Resistance of collard greens to Ascia monuste orseis (Lepidoptera: Pieridae)

Ascia monuste orseis (Godart) (Lepidoptera: Pieridae) is a production limiting pest in collard greens, Brassica oleracea (L.) var. acephala, cultivation. Because of the overuse and harmful effects of synthetic insecticides on nontarget species, the use of insect-resistant cultivars can be a valuable strategy in pest control. In this study, newly hatched A. monuste orseis larvae were confined to the leaves of 29 collard greens cultivars under a controlled environment to investigate plant resistance. We evaluated the incubation period, duration of instars, total duration of the immature and pupal phases, the egg to adult life cycle duration, mortality per instar, total weight of fifth instar larvae and pupae (age = 24 h) and larval and pupal survival and eclosion. Antibiosis and/or antixenosis were observed in selected cultivars. The results show that Gigante I-915 (E) exhibited high larval mortality and that the Pires 1 de Campinas cultivar (P) prevented all pupae from proceeding to the adult stage. The Introdu double dagger es do municipio de Arthur Nogueira Z (Y), Cabocla (AA), Japonesa (R) and Manteiga de Mococa (M) cultivars prolonged the larval stage. Japonesa (R) and Introdu double dagger es do municipio de Arthur Nogueira Z (Y) increased the egg to adult developmental period, and the Japonesa (R) cultivar also prolonged the pupal stage. The Verde-escura (O), Crespa de Capo Bonito (I), Couve de folhas Manteiga 900 Legitima P, Alto (AB), Gigante I-915 (E) and Manteiga Ribeiro Pires I-2446 (H) cultivars reduced the larval weight of A. monuste orseis.

ASSESSING Bemisia tabaci (GENN.) BIOTYPE B RESISTANCE IN SOYBEAN GENOTYPES: ANTIXENOSIS AND ANTIBIOSIS

Since it was first reported in Brazil in the 1990s, the B biotype of silverleaf whitefly (Bemisia tabaci [Genn.], Hemiptera: Aleyrodidae) has been recognized as an important pest in soybeans (Glycine max L.), reducing the productivity of this legume species in some areas of the country. As an alternative to chemical control, the use of resistant genotypes represents an important tool for integrated pest management (IPM). This study evaluated the performance of 10 soybean genotypes prior to whitefly infestation, by testing attractiveness and preference for oviposition in the greenhouse and antibiosis in the laboratory. In a multiple-choice test, 'IAC-17' was the least attractive to insects. In a no-choice test, 'IAC-17' was the least attractive for, egg deposition, indicating the occurrence of non-preference for oviposition on this genotype. Trichome density was positively correlated with the oviposition site and may be associated with the resistance of 'IAC-17' to infestation. The genotypes 'IAC-PL1', 'IAC-19', 'Conquista', 'IAC-24' and 'IAC-17' extended the insect's life cycle, indicating the occurrence of a small degree of antibiosis and/or non-preference for feeding.

Tradeoffs associated with constitutive and induced plant resistance against herbivory

Several prominent hypotheses have been posed to explain the immense variability among plant species in defense against herbivores. A major concept in the evolutionary ecology of plant defenses is that tradeoffs of defense strategies are likely to generate and maintain species diversity. In particular, tradeoffs between constitutive and induced resistance and tradeoffs relating these strategies to growth and competitive ability have been predicted. We performed three independent experiments on 58 plant species from 15 different plant families to address these hypotheses in a phylogenetic framework. Because evolutionary tradeoffs may be altered by human-imposed artificial selection, we used 18 wild plant species and 40 cultivated garden-plant species. Across all 58 plant species, we demonstrate a tradeoff between constitutive and induced resistance, which was robust to accounting for phylogenetic history of the species. Moreover, the tradeoff was driven by wild species and was not evident for cultivated species. In addition, we demonstrate that more competitive species—but not fast growing ones—had lower constitutive but higher induced resistance. Thus, our multispecies experiments indicate that the competition–defense tradeoff holds for constitutive resistance and is complemented by a positive relationship of competitive ability with induced resistance. We conclude that the studied genetically determined tradeoffs are indeed likely to play an important role in shaping the high diversity observed among plant species in resistance against herbivores and in life history traits.

