Herbivorous insects use diverse feeding strategies to obtain nutrients from their host plants. Rather than acting as passive victims in these interactions, plants respond to herbivory with the production of toxins and defensive proteins that target physiological processes in the insect. Herbivore-challenged plants also emit volatiles that attract insect predators and bolster resistance to future threats. This highly dynamic form of immunity is initiated by the recognition of insect oral secretions and signals from injured plant cells. These initial cues are transmitted within the plant by signal transduction pathways that include calcium ion fluxes, phosphorylation cascades, and, in particular, the jasmonate pathway, which plays a central and conserved role in promoting resistance to a broad spectrum of insects. A detailed understanding of plant immunity to arthropod herbivores will provide new insights into basic mechanisms of chemical communication and plant-animal coevolution and may also facilitate new approaches to crop protection and improvement.

Plant Immunity to Insect Herbivores

▪ Abstract Plants respond to herbivore attack with a bewildering array of responses, broadly categorized as direct and indirect defenses, and tolerance. Plant-herbivore interactions are played out on spatial scales that include the cellular responses, well-studied in plant-pathogen interactions, as well as responses that function at whole-plant and community levels. The plant's wound response plays a central role but is frequently altered by insect-specific elicitors, giving plants the potential to optimize their defenses. In this review, we emphasize studies that advance the molecular understanding of elicited direct and indirect defenses and include verifications with insect bioassays. Large-scale transcriptional changes accompany insect-induced resistance, which is organized into specific temporal and spatial patterns and points to the existence of herbivore-specific trans-activating elements orchestrating the responses. Such organizational elements could help elucidate the molecular control over the d...

Plant responses to insect herbivory: The emerging molecular analysis

The role of biotic interactions in shaping plant flowering phenology has long been controversial; plastic responses to the abiotic environment, limited precision of biological clocks and inconsistency of selection pressures have generally been emphasized to explain phenological variation. However, part of this variation is heritable and selection analyses show that biotic interactions can modulate selection on flowering phenology. Our review of the literature indicates that pollinators tend to favour peak or earlier flowering, whereas pre-dispersal seed predators tend to favour off-peak or later flowering. However, effects strongly vary among study systems. To understand such variation, future studies should address the impact of mutualist and antagonist dispersal ability, ecological specialization, and habitat and plant population characteristics. Here, we outline future directions to study how such interactions shape flowering phenology.

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Time after time: flowering phenology and biotic interactions

Preface.- List of contributors.- 1. Introduction to allelopathy Chou,C-H.- 2. Basic pathways for the origin of allelopathic compounds Seigler, D.S.- Methodological aspects. 3. Clues in the search of new herbicides Dayan, F.E., Duke, S.O.- 4. Distinguishing allelopathy from resource competition: the role of density Weidenhamer, J.D.- 5. Toxicity in allelopathy: in silico approach Lo Piparo, E. et al.- Physiological aspects of allelopathy. 6. Allelochemicals and photosynthesis Zhou, Y.H., Yu, J.Q.- 7. Cell cycle analyses for understanding growth inhibition Sanchez-Moreiras, A.M. et al. 8. Detoxification of allelochemicals. The case of bezoxazolin-2(3H)-one (BOA) Schulz, M. et al.- 9. Allelopathy and abiotic stress Pedrol, M.N. et al.- 10. Allelopathy and biotic stresses Gawronska, H., Golisz, A.- 11. Plant secondary metabolites. Targets and mechanisms of allelopathy Lotina-Hennsen, B. et al.- 12. Mitochondria as a site of allelochemical action Ishii-Iwamoto, E.L. et al.- Ecophysiology and allelopathy. 13. Weed germination, seedling growth and their lesson for allelopathy in agriculture Aliotta, A. et al.- 14. Allelopathy: A soil system perspective Blum, U.- 15. Microorganisms and allelopathy: A one-sided approach Vokou, D. et al.- Ecological aspects of allelopathy. 16. Ecological relationships and allelopathy Sinkkonen, A.- 17. Resistance and susceptibility of plant communities to invasion:revisiting rabotnov's ideas about community homeostasis Callaway, R.M., Hierro, J.L.- Allelopathy in different environments. 18. Allelopathy in marine ecosystems Graneli, E., Pavia, H.- 19. Allelopathy in aquatic environments Erhard, D.- 20. Forest ecosystems and allelopathy Reigosa, M.J., Gonzalez, L.- 21. Allelopathic interactions in agroecosystems Kohli, R.K. et al.- Applied aspects of allelopathy. 22. Playing with chemistry: studies on Orobanche spp. germination stimulants Macias, F.A. et al.- 23. Modes of action of phytotoxins from plants Duke, S.O., Dayan, F.E.- 24. Allelopathy in ecological sustainable agriculture Narwal, S.S.- 25. Parasitic weeds and allelopathy: from the hypothesis to the proof Qasem, J.R.-

