Kinetic Characterization of Anticarsia gemmatalis Digestive Serine- Proteases and the Inhibitory Effect of Synthetic Peptides

BACKGROUND

Enzyme kinetics contributes to understanding the structure and function of insect digestive serine proteases. Kinetic parameters allow to understanding active sites and mechanisms of enzymes efficacy, identifying the inhibition of the insects digestive protease system by inhibitors produced by plants, or via the application of synthetic inhibitors Objectives: The aim of this study was to purify digestive serine proteases of A. gemmatalis, determining their kinetic properties using the chromogenic substrates tripeptidyl and characterizing the effects of synthetic inhibitors on their activity. In order to provide new opportunities for sustainable pest management through the development of protease inhibitors.

METHODS

The enzymes were purified on p-aminobenzamidine agarose affinity column in an FPLC system using electrophoresis with 12.5% polyacrylamide gel. Michaelis-Menten constants and the inhibition model were determined according to the Dixon methodology and Lineweaver-Burk's double reciprocal.

RESULTS

The KM values and catalytic constants of peptide substrates show that A. gemmatalis trypsin- like has a higher affinity for substrates with arginine in the P1 position. Inhibition by Gor 3, Gor 4, and Gor 5, in the presence of L-BApNA, was linear competitive. The inhibition constant for the Gor 5 peptide was higher due to its strong interaction with hydrophobic residues in the secondary site region of A. gemmatalis trypsin-like.

CONCLUSION

It is observed that among the three peptides analyzed, the Gor 5 presented lower inhibition constant and therefore, the most potent among the tested ones. The predominance of hydrophobic residues in the region of the secondary site of the enzymes favored the interaction of the peptide. After characterization by three different types of graphs profiles, it is possible to verify that the inhibition model of the trypsin-like enzymes for the tested peptides is of the linear competitive type, in the concentration range of inhibitors and substrates analyzed. However, by the graphing profiles it is observed that the inhibition occurred due to the interaction of the peptides at the secondary site S2' in the hydrophobic cavity of the enzymes analyzed.

Does mechanical damage on soybean induces the production of flavonoids?

The response of plants to grazing includes the production of chemical defense compounds such as proteases inhibitors and secondary metabolites as flavonoids, which makes them less palatable to feeding and negatively affecting the physiology of insects. The aim of this study was to evaluate the phytochemical response of soybean cultivars (Glycine max (L.) Merrill) resistant (IAC-17, IAC-24) and susceptible (IAC-P1) to insects after mechanical damage. These cultivars were mechanically injured, and after 24 hours samples of these plants were analyzed by HPLC to identify and quantify flavonoids. The flavonoids daidzein, quercetin, and rutin were quantified, with the highest concentration of daidzin in soybean cultivars after mechanical damage. Rutin was biosynthesized by IAC-24. The cultivars IAC-PL1, IAC-17, and IAC-24 did not show a flavonoid response to mechanical damage. The soybean cultivars are not dependent on mechanical damage to produce flavonoids.

Protease inhibitory, insecticidal and deterrent effects of the trypsin-inhibitor benzamidine on the velvetbean caterpillar in soybean

The recognition of protease inhibitors with insecticidal activity is important as a basis for the development of mimetic peptides with potential use as biorational insecticides. We sprayed benzamidine on soybean plants and assessed whether this potent synthetic trypsin-inhibitor has protease inhibitory, insecticidal and deterrent effects on the velvetbean caterpillar Anticarsia gemmatalis Hübner (Lepidoptera: Erebidae). Activity of trypsin inhibition in soybean leaves was increased and total proteolytic activity in the midgut extract from larvae fed on these leaves was reduced by benzamidine. Different concentrations of benzamidine sprayed on the plant caused approximately 50 % of larval mortality, and larval choice and moth preference and oviposition were all negatively affected. Low concentrations of benzamidine increased mortality and hindered insect choice and oviposition as well as higher doses. Since many synthetic protease inhibitors are usually expensive, small doses of benzamidine may be effective to protect soybean against A. gemmatalis attack. Our results highlight the potential of synthetic protease inhibitors for insecticidal and deterrent purposes in insect pest management.

Sulfur fertilization increases defense metabolites and nitrogen but decreases plant resistance against a host-specific insect

We tested the sulfur-modulated plant resistance hypothesis using potted cabbage (Brassica oleracea var. capitata) plants that were grown without and with increasing levels of sulfur fertilization. Changes in plant chemical traits were assessed and developmental performance of Plutella xylostella, a highly host-specific leaf-chewing insect, was followed. Leaf sulfur concentration gradually increased with growing addition of sulfur in soil; however, there was a generalized saturation response curve, with a plateau phase, for improvements in total leaf nitrogen, defense glucosinolates and insect performance. Plutella xylostella performed better in sulfur-fertilized cabbage probably because of the higher level of nitrogen, despite of the higher content of glucosinolates, which are toxic for many non-specialized insects. Despite the importance of sulfur in plant nutrition and production, especially for Brassica crops, our results showed that sulfur fertilization could decrease plant resistance against insects with high feeding specialization.

