Greenhouse and field screening of wild<Emphasis Type="Italic">Lycopersicon</Emphasis> germplasm for resistance to the whitefly<Emphasis Type="Italic">Bemisia argentifolii</Emphasis>

Thirty-two accessions of wild tomato (Lycopersicon spp.) germplasm were evaluated for resistance to the whiteflyBemisia argentifolii Bellows ⇐p; Perring in a greenhouse choice bioassay. Density data were recorded for the adaxial and abaxial leaf surfaces for (i) all life stages ofB. argentifolii and (ii) types I, IV, V and VI trichomes. Individual plant selections (33 from 22 wild tomato accessions) with high resistance were subsequently tested in the field to verify the resistance found in the greenhouse screening. Resistance was defined by the density of all life stages of the whitefly observed on the eight leaflets sampled at nodes 5 and 7. Only type IV trichomes had a consistent (but low) and significant negative correlation between trichome density and whitefly density for various life stages. The highest whitefly resistance was observed inLycopersicon pennellii accessions LA 716, LA 1340 and LA 2560. The most resistantL. hirsutum f.typicum accessions were LA 1777 and LA 1353. http://www.phytoparasitica.org posting Dec. 9, 2002.     

Tomato partial resistance to Rhizoctonia solani involves antioxidative defense mechanisms

The resistance interactions of four tomato cultivars to five Rhizoctonia solani isolates, causing foot rot, were investigated. Priming H₂O₂ accumulation, peroxidase and ascorbate peroxidase activity was observed in CH Falat, as a partially resistant cultivar. Maximum peroxidase activity in CH Falat was observed at 24 h post inoculation (hpi) which was earlier than maximum phenolics accumulation at 72 hpi. In addition, application of peroxidase inhibitor reduced phenolics level. Therefore, peroxidase might be involved in phenolics production, as an effective resistance mechanism in our pathosystem.     

Purification of an infection-related acidic peroxidase from pearl millet seedlings

Higher basal level of peroxidase activity was observed in highly resistant pearl millet cultivar IP 18292. Upon inoculation with downy mildew pathogen, Sclerospora graminicola, up to 60% increase in peroxidase activity was observed in highly resistant seedlings over the period of time. Iso-electric focusing analysis revealed that, two acidic isozymes of peroxidase with the pI of 5.9 and 5.1 present only in IP 18292 pearl millet seedlings. Upon inoculation with downy mildew pathogen, accumulation of these to isozymes was increased. These results indicated the possible involvement of acidic peroxidase in pearl millet defense. To study the nature of the acidic peroxidase which increases upon inoculation was purified from seedlings of highly resistant pearl millet cultivar using DEAE–Sepharose and Sephadex G-100 columns. The purified enzyme has a molecular weight of 21.8 kDa on SDS–PAGE and has a pI of 5.1. The optimum pH for maximum peroxidase activity was found to be at pH 7.0 and was resistant to high temperature (27–60 °C). The K_m for H₂O₂ and V_max of the enzyme reaction were 5.26 mM and 322.58 units, respectively. Purified peroxidase enzyme was found to be CaCl₂ dependent and both MgCl₂ and ZnCl₂ showed inhibitory effect on enzyme activity. Sodium azide and EDTA inhibited the enzyme and EGTA found to be specific inhibitor of peroxidase.     

Resistance among accessions of the generalycopersicon andsolanum to four of the main insect pests of tomato in Israel

Laboratory, greenhouse and field experiments were conducted to identify plant accessions of the genusLycopersicon and ofSolarium pennellii resistant toSpodoptera littoralis (Boisd.),Plusia chalcites (Esp.),Heliothis armigera (Hbn.), andPhthorimaea operculella (Zeu.), four insect pests of the cultivated tomato. Percent survival, larval weight, duration of development, damage scores, etc., were the criteria used to determine the relative resistance of the examined accessions. TheL. hirsutum accession LA 1777 and theL. hirsutum f.glabratum accession LA 407 were found to be highly resistant to all four insect pests. TheS. pennellii accession LA 716 was found to be resistant to the first three insects, but only partially resistant toP. operculella. The possible mechanisms of resistance,i.e., the physical entrapment of larvae and the toxic action of phytochemicals in the various accessions, are discussed.     

