Salicylic Acid Induces Resistance to Alternaria solani in Hydroponically Grown Tomato

ABSTRACT Alternaria solani is the causal agent of early blight disease in tomato and is responsible for significant economic losses sustained by tomato producers each year. Because salicylic acid (SA) is an important signal molecule that plays a critical role in plant defense against pathogen invasion, we investigated if the exogenous application of SA would activate systemic acquired resistance (SAR) against A. solani in tomato leaves. The addition of 200 muM SA to the root system significantly increased the endogenous SA content of leaves. Free SA levels increased 65-fold over basal levels to 5.85 mug g(-1) fresh weight (FW) after 48 h. This level of SA had no visible phytotoxic effects. Total SA content (free SA + SA-glucose conjugate) increased to 108 mug g(-1) FW after 48 h. Concomitant with elevated SA levels, expression of the tomato pathogenesis-related (PR)-1B gene was strongly induced within 24 h of the addition of 200 muM SA. PR-1B expression was still evident after 48 h; however, PR-1B induction was not observed in plants not receiving SA treatment. Challenge inoculation of SA-treated tomato plants using conidia of A. solani resulted in 83% fewer lesions per leaf and a 77% reduction in blighted leaf area as compared with control plants not receiving SA. Our data indicate that root feeding 200 muM SA to tomato plants can (i) significantly elevate foliar SA levels, (ii) induce PR-1B gene expression, and (iii) activate SAR that is effective against A. solani.     

Effect of salicylic acid treatment on tomato plant physiology and tolerance to potato virus X infection

Salicylic acid (SA) is an inducer of systemic acquired resistance (SAR) and could be a potential candidate in the control of plant virus diseases. In this study we assayed under controlled conditions the potential effect of three doses of exogenous SA treatment on tomato plants infected with Potato virus X (PVX) and measured their effects on: different physiological parameters (gas exchange, stable isotopes, chlorophyll content), the activation of secondary metabolism, viral accumulation and induction of the expression of pathogenesis-related proteins (PRs) such as ß-1, 3-glucanase (PR2) and chitinase (PR3). SA treatment increased the expression of PR2, the activity of phenylalanine ammonia lyase (PAL) and the concentration of antioxidant compounds at 7 days post-treatment. Earlier expression of PR3 compared to PR2 was observed. SA treatment delayed the detection of PVX by ELISA in uninoculated leaves of mechanically infected tomato plants. Although the effect of PVX infection on physiological parameters was weak, moderate SA treatments showed enhanced photosynthesis, particularly for infected plants. The results obtained confirm that SA promotes major changes in the induction of resistance in tomato plants and suggest that treatment with exogenous SA could be considered to reduce the infections caused by PVX.     

Up-regulation of antioxidants in tobacco by low concentrations of H₂O₂ suppresses necrotic disease symptoms

Pretreatment of tobacco leaves with low concentrations (5 to 10 mM) of H?O? suppressed hypersensitive-type necrosis associated with resistance to Tobacco mosaic virus (TMV) or Pseudomonas syringae pv. phaseolicola. The same pretreatment resulted in suppression of normosensitive necrosis associated with susceptibility to Botrytis cinerea. This type of H?O?-mediated, induced disease symptom resistance correlated with enhanced host antioxidant capacity, i.e., elevated enzymatic activities of catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (POX) after viral and bacterial infections. Induction of genes that encode the antioxidants superoxide dismutase (SOD), CAT, and APX was also enhanced early after TMV infection. Artificial application of SOD and CAT suppressed necroses caused by viral, bacterial, or fungal pathogens similarly as H?O? pretreatment, implying that H?O?-mediated symptom resistance operates through enhancement of plant antioxidant capacity. Pathogen multiplication was not significantly affected in H?O?-pretreated plants. Salicylic acid (SA), a central component of plant defense, does not seem to function in this type of H?O?-mediated symptom resistance, indicated by unchanged levels of free and bound SA and a lack of early up-regulation of an SA glucosyltransferase gene in TMV-infected H?O?-pretreated tobacco. Taken together, H?O?-mediated, induced resistance to necrotic symptoms in tobacco seems to depend on enhanced antioxidant capacity.     

