic characteristics and three chemical components. The selections were derived from crosses between burley tobacco isolines possessing the genetic factors, transferred from N. glutinosa L. and N. longiflora Cav., for resistance to TMV and wildfire, respectively. The four selections, the two isolines, and the original susceptible parent of each of the three families were grown for 2 years at Lexington, KY. Since the selections showing susceptible reactions to both mosaic and wildfire performed essentially the same as the original susceptible cultivars, we concluded that the parental genotypes were recovered and that comparisons of the resistant selections with their corresponding isolines and parental cultivars were valid. Consistent differences between the selections and the original cultivar were noted only in one family, and the majority of those differences favored the selections. The wildfire resistant isoline in one family was significantly different from the selections and the parental cultivar. The effect of the introgressed genetic factors for TMV and wildfire resistance on the traits evaluated depended on genetic background. However, the presence of both TMV and wildfire resistance in one breeding line should be considered no obstacle to the further improvement of most other traits.

Quantity production of anther-derived haploids from a multiple disease resistant tobacco hybrid. I. Frequency of plants with resistance or susceptibility to tobacco mosaic virus (TMV), potato virus Y (PVY), and root knot (RK)

Methods are described for producing large numbers of haploid plantlets from anthers of a flue-cured tobacco hybrid with monogenic resistance to tobacco mosaic virus, (TMV), potato virus Y (PVY) and root knot (RK), respectively. Additional details are given on colchicine treatment for converting haploids to doubled haploids (DH's) and on the frequency of spontaneous DH's among untreated plantlets. Disparate genetic ratios of TMV-resistant to TMV-susceptible plants were obtained among colchicine-treated haploid plantlets, induced DH's and untreated haploids when compared with F2 and BC1 progenies. Haploids (gametes) with the gene for TMV resistance occurred more frequently than expected and plantlets with the gene for RK resistance occurred less frequenctly than expected. Transmission of the gene for PVY resistance differed only slightly from Mendelian expectations. These unexpected ratios, in addition to the frequent occurrence of plastid chimeras among anther-derived plantlets, strengthened our conviction that haploidy is somehow associated with mutation.

The establishment of a new strain of tobacco mosaic virus resulting from the use of resistant varieties of tomato

SUMMARYTomato varieties with monogenic resistance (tolerance) to tobacco mosaic virus (TMV) were used by British growers for the first time in 1966. Samples of TMV collected before 1966, and in 1967 and 1968, were tested for their ability to produce virus symptoms on a series of isogenic tomato differential hosts differing in three factors for TMV resistance. Samples collected before 1966 yielded only strain o of TMV, which was unable to overcome any of the three factors. Strain 1, able to infect the newly‐introduced resistant varieties, was found with increased frequency after 1966. It was found less frequently on nurseries which had resumed growing susceptible varieties, suggesting that in these it may be unable to compete successfully with strain o. The dangers of indiscriminate release of varieties protected from disease by only one gene are discussed.