Evaluation of molluscicidal activities of Arisaema tubers extracts on the snail Oncomelania hupensis

Four extracts of Arisaema erubescens tubers by acetic acetal (AAE), benzinum (BZE), n-butanol (NBE) and chloroform (CFE) were obtained to evaluate their molluscicidal activities against the snail Oncomlania hupensis. The responses of choline esterase (ChE), alkaline phosphatase (ALP), esterase (EST), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) to the extracts (NBE) were also investigated. In the four extracts (AAE, BZE, NBE and CFE), NBE showed the highest toxicity on the snails after 48 h exposure. NBE also showed the time- and concentration-dependent effect, for example, the LC₉₀ values of the NBE were decreased from 365.5 mg/L (24 h) to 36.4 mg/L (96 h). At the end of exposure to NBE (LC₅₀ concentration), the activities of ChE and ALP in snail tissues (cephalopodium and liver) decreased significantly. Isozyme electrophoresis profiles indicated that responses of isozymes (EST, SOD and GSH-Px) to NBE were more intense in liver than in cephalopodium. After 72 h exposure to NBE, the EST activity in snail liver decreased and some enzyme bands (EST1 and EST4) disappeared. But the activities of SOD 1 and GSH 2 in liver increased after 48 h exposure. The results indicated that NBE was the highest toxic component in the four extracts. The decline of the detoxification ability and the oxidative damage in snail tissues might be the main reason for the molluscicidal activities.     

Induction of disease resistance against Botrytis cinerea by heat shock treatment in melon (Cucumis melo L.)

The present study investigated resistance against Botrytis cinerea after heat shock treatment in melon plants. Heat shock at 50 °C for 20 s 0–24 h before inoculation resulted in maximal B. cinerea symptom reduction and peroxidase gene expression, which peaked 12 and 72 h post-treatment and decreased 24–48 h post-treatment, suggesting pathogenesis-related protein expression priming. Hot water dipping did not directly inhibit mycelia growth. Plants treated with 2-benzisothiazol-3(2H)-one 1,1-dioxide, which induces systemic acquired resistance, demonstrated higher peroxidase gene expression but no B. cinerea resistance, indicating possible involvement of additional novel mechanisms in heat shock-activated resistance of melon against B. cinerea.     

Genetic analysis of abamectin resistance in Tetranychus cinnabarinus

The carmine spider mite is the most serious crop mite pests in China. Abamectin has been used to control insects and mites worldwide but carmine spider mites, Tetranychus cinnabarinus, had developed resistance to it. Genetic research on insecticide resistance has been fundamental for understanding the resistance development, studying resistance mechanisms, and designing appropriate resistance management strategies to control insect pests. A resistant colony of T. cinnabarinus, RRG42, was established to examine the inheritance of abamectin resistance in T. cinnabarinus. The females of T. cinnabarinus were selected for bioassay using a slide dip method. After 42 generations of selection, the RRG42 strain was 8.7-fold resistant to abamectin compared with the susceptible strain (SS). The logarithm (log) concentration–probit response curve for F₁s from reciprocal crosses, of F₁RS and F₁SR, were inclined to that for SS and the degree of dominance (D) values for F₁s were −0.81 and −0.17. There was a significant difference in values of LC₅₀ and slope of log concentration–probit lines between F₁RS and F₁SR. The observed mortalities of BC1 (F₁RS♀ × RRG42♂) and BC1′ (F₁SR♀ × SS♂) were significantly different from the expected mortalities based on a monogenic resistance in the chi-square tests. The inheritance of abamectin resistance in T. cinnabarinus is incompletely recessive and may be controlled by more than one gene. The maternal or cytoplasmic effect may exist in the inheritance of resistance to abamectin in T. cinnabarinus.     