黄瓜花叶病毒M株系引致烟草症状恢复的初步研究

 黄瓜花叶病毒M株系在白肋烟上具有典型的症状恢复现象,本研究用提纯病毒和DAS-ELISA建立了CMV-M病毒定量检测方法。研究发现,症状严重程度与叶片中的病毒浓度呈正相关:最早发病的黄化叶每克病组织中病毒可高达790 μg,而恢复叶片上部再发病的花叶症状病叶中每克病组织中病毒也可高达508 μg,恢复叶片中病毒含量很低,每克叶片中最高也没有超过6 μg,仅为发病叶片病毒浓度的1/85~1/135,远低于根和茎中的病毒浓度。RT-PCR和生物学检测结果表明恢复叶片中确实存在具侵染活性的病毒,而且病毒在长达半个月以上一直保持很低浓度。结果表明恢复叶中可能存在有效的病毒防御机制,其具体机理有待进一步研究。     

Biological and molecular analyses of the acibenzolar-S-methyl-induced systemic acquired resistance in flue-cured tobacco against Tomato spotted wilt virus

Tomato spotted wilt virus (TSWV) is an economically important virus of flue-cured tobacco. Activation of systemic acquired resistance (SAR) by acibenzolar-S-methyl (ASM) in flue-cured tobacco was studied under greenhouse conditions by challenge inoculation with a severe isolate of TSWV. ASM restricted virus replication and movement, and as a result reduced systemic infection. Activation of resistance was observed within 2 days after treatment with ASM and a high level of resistance was observed at 5 days onward. Expression of the pathogenesis-related (PR) protein gene, PR-3, and different classes of PR proteins such as PR-1, PR-3, and PR-5 were detected at 2 days post-ASM treatment which inversely correlated with the reduction in the number of local lesions caused by TSWV. Tobacco plants treated with increased quantities of ASM (0.25, 0.5, 1.0, 2.0, and 4.0 g a.i./7,000 plants) showed increased levels of SAR as indicated by the reduction of both local and systemic infections by TSWV. The highest level of resistance was at 4 g a.i., but this rate of ASM also caused phytotoxicity resulting in temporary foliar spotting and stunting of plants. An inverse correlation between the TSWV reduction and phytotoxicity was observed with the increase of ASM concentration. ASM at the rate of 1 to 2 g a.i./7,000 plants activated a high level of resistance and minimized the phytotoxicity. Use of gibberellic acid in combination with ASM reduced the stunting caused by ASM. Present findings together with previous field experiments demonstrate that ASM is a potential option for management of TSWV in flue-cured tobacco.     

利用RNA介导的抗病性获得高度抗马铃薯Y病毒的转基因烟草

 以马铃薯Y病毒坏死株系(PVY^N)的RNA为模板,应用反转录-聚合酶链式反应(RT-PCR)方法,扩增出长度为801 bp的非翻译的马铃薯Y病毒外壳蛋白基因。将扩增的片段克隆到pBSK的BamHI和KpnI之间并进行了序列测定。用BamHI和KpnI从重组克隆载体上切下该基因并插入到质粒pROKII内得到植物表达载体pPVYCP。通过根癌农杆菌(LBA4404)介导的方法转化烟草NC89,经卡那霉素抗性筛选、PCR和Southern blot检测,获得82株转基因植株。Northern blot和Western blot分析表明,转基因植株只在RNA水平上得到了表达。抗病性试验表明转基因植株之间抗性水平存在着差异,其中有7株是对PVY^N高度抗病性的植株。转基因植株抗病特异性试验初步表明,对PVY^N表现高度抗病的植株对PVY^O也具有高度抗病性。     

The effects of co‐infection by different Potato virus Y (PVY) isolates on virus concentration in solanaceous hosts and efficiency of transmission

