Hamed Doostdar

Elicitors of Plant Defensive Systems Reduce Insect Densities and Disease Incidence

Some elicitors of plant defensive systems can induce biochemical changes that enable the plant to reduce disease incidence; however, little is known about the effect of these induced responses on insect herbivores. We approached this problem using exogenous field applications of several abiotic elicitors of defensive systems in tomatoes (Lycopersicon esculentum), and evaluated the ability of the elicitors [benzo(1,2,3)thiadiazole-7-carbothioic acid (S)-methyl ester (BTH, Actigard); Probenazole; chitosan; salicylic acid; KeyPlex 350; KeyPlex DP2; and KeyPlex DP3] to reduce pest densities and to provide cross-resistance against various insect herbivores and pathogens. Only BTH provided cross-resistance and significantly reduced the incidence of bacterial spot (Xanthomonas campestris pv. vesicatoria), early blight (Alternaria solani), leaf mold (Fulvia fulva), and leafminer larval densities (Liriomyza spp.). The effects on leafminer larval densities were more pronounced during the early stages of plant development. A trend of reduced densities of whiteflies (Bemisia argentifolii) and powdery mildew (Oidium sp.), although not significant, was also found on the BTH-treated plants. Other elicitors had no significant effect on insect populations, but Probenazole and KeyPlex 350 significantly reduced bacterial spot and early blight incidence. The antiherbivore effects of BTH on leafminers was confirmed in a laboratory two-choice experiment. Adult leafminers preferred untreated plants to the BTH-treated tomatoes as ovipositioning host plants, generally corresponding with larval performance. BTH induced high levels of pathogenesis-related proteins in tomato plants including peroxidase, lysozymes, chitinase, and β-1,3-glucanases. The possible cross-resistance role of these proteins is discussed. The demonstration that exogenous induction of plant defensive systems in the field can result in lower damage caused by various pathogens and insects, supports the hypothesis that plant defensive systems may be general.

The Role of Plant Rapidly Induced Responses in Asymmetric Interspecific Interactions Among Insect Herbivores

The role of induced responses of tomato, Lycopersicon esculentum, in interspecific interactions between two polyphagous herbivores, the silverleaf whitefly, Bemisia argentifolii (WF), and the vegetable leafminer, Liriomyza trifolii (LM), was characterized in laboratory and field experiments. Feeding by LMs and WFs induced local and systemic production of putative defensive proteins, i.e., chitinases, peroxidases, β-1,3-glucanases, and lysozymes. The magnitude of the induction for each defensive protein varied between species. Unlike WFs, LMs caused a 33% local reduction in total foliar protein content. In a whole-plant choice experiment, adult LM feeding, oviposition, and larval survival were reduced by 47.7%, 30.7%, and 26.5%, respectively, for the WF-infested host compared with the controls. Early WF infestations also had negative systemic (plant-mediated) effects on LMs. Adult LMs preferred leaves from control plants to leaves of plants that had been previously infested with WFs; no reciprocal effect of LMs on WFs were found. Feeding by Helicoverpa zea larvae, which has been shown previously to affect LM performance, had no effect on WF survival and development. LM natural population dynamics were monitored on WF-preinfested and control plants in a field experiment. WF-infested plants were less suitable for LM development with an overall 41% reduction in LM population density. These results demonstrate asymmetric direct and plant-mediated interspecific interactions between generalist herbivores feeding simultaneously on the same host. Possible mechanisms by which WFs overcome plant defenses are suggested. This ability may also contribute to WF success that makes them a major pest worldwide. The study supports the idea that over an evolutionary time scale, herbivores sharing the same host plant will automatically compete.

Induction of systemic acquired resistance in cotton by BTH has a negligible effect on phytophagous insects

Whether or not chemical changes in plants in response to pests (insects and pathogens) are general or specific remains unclear. Some evidence indicates that an induced response (IR) to arthropods via the octadecanoid pathway represents a distinct mechanism from the salicylic acid‐based pathway of systemic acquired resistance (SAR) to pathogens. To further test this hypothesis, young cotton seedlings were activated with benzo (1,2,3) thiadiazole‐7‐carbothioic acid (S) methyl ester (BTH), an elicitor of SAR. The enzymatic activities of a number of pathogenesis‐related (PR) proteins in young and old leaves of control and BTH treated plants were measured. BTH applications elicited marked increases in the activity levels of chitinase, peroxidase, and β‐1,3‐glucanase both locally and systemically. The highest levels of induction were detected systemically in young leaves. Except for some local effects on whitefly oviposition, the induction of SAR by BTH had no effect on either host preference of whiteflies Bemisia tabaci (Gennadius) or on feeding efficiency of cotton bollworms Helicoverpa armigera (Hübner). We conclude that SAR induction via the salicylic acid pathway in ‘Acala’ cotton has negligible effect on the tested insect herbivores.