The role of salicylic acid in defense response of tomato to root-knot nematodes

Salicylic acid (SA) is involved in hypersensitive reactions of plants to incompatible pathogens and in systemic acquired resistance (SAR) after the attack of necrosis-inducing pests. The possible involvement of SA in defense responses of tomato to root-knot nematodes (Meloidogyne spp., RKNs) was investigated. SA was found not to be responsible for the inhibition of catalase (CAT) detected in the early stages of Meloidogyne-tomato incompatible interactions. CAT extracted from leaves was inhibited only after treatment of the seedlings with SA concentrations as high as 4 mM. Most of the amount of free SA found in plants after SA treatment was detected in the leaves. SA (0.2 mM) was found to cause a competitive inhibition of CAT only at high substrate (H₂O₂) concentrations. Under different conditions it did not affect, or even enhanced, the enzyme activity. Therefore, it is suggested that SA-mediated CAT inhibition does not operate early in resistance against RKN in tomato, although it might have a role in the consequent lesion formation. Plant uptake of SA was detected by immersion of roots of 1-month-old seedlings in aqueous solutions of SA and SA plus a soil humic acid. Considering the low level of free SA retained by roots, the capacity of exogenously provided SA to act as an elicitor of resistance to root pests is considered unlikely.     

Agrobacterium induces plant cell death in wheat (Triticum aestivum L.)

The symptoms of gall or hairy root do not occur in the interactions between wheat (Triticum aestivum L.) and other monocotyledonous plants, with Agrobacterium tumefaciens or Agrobacterium rhizogenes. However, both bacteria colonized wheat root surfaces at similar levels (2.0 × 10⁷ colony forming U g⁻¹ root) and grew without inhibition in suspension with intact or wounded wheat embryos or root segments present. Suspension-cultured wheat embryo cells grown in 7.4 m M O₂ displayed 23% cell death after 1 h exposure to Agrobacterium cells, while the extent of cell death with 2.1 m M O₂ averaged 8%. Cultured wheat embryo and root cells rapidly produced hydrogen peroxide (H₂O₂) when contacted with A. tumefaciens or A. rhizogenes. Production of H₂O₂ was lower at 2.1 m M O₂ than 7.4 mM O₂. Browning and autofluorescence of epidermal cells of callus derived from wheat embryos and wheat roots was observed after inoculation with Agrobacterium. An increase in ferulic acid was detected in the walls of roots exposed to Agrobacterium. However, neither lignin nor callose was detected by diagnostic staining methods. These findings suggest that Agrobacterium induced a resistance-like response in wheat that may reduce the efficacy of transformation and limit the normal symptom formation.     

Mechanisms of resistance in<Emphasis Type="Italic">Lycopersicon</Emphasis> germplasm to the whitefly<Emphasis Type="Italic">Bemisia argentifolii</Emphasis>