The influence of co‐infection on concentration and accumulation of genetically different isolates of Potato virus Y (PVY) in potato and tobacco plants and the efficiency of transmission by Myzus persicae of PVY isolates from doubly versus singly infected plants were evaluated. The vector ability to simultaneously transmit two virus isolates was examined. Eight PVY isolates represented three strain groups: PVY^O (pathotype and serotype O), PVY^NW (pathotype N and serotype O), and PVY^NTN (pathotype and serotype N). Different diagnostic methods, including DAS‐ELISA, multiplex RT‐PCR, aphid transmission tests and bioassays, were applied to detect the presence of PVY isolates in source and assay plants. Significant reductions in concentrations of certain PVY isolates during co‐infection with other isolates were found both in potato and tobacco plants. The observed effects were both isolate‐ and host‐dependent in form. The highest rates of virus transmission by single aphids were recorded with PVY^NTN isolates, and the lowest ones with PVY^O isolates. Individual aphids of M. persicae were able to simultaneously transmit two PVY isolates. The frequency of transmission was generally low, but it reached as high as 20% for one of the isolate combinations. The findings presented in the work provide proof for antagonistic within‐plant interactions between isolates of PVY, with some implications of these interactions for virus transmission by aphid vectors. Consequently, this research contributes to a better understanding of the epidemiology of the disease caused by PVY.     

The movement of potato virus Y (PVY) in the vascular system of potato plants

Potato virus Y (PVY) is responsible for major viral diseases in most potato seed areas. It is transmitted by aphids in a non-persistent manner, and it is spread in potato fields by the winged aphids flying from an infected source plant to a healthy one. Six different PVY strains groups affect potato crops: PVY^C, PVY^N, PVY^O, PVY^N:O, PVY^NTN, and PVY^N-Wi. Nowadays, PVY^NTN and PVY^N-Wi are the predominant strains in Europe and the USA. After the infection of the leaf and accumulation of the virus, the virus is translocated to the progeny tubers. It is known that PVY^N is better translocated than PVY^O, but little is known about the translocation of the other PVY strains. The translocation of PVY occurs faster in young plants than in old plants; this mature plant resistance is generally explained by a restriction of the cell-to-cell movement of the virus in the leaves. The mother tuber may play an important role in explaining mature plant resistance. PVY is able to pass from one stem to the other stems of the same plant through the vascular system of the mother tuber, but it is unknown whether this vascular link between stems is permanent during the whole life of the plant. Two greenhouse trials were set up to study the spread of PVY in the vascular system of the potato plant. The PVY-susceptible cultivar Charlotte was used for both trials. It was demonstrated that all stems growing from a PVY-infected tuber will become infected sooner or later, and that PVY^N-Wi translocates more efficiently to progeny tubers than PVY^NTN. It was also demonstrated that the progressive decay of the mother tuber in the soil reduces the possibility for virus particles to infect healthy stems through the vascular system of the mother tuber. This new element contributes to a better understanding of the mechanism of mature plant resistance.     

Systemic resistance induced in Arabidopsis thaliana by Trichoderma asperellum SKT-1, a microbial pesticide of seedborne diseases of rice

BACKGROUND: Trichoderma asperellum SKT-1 is a microbial pesticide of seedborne diseases of rice. To investigate the mechanisms of disease suppression in SKT-1, the ability to induce systemic resistance by SKT-1, or its cell-free culture filtrate (CF), was tested using Arabidopsis thaliana Col-0 plants. RESULTS: Both SKT-1 and its CF elicit an induced systemic resistance against the bacterial leaf speck pathogen Pseudomonas syringae pv. tomato DC3000 in Col-0 plants. Involvement of plant hormones in the induced resistance by SKT-1 and CF was assessed using Arabidopsis genotypes such as the jasmonic acid (JA)-resistant mutant jar1, the ethylene (ET)-resistant mutant etr1, the plant impaired in salicylic acid (SA) signalling transgenic NahG and the mutant npr1 impaired in NPR1 activity. In soil experiments using SKT-1, no significant disease suppression effect was observed in NahG transgenic plants or npr1 mutant plants. Expression levels of SA-inducible genes such as PR-1, PR-2 and PR-5 increased substantially in the leaves of Col-0 plants. Expression levels of JA/ET-induced genes such as PDF1.2a, PR-3, PR-4 and AtVsp1 were also induced, but the levels were not as high as for SA-inducible genes. In a hydroponic experiment using CF from SKT-1, all Arabidopsis genotypes showed an induced systemic resistance by CF and increased expression levels of JA/ET- and SA-inducible genes in leaves of CF-treated plants. CONCLUSION: The SA signalling pathway is important in inducing systemic resistance to colonisation by SKT-1, and both SA and JA/ET signalling pathways combine in the signalling of induced resistance by CF. These results indicate that the response of A. thaliana is different from that found in root treatments with barley grain inoculum and CF from SKT-1. Copyright © 2011 Society of Chemical Industry     

