Host Plant Defense Research Articles

Human beta-defensin-1: an antimicrobial peptide of urogenital tissues.

Antimicrobial peptides are widely distributed mediators of innate host defense in animals and plants. A 36 amino acid antimicrobial peptide belonging to the defensin family, and named human beta-defensin-1 (HBD-1), was purified recently from hemodialysate fluid, but its tissue sources were not identified. By Northern blotting, we found the highest concentrations of HBD-1 mRNA in the kidney and the female reproductive tract. In situ hybridization localized the HBD-1 mRNA in the epithelial layers of the loops of Henle, distal tubules, and the collecting ducts of the kidney and the epithelial layers of the vagina, ectocervix, endocervix, uterus, and fallopian tubes in the female reproductive tract. Using a novel technique designed to detect cationic peptides in urine, we recovered several forms of HBD-1 ranging in length from 36 to 47 amino acid (aa) residues and differing from each other by amino terminal truncation. The total concentration of HBD-1 forms in voided urine was estimated at 10-100 microg/liter, with individual variations in the total amount of HBD-1 peptides and the relative proportion of HBD-1 forms. Multiple forms of HBD-1 (size 36-47 aa) were also found in the blood plasma, bound to carrier macromolecules that released the peptide under acid conditions, and in vaginal mucosal secretions (39, 40, and 44 aa). By immunostaining, HBD-1 was located in the kidney within the lumen of the loops of Henle, but no intracellular storage sites were identified in renal or female reproductive tissues. Recombinant HBD-1 forms (36, 39, and 42 aa) and natural HBD-1 forms were antimicrobial to laboratory and clinical strains of Escherichia coli at micromolar concentrations. HBD-1 activity was not changed appreciably by low pH, but was inhibited by high salt conditions. Some of the HBD-1 peptides retained their activity against E. coli in unconcentrated (low conductance) urine, and the 36 aa form was microbicidal even in normal (high conductance) urine. Production of HBD-1 in the urogenital tract could contribute to local antimicrobial defense.

Plant cell wall-degrading enzymes of Rhizobium leguminosarum bv. viciae: Their role in avoiding the host-plant defence response

Pathogenic bacteria and fungi are characterized by the production of enzymes capable of indiscriminate degradation of plant cel1 walls (Cooper, 1981, 1983). However, the host being invaded may be able to detect breakdown products of cel1 wal1 modification which can signal the operation of plant defence systems (Mansfield, 1982; Sharp et al., 1984; Collmer and Keen, 1986; Ryan and Farmer, 1991). The host-plant defence response (HPDR) to infection is wel1 documented in legume and non-legume systems (Davis et al., 1984). Plant cel1 wall-modifying enzymes capable of eliciting a HPDR are polygalacturonase, pectin lyase, pectate lyase, and glucanase. In addition, enzymes which are not recorded as being able to elicit a HPDR, but are often associated with phytopathogen ingress, include pectin methyl esterase and protease. The mechanism by which legume host-root cel1 walls are penetrated poses the following question: HOW do host compatible rhizobia manage to breach the host cel1 wal1 and evade a HPDR? (McKhann and Hirsch, 1994). Rhizobia do appear to degrade the host-root cel1 wal1 at infection sites, although which enzymes are responsible is not clear. Rhizobia do have the ability to produce pecteolytic enzymes (Plazinski and Rolfe, 1985; Prasuna and Ali, 1987; Mateos et al., 1992); however, this activity is not secreted freely but tightly cell-bound (Mateos et al., 1992). Cellulase and hemicellulase enzyme activities produced by rhizobium have been detected, but it is unlikely that these enzymes alone facilitate cel1 wal1 penetration (Verma et al., 1978; Martinez-Molina et al., 1979; Al-Mallah et al., 1987; Chalifour and Benhamou, 1989; Cooper, 1989; Sprent, 1994). It may be that rhizobial cel1

Glycine in Digestive Juice: a Strategy of Herbivorous Insects Against Chemical Defense of Host Plants

