The effects of Phaseolus vulgaris erythro- and leucoagglutinating isolectins (PHA-E and PHA-L) delivered via artificial diet and transgenic plants on the growth and development of tomato moth (Lacanobia oleracea) larvae; lectin binding to gut glycoproteins in vitro and in vivo

Research advances and prospects of legume lectins

Lectins are widely distributed proteins having ability of binding selectively and reversibly with carbohydrates moieties and glycoconjugates. Although lectins have been reported from different biological sources, the legume lectins are the best-characterized family of plant lectins. Legume lectins are a large family of homologous proteins with considerable similarity in amino acid sequence and their tertiary structures. Despite having strong sequence conservation, these lectins show remarkable variability in carbohydrate specificity and quaternary structures. The ability of legume lectins in recognizing glycans and glycoconjugates on cells and other intracellular structures make them a valuable research tool in glycomic research. Due to variability in binding with glycans, glycoconjugates and multiple biological functions, legume lectins are the subject of intense research for their diverse application in different fields such as glycobiology, biomedical research and crop improvement. The present review specially focuses on structural and functional characteristics of legume lectins along with their potential areas of application.

Effects of the Agglutinins Extracted From Rhizoctonia solani (Cantharellales: Ceratobasidiaceae) on Pieris brassicae (Lepidoptera: Pieridae)

Lectins are widespread proteins found in plants, fungi, bacteria, and vertebrates, and they play the critical roles in many physiological functions. Two lectin molecules (namely, RSAI and RSAII) were extracted from Rhizoctonia solani Kuhn and their effects on Pieris brassicae L. larvae were determined by larval survival rate, body mass, nutritional indices, digestive enzyme activities, and caspase-3 gene expression. The highest mortality caused by RSA treatment was recorded up to 80%, the larval weight decreased to 0.05 g and Similarly, RSAs significantly decreased nutritional indices including conversion efficiency of ingested food (ECI), conversion efficiency of digested food (ECD), approximate digestibility (AD), relative consumption rate (RCR), and relative growth rate (RGR) in a dose-dependent manner. Activities of α-amylase and α- and β-glucosidases significantly decreased in the larvae fed with RSA-treated diets. Also, activities of TAG-lipase and proteases significantly reduced after feeding with different concentrations of RSAs. Gene expression analysis of caspase-3 in control and treated larvae revealed significant increment of its expression in the larvae fed with RSAI and RSAII, respectively, 9.52- and 1.47-fold compared to control. These results clearly demonstrated insecticidal effects of R. solani lectins on P. brassicae via several physiological pathways, thus rendering RSA as a good target for furthering our knowledge and suggesting new strategies to overcome pesticide side effects.

Potential of the Lectin/Inhibitor Isolated from Crataeva tapia Bark (CrataBL) for Controlling Callosobruchus maculatus Larva Development

Callosobruchus maculatus is an important predator of cowpeas. Due to infestation during storage, this insect affects the quality of seed and crop yield. This study aimed to investigate the effects of CrataBL, a multifunction protein isolated from Crataeva tapia bark, on C. maculatus larva development. The protein, which is stable even in extreme pH conditions, showed toxic activity, reducing the larval mass 45 and 70% at concentrations of 0.25 and 1.0% (w/w), respectively. Acting as an inhibitor, CrataBL decreased by 39% the activity of cysteine proteinases from larval gut. Conversely, the activity of serine proteinases was increased about 8-fold. The toxic properties of CrataBL may also be attributed to its capacity of binding to glycoproteins or glycosaminoglycans. Such binding interferes with larval metabolism, because CrataBL-FITC was found in the fat body, Malpighian tubules, and feces of larvae. These results demonstrate the potential of this protein for controlling larva development.

Polymers for the stabilization and delivery of proteins topically and per os to the insect hemocoel through conjugation with aliphatic polyethylene glycol