Biocontrol activity and primed systemic resistance by compost water extracts against anthracnoses of pepper and cucumber

We investigated direct and indirect effects of compost water extracts (CWEs) from Iljuk-3, Iljuk-7, Shinong-8, and Shinong-9 for the control of anthracnoses caused by Colletotrichum coccodes on pepper and C. orbiculare on cucumber. All tested CWEs significantly (P < 0.05) inhibited in vitro conidial germination and appressorium formation of the fungal pathogens; however, DL-β-amino-n-butyric acid (BABA) failed to inhibit the conidial development of the pathogens. Direct treatments of the CWEs and BABA on pepper and cucumber leaves at 1 and 3 days before or after inoculation significantly (P < 0.05) reduced anthracnose severities; Iljuk-3, Shinong-9, and BABA for pepper and Iljuk-7 for cucumber had more protective activities than curative activities. In addition, root treatment of CWEs suppressed anthracnoses on the plants by the pathogens; however, CWE treatment on lower leaves failed to reduce the diseases on the upper leaves of the plants. The CWE root treatments enhanced not only the expression of the pathogenesis-related (PR) genes CABPR1, CABGLU, CAChi2, CaPR-4, CAPO1, and CaPR-10 in pepper and PR1-1a, PR-2, PR-3, and APOX in cucumber but also the activity of β-1,3-glucanase, chitinase, and peroxidase and the generation of hydrogen peroxide in pepper and cucumber under pathogen-inoculated conditions. However, the CWE treatments failed to induce the plant responses under pathogen-free conditions. These results indicated that the CWEs had direct effects, reducing anthracnoses by C. coccodes on pepper leaves and C. orbiculare on cucumber leaves through protective and curative effects. In addition, CWE root treatments could induce systemic resistance in the primed state against pathogens on plant leaves that enhanced PR gene expression, defense-related enzyme production, and hydrogen peroxide generation rapidly and effectively immediately after pathogen infection. Thus, the CWEs might suppress anthracnoses on leaves of both pepper and cucumber through primed (priming-mediated) systemic resistance.     

Examination of the leaf-drop symptom of virus-infected potato using anther culture-derived haploids

ABSTRACT Necrotic lesions and vein necrosis characteristic of the hypersensitive response (HR) controlled by the dominant resistance gene Ny develop in potato cv. Pito after infection with potato virus Y ordinary strain (PVY masculine) at a low temperature (16/18 degrees C night/day). In contrast, at high temperatures (19/24 degrees C night/day), large coalesced lesions develop in the lower infected leaves, which wither and remain hanging from stems forming the leaf-drop symptom; mosaic symptoms with no necrosis also develop in the top leaves. The genetic basis of the leaf-drop symptom and its dependence on temperature were examined using a novel approach involving 58 haploids (2n = 24) derived from 'Pito' (2n = 48) through anther culture. These haploids and 'Pito' were graft-inoculated with PVY(O) at 19/24 to 25 degrees C (night/day). Necrotic symptoms were expressed in 28 haploids, of which 18 haploids (phenotype class N) developed top necrosis, vein necrosis, or both and necrotic lesions that are characteristic of HR. Ten haploids showed leaf drop similar to 'Pito' (phenotype class LD). Thirty haploids were susceptible and showed only mosaic symptoms (phenotype class S). These data indicated that necrosis was induced by a single dominant gene, Ny, in the simplex condition. However, the three distinct phenotypic classes (N, LD, and S) among the haploids grown under the same environmental conditions showed that another locus (gene) was involved in modifying the HR triggered by Ny. Data suggested that this locus contains a dominant temperature-dependent modifier (Tdm) gene that alters the expression of PVY-induced HR at higher temperatures, resulting in leaf drop.     