Bottom-Up Mechanisms Generate the Same Temporal Pattern of Attack by a Specialist and a Generalist Caterpillar on Short-Lived Plants

The local population dynamics of insect herbivores in ephemeral patches of short-lived plants are poorly known. We investigated whether a specialist and a generalist caterpillar exhibit contrasting temporal patterns of attack during plant development and also assessed bottom-up forces related to plant ontogeny that govern such population trends. Immature stages of the polyphagous Trichoplusia ni (Hübner) and the oligophagous Plutella xylostella (L.) were sampled throughout the development of cabbage (Brassica oleracea L. var. capitata L.) crops. We measured protein and glucosinolate contents and insect performance with regard to plant age and leaf strata. The populations of both caterpillar species changed in close parallel throughout plant development, and a nonlinear temporal pattern of egg laying was reproduced in sequential population patterns of the larval stages until pupation. Reduced protein availability and insect performance coincided with a decline in egg laying and subsequent larval abundance in mature plants. By standardizing the plant size, we found that young and nutritious plants support proportionately more insects than large and mature plants. In our models of the population oscillations, the interaction between plant size and quality provided a strong causal explanation for the densities of both oligophagous and polyphagous caterpillars. Patches of fast-growing herbaceous plants are very common worldwide in the form of crop fields, and a generalized temporal pattern of attack may be widespread among caterpillars, regardless of their feeding specialization. Our results highlight the role of bottom-up forces in shaping the population dynamics of caterpillars in such systems.

Cabbage Seasonal Leaf Quality Mediating the Diamondback Moth Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) Performance

Seasonal variation in plant quality may be intense enough to generate predictable patterns in insect herbivore populations. In order to explain seasonal oscillations in neotropical populations of the diamondback moth Plutella xylostella (L.), we tested the following: (1) if nutritional quality of cabbage (Brassica oleraceae var. capitata), a primary host plant of diamondback moth, adversely affects the performance of this insect in late spring and early summer, when populations decline and go extinct, and (2) if nutritional features of cabbage change with the seasons. We measured the performance of diamondback moth reared on leaves of cabbages grown during the four seasons of the year. Summer plants proved to be worse for the survival of the immature stages and subsequent adult fecundity, but there were no significant differences between the remaining seasons. Our results support the hypothesis that short-lived plants, grown in different seasons of the year in the tropics, have different nutritional and defensive attributes. We analyzed nutritional quality of cabbage leaves from the four seasons, but only total lipids were reduced in summer plants. Neotropical populations of diamondback moth collapse before plant quality decay in the summer. If the diamondback moth is well adapted to the seasonal deterioration of the habitat, including the reduction in the quality of host plants, it is expected that emigration happens before the mortality increases and natality decreases during the summer.

Photoperiodism and seasonality in neotropical population of Plutella xylostella L. (Lepidoptera: Yponomeutidae)

Neotropical populations of the diamondback moth Plutella xylostella L. have seasonal cycles of growth and decrease, and moth migration plays a fundamental role in generating such population dynamics. Since the oscillation of these populations is predictable, photoperiod might operate as a signal that triggers the migratory behaviour of the insect. Migration in insects is usually preceded by reproductive diapause, a photoperiodic response that can be characterised by morphological, physiological and behavioural alterations that permit to discriminate between migratory and non-migratory forms. In this study, I tested whether the pre-imaginal and reproductive development of P. xylostella from Minas Gerais (Brazil) is affected by artificial day-lengths that are equivalent to the periods of natural population growth or decrease. No evidence of photoperiodic response was found for the insect reared in laboratory on five different constant photoperiods, from 8h to 16h of light per day. There was no significant variation in survival and duration of egg, larva, and pupa stages or in pupal weight, adult size (forewing length), fecundity, and longevity. Although some species have geographically distinct photoperiodic responses, previous assumptions that cosmopolitan P. xylostella responds to photoperiod in temperate regions was questioned. Migratory and population seasonality among neotropical populations of P. xylostella certainly occurs independently of the photoperiodic announcement of seasonal changes in habitat quality.

Comparação da eficiência de modelos de armadilhas de interceptação de vôo na amostragem de Hymenoptera e outros insetos

Armadilhas de interceptação de vôo para insetos propostas por Townes (1972) e Masner & Goulet (1981) foram modificadas. Os modelos originais e os modificados foram comparados entre si em sua eficiência na amostragem de ordens de Insecta e famílias de Hymenoptera. O uso de bandejas pintadas na cor amarela aumentou significativamente a eficiência de captura das armadilhas de Masner & Goulet. A união, em uma única armadilha, da bandeja amarela, típica do modelo de Masner & Goulet, com o pote, típico das armadilhas Malaise, não reduziu a eficiência individual dos dois sistemas. A associação desses sistemas de captura por interceptação de vôo, mais a atratividade da cor amarela, permitiu o funcionamento, num mesmo local e ao mesmo tempo, de três técnicas que geralmente têm sido empregadas isoladamente. O resultado foi o somatório de amostras padronizadas, sem perda de eficiência individual. Um menor esforço humano é possível, porque reduz a quantidade de armadilhas. O custo financeiro também é menor, devido à economia de tecido utilizado. Como uma técnica compensa a menor eficiência da outra na captura de determinados taxa, um modelo de armadilha que associa todas elas pode ser particularmente útil em levantamentos faunísticos abrangentes.