Co-feeding of aliphatic polyethylene glycol (PEG), phospholipase A2, anionic and ionic detergents, and amphipathic glycoside with bovine serum albumin (BSA) as a model protein to fourth stadium tobacco budworms, Heliothis virescens, did not affect the levels of BSA in the hemolymph. Covalent conjugation of small proteins like the decapeptide trypsin modulating oostatic factor (TMOF) to polyethylene glycol was previously shown to protect the peptide from protease attack and enhance its accumulation in the insect hemocoel. Whether this polymer chemistry could do the same for larger proteins was examined. The chemistry for the synthesis of polydispersed aliphatic PEG350-insulin and monodispersed aliphatic PEG333-insulin are described herein. Insulin was used for this synthesis and not BSA to better control conjugation among the available free amine groups. When PEGylated insulin or free insulin were fed in artificial diet to fifth stadium budworms, greater concentrations of insulin using the PEGylated variants were found in the hemolymph than when free insulin was used (a 6.7 and 7.3-fold increase for the PEG350 and PEG333 conjugates, respectively). When insulin is topically applied to the dorsum of H. virescens, no insulin is found in the hemolymph. However, after topical application of the PEGylated insulins, PEG350-insulin and PEG333-insulin were detected in the hemolymph. After injections of insulin into the hemocoel of fourth stadium H. virescens, insulin is completely cleared from the hemolymph in 120min. In comparison, PEG350-insulin and PEG333-insulin were present in the hemolymph for 300 and 240min after injection, respectively, translating to a 3.3 and 2.7-fold increase in the length of time insulin remains in the hemolymph after injection.

Expression of soybean lectin in transgenic tobacco results in enhanced resistance to pathogens and pests

Lectins are proteins of non-immune origin that specifically interact with carbohydrates, known to play important roles in the defense system of plants. In this study, in order to study the function of a new soybean lectin (SBL), the corresponding encoding gene lec-s was introduced into tobacco plants via Agrobacterium-mediated transformation. Southern blot analyses had revealed that the lec-s gene was stable integrated into the chromosome of the tobacco. The results of the reverse transcription polymerase chain reaction (RT-PCR) also indicated that the lec-s gene in the transgenic tobacco plants could be expressed under the control of the constitutive CaMV35S promoter. Evaluation agronomic of the performance had showed that the transgenic plants could resist to the infection of Phytophthora nicotianae. Insect bioassays using detached leaves from transgenic tobacco plants demonstrated that the ectopically expressed SBL significantly (P.0.05) reduced the weight gain of larvae of the beet armyworm (Spodoptera exigua). Further on, the lectins retarded the development of the larvae and their metamorphosis. These findings suggest that soybean lectins have potential as a protective agent against pathogens and insect pests through a transgenic approach.

Antimetabolic effect of phytohemagglutinin to the grain aphid Sitobion avenae fabricius

The insecticidal activity of plant lectins against a wide range of insect species have been intensively studied. Understanding the mechanism of the toxicity of lectins is one of the studied aspects. In the present research, the first step was determine the effect of phytohemagglutinin (PHA) on the development, fecundity and mortality of grain aphid. Next, the effect of PHA lectin on the activity of such enzymes as: α- and β-glucosidases, alkaline (AkP) and acid (AcP) phosphatases, aminopeptidase N and cathepsin L involved in the metabolism of sugar, phosphorus and proteins of an adult apterae aphids was investigated. The PHA lectin added into the liquid diet increased the pre-reproductive period, mortality of Sitobion avenae, the time of generation development and decreased its fecundity and the intrinsic rate of natural increase. In addition, activity of α-glucosidase, alkaline phosphatase and aminopeptidase N of adult apterae exposed to PHA were reduced. The results indicate that the insecticidal activity of PHA on S. avenae may involve changes in activity of the enzymes in the midgut and it may be part of its toxicity.

The fate of vicilins, 7S storage globulins, in larvae and adult Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

The fate of vicilins ingested by Callosobruchus maculatus and the physiological importance of these proteins in larvae and adults were investigated. Vicilins were quantified by ELISA in the haemolymph and fat body during larval development (2nd to 4th instars), in pupae and adults, as well as in ovaries and eggs. Western blot analysis demonstrated that the majority of absorbed vicilins were degraded in the fat body. Tracing the fate of vicilins using FITC revealed that the FITC-vicilin complex was present inside cells of the fat body of the larvae and in the fat bodies of both male and female adult C. maculatus. Labelled vicilin was also detected in ovocytes and eggs. Based on the results presented here, we propose that following absorption, vicilins accumulate in the fat body, where they are partially degraded. These peptides are retained throughout the development of the insects and eventually are sequestered by the eggs. It is possible that accumulation in the eggs is a defensive strategy against pathogen attack as these peptides are known to have antimicrobial activity. Quantifications performed on internal organs from larvae of C. maculatus exposed to extremely dry seeds demonstrated that the vicilin concentration in the haemolymph and fat body was significantly higher when compared to larvae fed on control seeds. These results suggest that absorbed vicilins may also be involved in the survival of larvae in dry environments.