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.     

Identification of the molecular make-up of the Potato virus Y strain PVY(Z): genetic typing of PVY(Z)-NTN

Potato virus Y (PVY) strains were originally defined by interactions with different resistance genes in standard potato cultivars. Five distinct strain groups are defined that cause local or systemic hypersensitive responses (HRs) in genetic background with a corresponding N gene: PVY(O), PVY(N), PVY(C), PVY(Z), and PVY(E). The nucleotide sequences of multiple isolates of PVY(O) and PVY(N) differ from each other by ≈8% along their genomes. Additionally, complete genome sequences of multiple recombinant isolates are composed of segments of parental PVY(O) and PVY(N) sequences. Here, we report that recombinant isolate PVY-L26 induces an HR in potato 'Maris Bard' carrying the putative Nz gene, and is not recognized by two other resistance genes, Nc and Ny(tbr). These genetic responses in potato, combined with the inability of PVY-L26 to induce vein necrosis in tobacco, clearly define it as an isolate from the PVY(Z) strain group and provide the first information on genome structure and sequence of PVY(Z). The genome of PVY-L26 displays typical features of European NTN-type isolates with three recombinant junctions (PVY(EU-NTN)), and the PVY-L26 is named PVY(Z)-NTN. Three typical PVY(NTN) isolates and two PVY(N) isolates, all inducing vein necrosis in tobacco, were compared with PVY-L26. One PVY(NTN) isolate elicited HR reactions in Maris Bard, similar to PVY-L26, while two induced a severe systemic HR-like reaction quite different from the quasi-symptomless reaction induced by two PVY(N) isolates. 'Yukon Gold' potato from North America produced HR against several PVY(NTN) isolates, including PVY-L26, but only late and limited systemic necrosis against one PVY(N) isolate. Consequently, according to symptoms in potato indicators, both PVY(Z) and PVY(NTN) isolates appeared biologically very close and clearly distinct from PVY(O) and PVY(N) strain groups.     

Menadione sodium bisulphite: a novel plant defence activator which enhances local and systemic resistance to infection by Leptosphaeria maculans in oilseed rape

Pretreatment of the first true leaves of oilseed rape plants ( Brassica napus cv. Bristol) with menadione sodium bisulphite (MSB) locally and systemically induced resistance, as shown by reduced lesion size and number, to infection by the fungal pathogen Leptosphaeria maculans , the causal agent of stem canker. Using a known systemic activator of salicylic acid-dependent PR-1 induction, acibenzolar- S -methyl (BTH; S -methylbenzo[1,2,3]thiadiazole-7-carbothiate) as a comparison, real-time PCR expression analysis of genes encoding a pathogenesis-related protein 1 ( PR-1 ) and an ascorbate peroxidase ( APX ) demonstrated a systemic enhancement of APX expression in MSB-pretreated plants, with no effect on PR-1 expression, suggesting augmented reactive oxygen species production in MSB-pretreated plants. The results demonstrate MSB to be an effective resistance activator in oilseed rape, and potentially useful for the control of stem canker.     

Effect of pokeweed antiviral protein (PAP) on the infection of plant viruses

At a concentration of 0.4 μg purified pokeweed antiviral protein (PAP)/ml, the formation of local lesions on tobacco leaves caused by tobacco mosaic virus infection was completely inhibited and at 25 ng PAP/ ml. 68% inhibition was still obtained. PAP protected plants from infection by viruses from seven virus groups-five RNA viruses: tobacco mosaic virus, cucumber mosaic virus, alfalfa mosaic virus, potato virus X and potato virus Y; and two DNA viruses: African cassava mosaic virus (ssDNA) and cauliflower mosaic virus (dsDNA). Virus infection was probably blocked by PAP at a very early stage. PAP infiltrated into the intercellular spaces through the lower surfaces of leaves inhibited infection by virus inoculated on the upper leaf surface, and partially prevented PVY transmission by aphids. However. PAP did not show any activity against two bacterial and six fungal pathogens.     