Sexual dimorphism dominates divergent host plant use in stick insect trophic morphology

Background: Clear examples of ecological speciation exist, often involving divergence in trophic morphology. However, substantial variation also exists in how far the ecological speciation process proceeds, potentially linked to the number of ecological axes, traits, or genes subject to divergent selection. In addition, recent studies highlight how differentiation might occur between the sexes, rather than between populations. We examine variation in trophic morphology in two host-plant ecotypes of walking-stick insects (Timema cristinae), known to have diverged in morphological traits related to crypsis and predator avoidance, and to have reached an intermediate point in the ecological speciation process. Here we test how host plant use, sex, and rearing environment affect variation in trophic morphology in this species using traditional multivariate, novel kernel density based and Bayesian morphometric analyses. Results: Contrary to expectations, we find limited host-associated divergence in mandible shape. Instead, the main predictor of shape variation is sex, with secondary roles of population of origin and rearing environment. Conclusion: Our results show that trophic morphology does not strongly contribute to host-adapted ecotype divergence in T. cristinae and that traits can respond to complex selection regimes by diverging along different intraspecific lines, thereby impeding progress toward speciation.

Contrasting Effects of Ethylene Biosynthesis on Induced Plant Resistance against a Chewing and a Piercing-Sucking Herbivore in Rice

Ethylene is a stress hormone with contrasting effects on herbivore resistance. However, it remains unknown whether these differences are plant- or herbivore-specific. We cloned a rice 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene, OsACS2, whose transcripts were rapidly up-regulated in response to mechanical wounding and infestation by two important pests: the striped stem borer (SSB) Chilo suppressalis and the brown planthopper (BPH) Nilaparvata lugens. Antisense expression of OsACS2 (as-acs) reduced elicited ethylene emission, SSB-elicited trypsin protease inhibitor (TrypPI) activity, SSB-induced volatile release, and SSB resistance. Exogenous application of ACC restored TrypPI activity and SSB resistance. In contrast to SSB, BPH infestation increased volatile emission in as-acs lines. Accordingly, BPH preferred to feed and oviposit on wild-type (WT) plants—an effect that could be attributed to two repellent volatiles, 2-heptanone and 2-heptanol, that were emitted in higher amounts by as-acs plants. BPH honeydew excretion was reduced and natural enemy attraction was enhanced in as-acs lines, resulting in higher overall resistance to BPH. These results demonstrate that ethylene signaling has contrasting, herbivore-specific effects on rice defense responses and resistance against a chewing and a piercing-sucking insect, and may mediate resistance trade-offs between herbivores of different feeding guilds in rice.

Jasmonate-dependent depletion of soluble sugars compromises plant resistance to Manduca sexta

Jasmonates regulate plant secondary metabolism and herbivore resistance. How they influence primary metabolites and how this may affect herbivore growth and performance are not well understood. We profiled sugars and starch of jasmonate biosynthesis-deficient and jasmonate-insensitive Nicotiana attenuata plants and manipulated leaf carbohydrates through genetic engineering and in vitro complementation to assess how jasmonate-dependent sugar accumulation affects the growth of Manduca sexta caterpillars. We found that jasmonates reduce the constitutive and herbivore-induced concentration of glucose and fructose in the leaves across different developmental stages. Diurnal, jasmonate-dependent inhibition of invertase activity was identified as a likely mechanism for this phenomenon. Contrary to our expectation, both in planta and in vitro approaches showed that the lower sugar concentrations led to increased M. sexta growth. As a consequence, jasmonate-dependent depletion of sugars rendered N. attenuata plants more susceptible to M. sexta attack. In conclusion, jasmonates are important regulators of leaf carbohydrate accumulation and this determines herbivore growth. Jasmonate-dependent resistance is reduced rather than enhanced through the suppression of glucose and fructose concentrations, which may contribute to the evolution of divergent resistance strategies of plants in nature.