The movement of proteins across the insect and tick digestive system

The movement of intact proteins across the digestive system was shown in a number of different blood-feeding and non-blood-feeding insects in the orders Blattaria, Coleoptera, Diptera, Hemiptera, Lepidoptera, Orthoptera, Neuroptera and Siphonaptera, as well as in two tick families Ixodidae and Argasidae. Protein movement was observed for both normal dietary and xenobiotic proteins, which suggest that the mechanism for transfer is not substrate specific. The number of studies on the mechanism of movement is limited. The research so far suggests that movement can occur by either a transcellular or an intercellular pathway in the ventriculus with most of the research describing the former. Transfer is by continuous diffusion with no evidence of pinocytosis or vesicular transport common in mammalian systems. Proteins can move across the digestive system without modification of their primary or multimeric structure and with retention of their functional characteristics. Accumulation in the hemolymph is the result of the protein degradation rate in the gut and hemolymph and transfer rate across the digestive system and can be highly variable depending on species. Research on the development of delivery systems to enhance protein movement across the insect digestive system is in its infancy. The approaches so far considered with some success include the use of lipophilic-polyethylene glycol (PEG) polymers, the development of fusion proteins with lectins, reduced gut protease activity and the development of amphiphilic peptidic analogs. Additional research on understanding the basic mechanisms of protein delivery across the insect digestive system, the importance of structure activity in this transfer and the development of technology to improve movement across the gut could be highly significant to the future of protein and nucleic acid-based insecticide development as well as traditional chemical insecticidal technologies.

Virus-derived genes for insect-resistant transgenic plants

Insect viruses have evolved to counter physiological barriers to infection presented by the host insect. For the Lepidoptera (butterflies and moths), these barriers include (1) the peritrophic membrane (PM) lining the gut, which presents a physical barrier to virus infection of the midgut epithelial cells, (2) the basement membrane (BM) that overlies the gut thereby restricting secondary infection of other tissues, and (3) the immune system of the host insect. Hence, insect viruses provide a resource for genes that disrupt host physiology in a specific manner, and these genes in turn serve as a resource both for the study of physiological processes, and for disruption of these processes for pest management purposes. There are several examples of the application of genes used by an insect virus to overcome the PM barrier for production of insect-resistant transgenic plants. There are other examples of intrahemocoelic effectors, such as BM-degrading proteases that can only be used with an appropriate system for delivery of the agent from the gut into the hemocoel (body cavity) of the insect pest. In this chapter, we describe (1) baculovirus- and entomopoxvirus-derived genes that alter the physiology of the host insect, (2) use of these and homologous genes for production of insect-resistant transgenic plants, (3) other viral genes that have potential for use in development of insect-resistant transgenic plants, and (4) the use of plant lectins for delivery of intrahemocoelic toxins from transgenic plants. Plant expression of polydnavirus-derived genes is described by Gill et al. (this volume, pp. 393-426).

Presence of the storage seed protein vicilin in internal organs of larval Callosobruchus maculatus (Coleoptera: Bruchidae)

Variant vicilins (7S storage globulins) of cowpea seeds (Vigna unguiculata) are considered as the main resistance factor present in some African genotypes against the bruchid Callosobruchus maculatus. It has been suggested that the toxic properties of vicilins may be related to their recognition and interaction with glycoproteins and other membrane constituents along the digestive tract of the insect. However, the possibility of a systemic effect has not yet been investigated. The objective of this work was to study the fate of 7S storage globulins of V. unguiculata in several organs of larvae of the cowpea weevil C. maculatus. Results demonstrated binding of vicilins to brush border membrane vesicles, suggesting the existence of specific receptors. Vicilins were detected in the haemolymph, in the midgut, and in internal organs, such as fat body and malpighian tubules. There is evidence of accumulation of vicilins in the fat body of both larvae and adults. The absorption of vicilins and their presence in insect tissues parallels classical sequestration of secondary compounds.

Distribution and residual activity of two insecticidal proteins, avidin and aprotinin, expressed in transgenic tobacco plants, in the bodies and frass of Spodoptera litura larvae following feeding

To understand how a major cosmopolitan pest responds to two very different insecticidal proteins and to determine whether herbivorous insects and their frass could be environmental sources of recombinant proteins from transgenic plants, Spodoptera litura (Fab.) (Lepidoptera, Noctuidae) larvae were fed on tobacco leaves expressing either the biotin-binding protein, avidin, or the protease inhibitor, aprotinin. Control larvae received non-transgenic tobacco. Samples of larvae were taken after 5, 6 or 7 days' feeding and frass was collected after two 24-h periods at 6 and 7 days. Insects in all treatments grew significantly during the experiment, but the avidin-fed larvae were significantly smaller than the others on Day 7. Avidin was found in all samples of avidin-fed larvae (7.0+/-0.86 ng mg(-1), n=45), at a lower level than in their frass (31.9+/-5.08 ng mg(-1), n=30), and these frass levels were lower than those of the the leaves fed to the larvae (69.0+/-6.71 ng mg(-1), n=45). All of the avidin detected in these samples was capable of binding biotin. On average, between 10 and 28% of avidin was recovered with the methods used, whereas almost full recovery of aprotinin was effected. Aprotinin levels in larvae (8.2+/-0.53 ng mg(-1), n=45) were also lower than aprotinin levels in frass (77.4+/-6.9 ng mg(-1), n=30), which were somewhat lower than those in the leaves fed to the larvae (88.6+/-2.51 ng mg(-1), n=45). Approximately half the trypsin-binding ability of aprotinin was lost in larvae, and in frass, aprotinin had lost about 90% of its ability to bind trypsin.