Immersion of cucumber fruit in heated water alters chilling-induced physiological changes

Abstract Cucumber (Cucumis sativus cv. Dasher 3) fruit were immersed in water at 25, 38, or 42 °C for 30 min prior to storage at 12 (nonchilling) or 2.5 °C (chilling) for two weeks to determine the effects of these treatments on chilling-induced changes in electrolyte leakage, and CO2, and ethylene evolution. Storage at 2.5 °C resulted in a significant increase in electrolyte leakage, which decreased significantly as immersion temperature increased. Chilled fruit had higher rates of CO2 production than did nonchilled fruit following transfer to 21 °C, but there was no difference due to immersion temperature. Following transfer to 21 °C, nonchilled fruit produced no detectable ethylene whereas chilled fruit produced significant amounts of ethylene. The amount of chilling-induced ethylene production decreased with increased temperature of immersion, and this difference persisted for at least 72 h after transfer to 21 °C. The amount of 1-aminocyclopropane-1-carboxylic acid (ACC) was significantly higher in chilled than in nonchilled fruit although the amount of ACC in chilled fruit decreased significantly as immersion temperature increased. Nonchilled fruit had significantly higher ACC oxidase activity than did chilled fruit at the time of transfer to 21 °C, and ACC oxidase activity decreased as immersion temperature increased.

EFFECT OF DIETARY COPPER ON LARVAL DEVELOPMENT OF DIAPREPES ABBREVIATUS (COLEOPTERA: CURCULIONIDAE)

Abstract Larvae of the Diaprepes root weevil, Diaprepes abbreviatus (L.), were reared from hatching on an artificial diet containing four concentrations of two copper compounds, cupric sulfate (CuSO4) or cupric hydroxide [Cu(OH)2]. Negative effects of copper on insect development were observed only for early instars. Survival of larvae from hatching to 4 weeks of age was significantly affected by the copper compounds compared with the artificial diet alone, and greater mortality was associated with CuSO4 compared with Cu(OH)2. The two compounds had equivalent effects on larval weight gain of early instars. Weight gain was negatively correlated with increasing copper concentration. No effect of copper was observed on late instars maintained on these diets beyond the initial 4 weeks. Larval and pupal period, weight gain, and survival of late instars were statistically similar. No effect on larval survival or weight gain was observed when copper solutions were applied at nonphytotoxic levels to two varieties of citrus rootstock. The potential for manipulating citrus tree copper content to control this pest is discussed.

SUSCEPTIBILITY OF DIAPREPES ABBREVIATUS (COLEOPTERA: CURCULIONIDAE) TO A COMMERCIAL PREPARATION OF BACILLUS THURINGIENSIS SUBSP. TENEBRIONIS

Abstract A commercial preparation of the microbial entomopathogen, Bacillus thuringiensis subsp. tenebrionis (Btt) was evaluated for biological activity against the Diaprepes root weevil, Diaprepes abbreviatus (L.). A reduction in survival was observed for neonatal larvae exposed to insect diet incorporated with Btt and in potted citrus treated with a Btt soil application. A treatment-induced, weight gain reduction for neonates was indicated only in the diet assay. Larvae exposed at 5 weeks old to diet treated with Btt demonstrated a dose-dependent mortality response. The mean ages for larval death ranged from 111 to 128 days among treatments. The LC50 for larvae in this age group was 6.2 ppm [AI] and the slope of the probit line was 2.29. The mortality response of larvae exposed at 12 weeks old also was dose dependent and the mean ages for larval death ranged from 130 to 141 days among treatments. The LC50 for larvae in this age group was 25.4 ppm [AI] and the slope of the probit line was 2.75. The delayed patterns of mortality that we observed among larvae treated at 5 and 12 weeks old indicates that disease is slow to develop in older larvae but that death occurs before maturation is completed.

Biochemical, Molecular Genetic, and Immunological Characterization of a β-1,3-Endoglucanase from ‘Valencia’ Orange Callus

Summary We have purified a β-1,3-endoglucanase (EC 3.2.1.39) from nonembryogenic Citrus sinensis (L.) Osbeck cv. Valencia callus to electrophoretic homogeneity by means of pH precipitation and ion exchange chromatography. The protein has an apparent Mr of 32,000, a pl > pH 10 and is serologically similar to a potato leaf glucanase induced by Phytophthora infestans infection. The enzyme hydrolyzes laminarin (Lam- inaria digitata) optimally at pH 5 and 50 °C. The enzyme will hydrolyze pachyman and laminarin extensively and yeast glucan slightly, but does not hydrolyze lichenin, barley glucan, cellulose, or starch. Product characterization by thin-layer chromatography indicates that the enzyme is an endohydrolase. The protein is N-terminal blocked, however, partial internal amino acid sequence analysis revealed that the peptide shared homology with a number of β-1,3-endoglucanases. Antibody to the purified protein was raised in a rabbit and used to screen an amplified cDNA library prepared from Citrus sinensis (L.) Osbeck cv. Valen- cia callus. A positive clone (pBGVC-1) containing a 1,249 by insert was isolated. A full length sequence of the clone was obtained and it contained a 1,229 by open reading frame starting at nucleotide 20. Sequence analysis indicated that the clone is homologous to other 0-1,3-endoglucanase genes. The predicted amino acid sequence was homologous with other 0-1,3-glucanases, contained both N- and C-terminal signal sequences, the glycosyl hydrolase family 17 signature sequence, and the sequence identical to the peptide that was sequenced from the purified protein.