The role of the secondary cell wall in plant resistance to pathogens

Plant resistance to pathogens relies on a complex network of constitutive and inducible defensive barriers. The plant cell wall is one of the barriers that pathogens need to overcome to successfully colonize plant tissues. The traditional view of the plant cell wall as a passive barrier has evolved to a concept that considers the wall as a dynamic structure that regulates both constitutive and inducible defense mechanisms, and as a source of signaling molecules that trigger immune responses. The secondary cell walls of plants also represent a carbon-neutral feedstock (lignocellulosic biomass) for the production of biofuels and biomaterials. Therefore, engineering plants with improved secondary cell wall characteristics is an interesting strategy to ease the processing of lignocellulosic biomass in the biorefinery. However, modification of the integrity of the cell wall by impairment of proteins required for its biosynthesis or remodeling may impact the plants resistance to pathogens. This review summarizes our understanding of the role of the plant cell wall in pathogen resistance with a focus on the contribution of lignin to this biological process.

PoolSeq analysis of the selection of the Rhizobium genotypes by the legume host plant

Rhizobium leguminosarum establishes highly specific nitrogen-fixing symbioses. We have applied a Pool-Seq approach to study plant host selection of genotypes. Our results confirm, at the genomic level, previous observations regarding plant selection of specific genotypes

Quantitative trait loci and metabolic pathways: genetic control of the concentration of maysin, a corn earworm resistance factor, in maize silks.

Interpretation of quantitative trait locus (QTL) studies of agronomic traits is limited by lack of knowledge of biochemical pathways leading to trait expression. To more fully elucidate the biological significance of detected QTL, we chose a trait that is the product of a well-characterized pathway, namely the concentration of maysin, a C-glycosyl flavone, in silks of maize, Zea mays L. Maysin is a host-plant resistance factor against the corn earworm, Helicoverpa zea (Boddie). We determined silk maysin concentrations and restriction fragment length polymorphism genotypes at flavonoid pathway loci or linked markers for 285 F2 plants derived from the cross of lines GT114 and GT119. Single-factor analysis of variance indicated that the p1 region on chromosome 1 accounted for 58.0% of the phenotypic variance and showed additive gene action. The p1 locus is a transcription activator for portions of the flavonoid pathway. A second QTL, represented by marker umc 105a near the brown pericarp1 locus on chromosome 9, accounted for 10.8% of the variance. Gene action of this region was dominant for low maysin, but was only expressed in the presence of a functional p1 allele. The model explaining the greatest proportion of phenotypic variance (75.9%) included p1, umc105a, umc166b (chromosome 1), r1 (chromosome 10), and two epistatic interaction terms, p1 x umc105a and p1 x r1. Our results provide evidence that regulatory loci have a central role and that there is a complex interplay among different branches of the flavonoid pathway in the expression of this trait.

Intra-specific variation for host plant use in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae): Implications for management

The polyphagous moth Helicoverpa armigera (Hubner) is one of the world's most important agricultural pests. A number of existing approaches and future designs for management of H. armigera rely on the assumption that moths do not exhibit either genetically and/or non-genetically based variation for host plant utilization. We review recent empirical evidence demonstrating that both these forms of variation influence host plant use in this moth. The significance of this variation in H. armigera in relation to current and future pest management strategies is examined. We provide recommendations on future research needs and directions for sustainable management of H. armigera, under a framework that includes consideration of intra.-specific variation for host use relevant in this and other similar pest species. (C) 2004 Elsevier Ltd. All rights reserved.

The onset and effectiveness of Adult Plant Resistance (APR) in Tallon barley

Field observations of net blotch epidemics indicated that Tallon barley was quite resistant to infection during later stages of growth despite being susceptible as a seedling. A glasshouse experiment was conducted to determine the effectiveness of this resistance and when it became operative. Three cultivars – Gilbert (very susceptible), Patty (resistant) and Tallon – were inoculated at various stages of growth with conidia of Pyrenophora teres f. teres and the infection response and leaf area diseased, recorded 13 days later. The response of Tallon clearly changed from susceptible to moderately susceptible at growth stage 33. Plants sown two weeks earlier were susceptible and plants sown two weeks later were moderately resistant. The response of the other two cultivars at similar growth stages paralleled their seedling responses. The resistance of Tallon appeared to increase with maturity so that, at its most resistant growth stage, the leaf area diseased was just 10% that of the susceptible, Gilbert. While this resistance appears pathotype specific, this experiment demonstrated very effective APR to net blotch. As most losses to this disease occur during the later stages of plant development, APR offers a valuable source of resistance.