A large amount of free glycine was found in the digestive juice of several Lepidoptera larvae and appears to be secreted. The amount of glycine differs greatly among species and seems to correlate with the host plant. Several species of Lepidoptera from different famileis which feed on the privet tree, Ligustrum obtusifolium, show extremely large amounts of free glycine. The concentration of glycine reached 57 μmoles/g (0.4%). The leaves of the privet tree have an extremely strong protein denaturing activity, enough to make protein completely non-nutritive. This denaturing property closely resembles the activity of polyphenolics, digestion inhibitory compounds common among plants. In vitro studies showed that free glycine completely inhibits the denaturing activity of the privet leaves. These results suggest that insects secrete glycine to counter the chemical defenses of host plants. © 1997 Published by Elsevier Science Ltd. All rights reserved

Two wound-inducible soybean cysteine proteinase inhibitors have greater insect digestive proteinase inhibitory activities than a constitutive homolog.

Diverse functions for three soybean (Glycine max L. Merr.) cysteine proteinase inhibitors (CysPIs) are inferred from unique characteristics of differential regulation of gene expression and inhibitory activities against specific Cys proteinases. Based on northern blot analyses, we found that the expression in leaves of one soybean CysPI gene (L1) was constitutive and the other two (N2 and R1) were induced by wounding or methyl jasmonate treatment. Induction of N2 and R1 transcript levels in leaves occurred coincidentally with increased papain inhibitory activity. Analyses of kinetic data from bacterial recombinant CysPI proteins indicated that soybean CysPIs are noncompetitive inhibitors of papain. The inhibition constants against papain of the CysPIs encoded by the wound and methyl jasmonate-inducible genes (57 and 21 nM for N2 and R1, respectively) were 500 to 1000 times lower than the inhibition constant of L1 (19,000 nM). N2 and R1 had substantially greater inhibitory activities than L1 against gut cysteine proteinases of the third-instar larvae of western corn rootworm and Colorado potato beetle. Cysteine proteinases were the predominant digestive proteolytic enzymes in the guts of these insects at this developmental stage. N2 and R1 were more inhibitory than the epoxide trans-epoxysuccinyl-L-leucylamide-(4-guanidino)butane (E-64) against western corn rootworm gut proteinases (50% inhibition concentration = 50, 200, and 7000 nM for N2, R1, and E-64, respectively). However, N2 and R1 were less effective than E-64 against the gut proteinases of Colorado potato beetle. These results indicate that the wound-inducible soybean CysPIs, N2 and R1, function in host plant defense against insect predation, and that substantial variation in CysPI activity against insect digestive proteinases exists among plant CysPI proteins.

Possible involvement of lipoxygenase in a defense response in aflatoxigenic Aspergillus – cotton plant interactions

Damage to the carpel surface of the developing cotton boll such as that caused by infection with aflatoxigenic Aspergillus spp. results in activation of the lipoxygenase pathway. Lipoxygenase pathway activation generates a series of diverse, antifungal, volatile aldehydes that affect the growth of Aspergillus flavus and indirectly has an effect on aflatoxin production. Analysis of the head space volatile aldehydes by gas chromatography – mass spectrometry trapped during a 1-h sampling period of carpel surfaces of 23- to 35-day developing cotton bolls treated with exogenous lipase resulted in the identifications of C5–C9 alkanals and alkenals including hexanal and trans-2-hexenal. Hexanal concentrations generated in lipase-treated bolls resulted in a two-fold increase of hexanal production. The effects of trans-2-hexenal on aflatoxin production were determined on intact 30-day-old developing cotton bolls treated with 2.2–8.6 μmol amounts of trans-2-hexenal and inoculated with aflatoxigenic Aspergillus in three different treatment systems. Trans-2-hexenal was placed on the developing cotton boll (i) 24 h before, (ii) 24 h after, or (iii) simultaneously with fungal inoculation. After a 3-week incubation on the intact cotton plants, bolls were harvested and aflatoxin determinations were conducted on cottonseeds from the bolls. The results suggest that pretreatment of the boll with trans-2-hexenal initiates a strong antagonism to aflatoxin formation due to the antifungal effects on the growth of the fungus in developing cotton bolls. These results also demonstrate a series of host-plant defense responses that suggest the possible involvement of the lipoxygenase pathway in interactions between aflatoxigenic Aspergillus and the cotton plant. Keywords: aflatoxin, Aspergillus flavus, cotton (Gossypium hirsutum), lipoxygenase (LOX) pathway, lipids.