The silverleaf whiteflyBemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) [also known as strain B of the sweetpotato whiteflyB. tabaci (Gennadius)] is a major pest of tomatoes due to both feeding damage and transmission of plant viruses. Certain wild species ofLycopersicon have demonstrated high levels of resistance to the pest. Greenhouse studies were undertaken to quantify the effects on whitefly behavior and mortality of individual, resistant plants selected from three accessions ofL. pennellii (Corr.) D’Arcy (LA 1340, LA 1674 and LA 2560), five accessions ofL. hirsutum f.typicum Humb. & Bonpl. (LA 386, LA 1353, LA 1777, PI 127826 and PI 127827) and one accession ofL. hirsutum f.glabratum C.H. Mull. (PI 126449). In no-choice experiments, fewer adults settled on leaflets of the wild species and deposited 75–100% fewer eggs compared to the cultivated tomato,L. esculentum Mill. Adult mortality ranged from 77–100% on wild accessions but was only 1% onL. esculentum. Most dead adults were trapped in glandular trichome exudates. The effects of these resistant accessions onB. argentifolii were mechanically transferable by appressing the trichome exudates onto the leaves of the susceptible tomato, indicating an association between the factors mediating the resistance and the glandular trichomes. Laboratory studies evaluated the repellent, fumigant and residual toxic effects of representative constituents of trichome exudates onB. argentifolii adults by using selected concentrations and probit analyses. RC₅₀ values (estimated concentration to repel 50% of the adults) and LC₅₀ values for fumigant and residual toxicity indicated that 2-tridecanone had low levels of repellent and residual toxicity activity; that 2-undecanone had high levels of repellent and fumigant activity; and that ginger oil (composed, in part, of sesquiterpene hydrocarbons) had high levels of repellent and residual toxicity activity. These studies suggest that multi-factor resistance exists in wild tomato germplasm. By combining genetically the observed chemical constituents of resistance into a single germplasm, the resulting resistance may be more difficult forB. argentifolii to overcome. http://www.phytoparasitica.org     

Correlation between sensitivity of barley to Pyrenophora teres toxins and susceptibility to the fungus

Detached leaves of 25 barleys, ranging from highly susceptible to highly resistant to Pyrenophora teres f. teres and Pyrenophora teres f. maculata, were tested for their reaction to three phytotoxins isolated from cultures of the fungus: toxin A [ L, L - N -(2-amino-2-carboxyethyl)aspartic acid], toxin C (aspergillomarasmine A) and toxin B (anhydroaspergillomarasmine A). 0.75 m M toxin A caused mainly dark yellow chlorotic symptoms but little necrosis, whereas leaves treated with 0.25 m M toxin C developed distinct necrotic symptoms and zones of light yellow chlorosis. Toxin B is only weakly toxic, and toxin B and control solutions containing aspartic acid in the concentration of 0.75 m M did not cause any symptoms. The best differentiation between the barleys was obtained by scoring chlorosis after 120 h, and the optimal toxin concentrations for this differentiation were 0.75 m M toxin A and 0.25 m M toxin C, respectively. Results with different toxin concentrations inducing distinct variation in symptom expression indicate that the two toxins have different potencies as phytotoxins. The reaction of the barleys to toxins A and C correlated well with their reaction to infection by P. teres f. teres and P. teres f. maculata, suggesting that toxins A and C may be used to select resistant barley lines in the early stages of a breeding programme.     

Effect of miconazole,an antifungal agent,on allene oxide synthase: Inhibition,kinetics, and binding

We investigated the inhibition of allene oxide synthase (AOS), a key enzyme in jasmonic acid biosynthesis, by miconazole. Kinetic analysis indicated that miconazole was a mixed-type inhibitor of AOS with a K_i value of approximately 8.4 ± 0.2 μM. Analysis of the interactions between miconazole and AOS by optical difference spectroscopy revealed that miconazole binding induces type II binding spectra with a K_d value of approximately 6.0 ± 0.2 μM.     

Toxic effects of acephate on Paramecium caudatum with special emphasis on morphology,behaviour, and generation time