Effect of temperature on resistance to Potato virus Y in potato cultivars carrying the resistance gene Rychc

The effect of cultivation temperatures on the resistance reaction to three Potato virus Y strains (PVY^O, PVY^N and PVY^NTN) in potato cultivars carrying Ry_chc was examined. When potato plants carrying Ry_chc were cultivated at 22 °C, a few small necrotic spots developed on inoculated leaves by 5 days after mechanical inoculation (dpi), and systemic infection of a few symptomless plants was confirmed at 28 dpi by IC‐RT‐PCR. At 28 °C, distinct necrotic spots developed on inoculated leaves by 5 dpi, and systemic symptoms occasionally appeared at 28 dpi. Thus, high temperature weakens Ry_chc‐conferred resistance. However, the incidence of systemic infection and the titre of virus in resistant cultivars at 28 °C were lower than in a susceptible cultivar. In graft inoculation under high summer temperatures, some plants developed necrosis on the leaves and stem, but PVY was barely detected by RT‐PCR in leaves on potato carrying Ry_chc. When seedlings from progeny tubers of plants that were inoculated with PVY and grown in a greenhouse at >30 °C in the daytime were examined by ELISA and IC‐RT‐PCR, PVY was not detected in cultivars carrying Ry_chc. These results show that Ry_chc confers an extreme resistance to PVY strains occurring in Japan.     

Diversity Among Potato virus Y Isolates Obtained from Potatoes Grown in the United States

ABSTRACT Potato field isolates (Solanum tuberosum) of Potato virus Y (PVY) collected from the midwestern and western United States were characterized using serological, molecular, and biological assays. PVY field isolates were grouped into the previously defined categories: PVY(O), European PVY(NTN), North American PVY(NTN), and PVY(N:O) recombinant and four previously undefined groups. Studies reported here agree with published reports from Europe and elsewhere in North America as PVY isolates capable of causing veinal necrosis in tobacco indicator plants appear in high frequency. In contrast to European experiences, PVY tuber necrosis isolates have a PVY(O) coat protein rather than that of PVY(N). Several PVY(N:O) recombinant isolates induced potato tuber necrotic ringspot disease (PTNRD) in the highly susceptible potato cv. Yukon Gold. The PTNRD symptoms produced by these PVY(N:O) recombinants were atypical compared with lesions found on the same cultivar infected with either the European or North American PVY(NTN) isolates. These PVY(N:O) isolates produced a roughly circular, sunken necrotic lesion on the surface of the tuber instead of the typical external sunken ring pattern displayed by PVY(NTN) isolates. This study establishes the complex nature of PVY populations within the U.S. potato industry and clearly demonstrates the diverse nature of PVY in the United States.     

Influence of the amount of blackgram mottle virus in different tissues on transmission through the seeds of Vigna mungo

Blackgram mottle virus (BMoV) was transmitted via up to 16% of seeds of different cultivars of blackgram, as determined by seedling symptom tests. The percentage of seed infection by BMoV as determined by EL1SA was even higher. Seed transmission was highest in cv. PLU-277 (15.9%), followed by cvs T-9 (11.8%), PLU-213 (7.0%) and UH-81-7 (1.3%). Seed transmission was correlated with the amount of virus present in the embryonic axis and later in primary leaves. The presence of virus in the testa alone did not result in its transmission through seeds. Virus concentration in different tissues varied; the mean amount of virus in the three cultivars was found to be 48–1234 ng per embryonic axis, 15–24 ng per cotyledon, and 12–20 ng per testa. The infection of primary leaves through the seed also resulted in systemic infection if the amount of virus in primary leaves exceeded 100 ng/100 mg of tissue. Close agreement was found between the percentage of seedlings with systemic infection and the percentage of seeds and embryonic axes containing more than 100 ng virus. The cultivars that resisted seed transmission contained relatively small amounts of the virus in embryonic axes.