Biology and Conservation of the Monarch Butterfly

This book is a collection of papers pre sented as part of the Second International Conference on the Monarch Butterfly held at the Natural History Museum of Los Angeles County in Los Angeles, CA, in September 1986. In addition to fine introductory and concluding sections prepared by the editors, it contains 42 papers contributed by 50 authors representing Australia, Germany, Mexico, the United Kingdom, and the United States. The papers are arranged in eight subject-related sections: Systematics; Chemical Communication; Mating Behavior; Host Plant Use, Cardenolide Sequestration, and Defense Against Natural Enemies; Physiological Ecology and the Annual Cycle; Migration; Overwintering Biology; and Conservation. Almost two-thirds of the papers report the results of original research, and about onequarter are overviews or brief reviews, most of which contain some previously unpublished material and suggest new insights and possibilities. The remaining articles deal with various aspects of conservation.

Host plant-based territoriality in the white peacock butterfly,Anartia jatrophae (Lepidoptera: Nymphalidae)

Mate locating behavior of Anartia jatrophaewas studied near Ochopee, Collier County, Florida. Males were individually marked, and focal animal samples were used to determine activity, residency, and interactions with other butterflies. We recaptured more than half of 53 marked males within the study area on subsequent days. Marked males restricted non-feeding activity to roughly circular areas typically less than 15 m in diameter. Males showed considerable site fidelity and chased both conspecifics and a variety of other insects. Typically, encountered individuals flew away, and the resident peacock male returned to perch near his previous perch. Encounters with conspecific males averaged 38 sec, three times longer than with other insects. Such site fidelity combined with aggressive interactions indicates territorial behavior. When male density more than doubled, site fidelity decreased, resident turnover increased, and areas that had not been defended at lower density were defended. Without exception, defended sites contained the larval host, Bacopa monnieri.In butterfly mating systems, defense of host plants appears to be much less common than defense of natural features such as landmarks.

Effect of Plant Resistance, Competition, and Enemies on a Leaf-Galling Sawfly (Hymenoptera: Tenthredinidae)

The relative importance of three sources of larval mortality (host plant resistance, intraspecific competition, and natural enemies) was evaluated for a population of leafgalling sawflies, Pontania sp., near P. pacifica Marlatt, on arroyo willow, Salix lasiolepis Bentham, at a site in Flagstaff, Ariz., from 1981 to 1984. The ratio of the percentage of sawfly larvae killed by natural enemies to the percentage lost to host plant defenses (i. e., abortion, defined as an aborted formation of a gall resulting from death of the egg or small larva) was 1.53:1. Mortality from abortion was divided into a host plant resistance component (constitutive abortion, e.g., a plant resistance factor preventing establishment of the egg or small larva) and an intraspecific competition-mediated component (because abortion increased as within-leaf gall densities rose). Natural enemies caused twice as much mortality as plant resistance and 6.36 times greater mortality than intraspecific competition. These results support the hypothesis that third-trophic-level effects exert a stronger selective pressure on many insect populations than competition for limiting resources. There was evidence that intraspecific competition for limiting food resources occurred for these Pontania sp. sawflies; increased within-leaf gall densities had a density-dependent, linear, negative effect on gall size, and consequently, on larval biomass because sawfly size was positively correlated with gall size. However, only about 3% of the variation in gall size was explained by within-leaf gall density. Thus, gall (and larval) size were only weakly affected by intraspecific competition.

Molecular basis for evasion of plant host defence in bacterial spot disease of pepper

A fundamental objective in studying host-pathogen interactions is to determine how a pathogen evolves to overcome the defences of a previously resistant host. In mammalian systems, evolution of a pathogen population is often directed towards avoiding recognition by an immune system1–3. Although plants lack circulating antibodies and the memory response of mammalian immune systems, it is becoming increasingly clear that active resistance in plants involves recognition of the pathogen by its host4,5. In this paper we present the first molecular evidence to indicate that plant pathogens evolve to overcome resistance by evading host recognition and response. The bacterial pathogen Xanthomonas campestris pathovar vesicatoria mutates to overcome genetically defined resistance in pepper, Capsicum annuum, by the transposon-induced mutation of avrBs1 a bacterial gene that provokes the plant's resistance response.