The continuous increase in the number of new chemicals as well as the discharges of solid and liquid wastes triggered the need for simple and inexpensive bioassays for routine testing. In recent years, there has been increasing development of methods (particularly rapid tests) for testing environmental samples. This paper describes the quick toxic evaluation of an organophosphorus insecticide, acephate (O,S-dimethyl acetylphosphoramidothioate) on Paramecium caudatum for acute and sub-acute toxicity studies with reference to morphology, behaviour, and its generation time. The lethal concentrations for 10 min and 2 h were determined by probit method, as 500 mg L⁻¹ and 300 mg L⁻¹, respectively. Higher concentrations of 10 min exposure caused cell lysis with disintegration of cell membrane and precipitation of protoplasm. Combination of conventional light microscopy and computerized video tracking systems were used to study the locomotor behaviour of paramecia. The test organism was under stress and exhibited an initial increase and subsequent decrease in the swimming speed when exposed to 1/4, 1/2, 3/4, and LC₅₀ concentrations for 10 min (125, 250, 375, and 500 mg L⁻¹, respectively). Similar changes were also noticed when paramecia were exposed to LC₅₀ for 2 h. In a separate set of experiments, the number of generations and generation time in 24 h was evaluated with respect to the different sub-lethal concentrations (30, 60, 120, and 240 mg L⁻¹). The number of generations decreased and generation time extended significantly in a concentration dependent manner. The results indicate that the Paramecium toxicity assay could be used as a complimentary system to rapidly elucidate the cytotoxic potential of insecticides. The major advantages associated with these tests are: they are inexpensive, simple, user-friendly, space saving, and seem to be attractive alternatives to conventional bioassays.     

Comparative molluscicidal action of extract of Ginko biloba sarcotesta,arecoline and niclosamide on snail hosts of Schistosoma japonicum

In search for new local plant molluscicides for the control of the vectors of schistosomiasis, we compared the molluscicidal action of the extract of Ginkgo biloba sarcotesta by benzinum (EGSB) to that of arecoline (ARE) and niclosamide (NIC) against Oncomelania hupensis snails. NIC showed the highest toxicity on snails with 24 h LC₅₀ vales of 0.12 mg/L and LC₉₀ of 0.98 mg/L, while the LC₅₀ and LC₉₀ of EGSB were much lower than that of ARE. Sublethal in vivo 24 h exposure to 40% and 80% LC₅₀ of NIC, EGSB and ARE altered the activities of different enzymes in different body tissues of snails. EGSB could significantly inhibit Choline esterase (ChE), Alanine aminotransferase (ALT), Alkaline phosphatase (ALP) and Malic dehydrogenase (MDH) activities both in the cephalopodium and liver. ARE could significantly cause a reduction in ChE, ALP activities in the cephalopodium and ChE, ALT, ALP, Succinodehydrogenase (SDH), MDH activities in the liver. NIC significantly altered activities of ChE, ALT, ALP, SDH, and MDH in the cephalopodium and ChE, ALT, ALP, SDH activities in the liver. All molluscicides could not affect Lactate dehydrogenase (LDH) activity in the cephalopodium and the liver. Maximum inhibition of ALT and MDH activities was found in the cephalopodium and liver of snails treated with 80% of 24 h LC₅₀ of EGSB. However, NIC and ARE caused maximum reduction in ALP and SDH activities, respectively. The results indicated that molluscicidal action of EGSB was different to that of ARE and NIC in some extent.     

In vivo growth and pathotoxin production by the maize pathogen Cochliobolus carbonum

Conidia of Cochliobolus carbonum secrete a toxin (HC-toxin) during appressorium formation on maize leaves. Plasma desorption mass spectrometry revealed that approximately 70 ng of toxin per 10⁶ conidia were secreted during the first 16 h of morphogenesis. Growth of the fungus was monitored microscopically. Extensive fungal growth occurred in the susceptible interaction by 24 h. In spite of a substantial amount of HC-toxin, the fungus failed to become established in the resistant host even after 36 h. Results suggest that the resistance conditioned by Hm1, which encodes a toxin reductase, causes inactivation of the toxin early in the interaction.     