Binding of the insecticidal lectin Concanavalin A in pea aphid, Acyrthosiphon pisum (Harris) and induced effects on the structure of midgut epithelial cells

Concanavalin A (lectin from Canavalia ensiformis L., ConA) has previously been shown to act as a feeding inhibitor for Acyrthosiphon pisum, the pea aphid. In the present study a range of histochemical and biochemical techniques were used to elucidate the target tissues and binding sites of the lectin in the aphid. Diet uptake was evaluated using a radioactive tracer (14C-methylated inulin) and demonstrated that adults were capable of ingesting high quantities of the toxin (approx. 1 microg over a 48 h period). Electophoretic analysis and enzyme-linked immuno-sorbent assay of honeydew samples confirmed these results and further demonstrated that only small levels of ConA were excreted. Histofluorescence and immunolocalisation studies on nymphs revealed that the stomach was the primary target for ConA. At concentrations up to 400 microg ml(-1), lectin binding only occurred in the stomach region, however, at high concentrations (800 microg ml(-1)) the whole digestive tract was stained, although there was no evidence of binding in either the oesophagus or rectum. In addition to binding, there was evidence to suggest that ConA was also causing systemic effects in that the lectin appeared to cross the intestinal epithelial barrier. Immunohistochemical and electron microscopy studies revealed that ConA induced severe cellular swelling of the epithelial cells, accompanied by hypersecretion and a progressive detachment of the apical membrane; however, the striated border itself did not appear to be directly affected. Furthermore, there was no lysis of the epithelium, nor loss of integrity of the epithelial cells themselves. Our results suggest that ConA interacts with glycosylated receptors at the surface of the stomach epithelial cells, interfering with normal metabolism and cell function, resulting in a rapid feedback response on feeding behaviour. Whilst our results provide a much greater understanding regarding the modes of action of ConA in insects, they suggest that different lectins, including other mannose binding lectins, have different modes of action at the cellular levels, and thus generalizations should be treated with caution.

Antinutritional properties of plant lectins

Lectins are carbohydrate binding (glyco)proteins which are ubiquitous in nature. In plants, they are distributed in various families and hence ingested daily in appreciable amounts by both humans and animals. One of the most nutritionally important features of plant lectins is their ability to survive digestion by the gastrointestinal tract of consumers. This allows the lectins to bind to membrane glycosyl groups of the cells lining the digestive tract. As a result of this interaction a series of harmful local and systemic reactions are triggered placing this class of molecules as antinutritive and/or toxic substances. Locally, they can affect the turnover and loss of gut epithelial cells, damage the luminal membranes of the epithelium, interfere with nutrient digestion and absorption, stimulate shifts in the bacterial flora and modulate the immune state of the digestive tract. Systemically, they can disrupt lipid, carbohydrate and protein metabolism, promote enlargement and/or atrophy of key internal organs and tissues and alter the hormonal and immunological status. At high intakes, lectins can seriously threaten the growth and health of consuming animals. They are also detrimental to numerous insect pests of crop plants although less is presently known about their insecticidal mechanisms of action. This current review surveys the recent knowledge on the antinutritional/toxic effects of plant lectins on higher animals and insects.

Insect peptide hormones: a selective review of their physiology and potential application for pest control

Our knowledge on primary structure, synthesis, release, receptor binding, structure-activity relationships, mode of action and degradation of, mainly, neuropeptides from insects has increased dramatically during the last 10 years or so. Here, five case studies are presented, which deal selectively with effects on: reproduction (trypsin modulating oostatic factor in mosquito); energy metabolism, locomotion and the immune system (adipokinetic hormones); water and ion balance, and feeding behaviour (diuretic hormones, kinins, sulfakinins); sex attraction (pheromone biosynthesis activating neuropeptide); and growth and development, and muscle activity (allatostatins). The literature is reviewed in the context of how the knowledge on neuropeptides has been and can be used for the design of novel, safe and selective compounds to control pest insects in the foreseeable future.