Evolution of Territoriality by Herbivores in Response to Host Plant Defenses

Plants may play an active role in the evolution of territoriality and associated animal behaviors such as spacing and cannibalism. I hypothesize that these behaviors have in part evolved in direct response to plant defenses that would otherwise diminish individual success. These defenses limit resource availability and concentrate herbivores at specific sites where they then suffer from induced plant defenses, increased predation and competition. Plant traits that enhance the negative effects of competition and increased predation must be included in the suite of plant defenses against herbivory. In a specific example with Pemphigus gall aphids, plant defenses result in a strong selection pressure favoring territorial behavior. The negative effects of competition give territorial individuals a 2.3-fold advantage over non-territorial individuals. Induced defenses ( i.e. , density dependent leaf abscission) can be just as important as competition as a selection pressure for territorial behavior. With the addition of induced plant defenses territorial individuals realize a 4.4-fold advantage. Furthermore, rough approximations suggest that in the absence of territorial behavior predation would increase by 52%. The same traits which promote territoriality also encourage cannibalism, a surprisingly common herbivore behavior. To the extent cannibalism reduces the herbivore population, plant traits which promote this behavior may realize a selective advantage. Viewing plants as dynamic rather than passive in their interactions with pests may lead to a better understanding of animal behaviors.

Ecological genetics and evolution in insect pests: Implications for lower input agriculture

AbstractThis paper has three goals: (1) to convince ecologists and evolutionary biologists to study the evolution in insects of the ability to overcome crop protection measures, (2) to provide insights into the kinds of data needed to develop methods for retarding the evolution of such traits, and (3) to suggest that the study of these phenomena can further our understanding of evolution. The evolution of resistance to chemical insecticides often results in higher application rates and constant development of new classes of these potentially environmentally degrading toxicants. Moreover, it is important to understand resistance phenomena related to alternative crop protection measures involving plant genetic resistance and biological insecticides so that these less environmentally damaging control measures can be maintained. Insecticide resistance evolves in insect populations in response to selection by chemical compounds. Similarly, selection by host-plant defenses of resistant crops leads to the evolution of virulence to those varieties. The evolution of these traits constitutes an important subject of applied evolutionary biology. In the context of single-gene evolutionary models, this article reviews the most common strategies that have been suggested for retarding the evolution of insecticide resistance. These models are also used to illustrate the effects of ecological factors and genetical properties of insect populations on the evolution of resistance. Where appropriate, the relevance of these models to the evolution of virulence to resistant crop varieties is also described. Durability in the insecticidal effectiveness of a plant protective chemical is not incompatible with the requirement for health safety in the same material.

Identification and partial characterization of a major gut proteinase from larvae of the hessian fly, Mayetiola destructor (say) (Diptera: Cecidomyiidae)

A major digestive proteinase in the guts of larvae of biotype D of Mayetiola destructor is a chymotrypsin-like serine proteinase which can hydrolyze [ 3H]alpha-casein and benzoyl- l-tyrosine ethyl ester (BTEE). Hydrolysis of protein and ester by the gut proteinase were blocked by di isopropyl fluorophosphate (DFP) and inhibited by l-1-tosylamide-2-phenylethyl chloromethyl ketone (TPCK). Optimum pH for hydrolysis of BTEE by gut proteinase is 7.8. Hydrolysis of [ 3H]alpha-casein by gut proteinase reaches a maximum at pH 8.4 and then maintains a plateau until pH 10.8. Proteolytic activity is located extracellularly in the lumen contents of the gut. The nature of proteolytic activity appears to be consistent in all larval stages examined. The Bowman-Birk inhibitor (BBI) from soybeans is a strong inhibitor of the proteolytic and esterolytic activities of the gut proteinase. These results are discussed in the context of the potential application of naturally occurring proteinase inhibitors as engineered host plant defences against larvae of the Hessian fly.