A soluble carbohydrate elicitor from Blumeria graminis f. sp. tritici is recognized by a broad range of cereals

Wheat plants rapidly recognize pathogenic and non-pathogenic conidia of the powdery mildew fungusBlumeria (syn. Erysiphe)graminis on their leaf surfaces. This suggests that a chemical signal emanates from conidia at the pre-penetration stage of infection. Conidia of B. graminis f. sp. tritici were found to contain an elicitor that was easily washed off their surface. The elicitor activity is heat stable and could not be removed by phenol extraction. By contrast, elicitor activity is sensitive to periodate oxidation and partial acid hydrolysis suggesting that the elicitor activity resides in a carbohydrate moiety. Analysis of carbohydrates revealed mostly glucose, with smaller amounts of xylose and mannose. The glucosyl residues of the B. graminis elicitor were found to be linked (1 → 2)-, (1 → 4), and (1 → 6)-, with (1 → 4, 1 → 6)- branch point residues, and no 3-linked glucose residues were detected. As treatment with β -mannanase significantly reduced elicitor activity, mixed-linkage (1 → 4), (1 → 6)-mannosyl residues appeared to be important for elicitor activity. The B. graminis elicitor induced the expression of all defence-related genes tested in wheat and also induced resistance to subsequent attack by B. graminis f. sp. tritici. In contrast, a hypersensitive response was not induced by the elicitor in the absence or the presence of a challenging inoculum of B. graminis f. sp. tritici. The elicitor also induced the accumulation of thaumatin-like proteins in barley, oat, rye, rice and maize, but did not induce necrosis in any of these species. This suggests that the B. graminis elicitor represents a host non-specific determinant of non-self recognition in cereals activating general defence responses other than the hypersensitive reaction.     

Resistance to Botrytis cinerea in Solanum lycopersicoides is Dominant in Hybrids with Tomato, and Involves induced Hyphal Death

Botrytis cinerea causes gray mold disease and affects hundreds of plant species, including tomato (Lycopersicon esculentum). The wild nightshade, Solanum lycopersicoides, is cross compatible with tomato and is more resistant to B. cinerea, thus representing a potential source for crop improvement. Tests involving droplet inoculation of detached leaves and spray inoculation of entire seedlings demonstrated that resistance to B. cinerea varies among S. lycopersicoides accessions, with S. lycopersicoides LA2951 being the most resistant accession tested. Expression of resistance in the intergeneric hybrid (L. esculentum cv. 'VF36' × S. lycopersicoides LA2951) suggested that resistance is at least partially dominant in tomato. A green fluorescent protein-tagged B. cinerea strain was used for confocal microscopic comparison of infection in leaves of S. lycopersicoides and tomato. Even though S. lycopersicoides supported spore germination, there was evidence for hyphal lysis and death 3 days after inoculation, at a time when lesions were expanding on susceptible tomato plants. The reduced frequency of B. cinerea lesion spread on S. lycopersicoides explains why this fungus produced fewer spores in this wild nightshade than in tomato.     

Resistance to Taiwanese race 1 strains of Ralstonia solanacearum in wild tomato germplasm

A total of 252 wild Solanum accessions and one population of 49 introgression lines of LA716 were screened for resistance to a race 1/biovar 4/phylotype I strain Pss186 of Ralstonia solanacearum. Most wild tomato accessions were highly susceptible. However, five accessions of S. pennellii, i.e. LA1943, LA716, LA1656, LA1732 and TL01845 were resistant to strain Pss186. These accessions were then challenged against two other race 1/phylotpye I strains Pss4 and Pss190, which were more aggressive. All the five S. pennellii accessions were susceptible to Pss4, but displayed high to moderate resistance to Pss190 with a percentage of wilted plants ranging from 0% to 60%. Pss190 is an aggressive strain that made a resistant tomato line Hawaii 7996 susceptible. Thus, the results found in this study provide evidence of the presence of strain-specific resistance. LA3501, which has an introgression segment on chromosome 6, was found to be resistant to Pss186 among the screened introgression lines. This confirms the importance of resistance trait loci on chromosome 6 that have been identified by other studies. This is the first report of S. pennellii being resistant to bacterial wilt. These new resistant sources will provide breeders with more resources to breed for stable resistance to bacterial wilt of tomato.