Allelopathy : a physiological process with ecological implications

Evidence for the in planta defensive function of trypsin protease inhibitors (TPIs) comes from observations of enhanced herbivore resistance after heterologous TPI expression or the manipulation of signal cascades that activate numerous defense responses, including TPI production; no studies have altered the expression of an endogenous pi gene to examine defensive function. We isolated two genes with seven- and six-repeat TPI domains from Nicotiana attenuata from the potato (Solanum tuberosum) PI-II family. To determine whether endogenous TPIs in N. attenuata function defensively against the native herbivores, hornworm (Manduca sexta) and mirids (Tupiocoris notatus), we expressed 175 bp of the seven-domain pi from N. attenuata in an antisense orientation in a TPI-producing genotype to reduce TPI expression and expressed the full-length seven-domain pi in a sense orientation under control of a constitutive promoter to restore TPI activity in a natural genotype from Arizona unable to produce TPIs. Constitutive and inducible TPI production in two antisense lines were diminished by 80% to 90% and 33% to 52%, respectively, and sense expression restored 67% of the activity found in the TPI-producing genotype after caterpillar attack in the TPI-deficient A genotype. Hornworm larvae fed on genotypes with low or no TPI activity grew faster, had higher survivorship, and produced heavier pupae than those that fed on genotypes with high TPI activity. T. notatus showed higher preference for genotypes with low or no TPI activity than for genotypes with high TPI levels. We conclude that endogenous TPIs are an effective defense against these native herbivores.

http://www.plantphysiol.org/content/plantphysiol/134/3/1181.full.pdf

Manipulation of endogenous trypsin proteinase inhibitor production in Nicotiana attenuata demonstrates their function as antiherbivore defenses

Prior study of the effect of protease inhibitors (PIs) on diamondback moths suggests that moths are resistant to them, so PIs represent an ineffective defence against moths. However, our data suggest that diamondback moths do suffer lower growth rates when they consume plants transformed with potato protease inhibitor (PI2), but that effect is hidden by compensatory consumption. Plants, instead of gaining an advantage by lowering the insect growth rate, suffer a disadvantage as moths consume more tissue to mitigate the effect. Furthermore, PI2, when used in conjunction with another transgenic pesticidal protein, Bt (from Bacillus thuringiensis) counteracts the effectiveness of Bt at protecting plant tissue. Thus, transgenic PIs are not only less effective than previously thought in protecting Brassica plants from diamondback moths, they may actually lead to increased plant damage by the moths.

Diamondback moth compensatory consumption of protease inhibitor-transformed plants.

Garlic mustard (Alliaria petiolata) is an invasive, spring-germinating, obligately biennial plant. In a central Pennsylvania forest, two distinctive plant patch types exist simultaneously: patches with mixed juveniles and adults and patches that are exclusively juvenile. We experimentally tested two hypotheses to explain this peculiar distribution. The habitat limitation hypothesis proposes that garlic mustard, like other biennials, is limited to rarely available habitats and can only reach maturity in a subset of the patches where its seeds germinate. The intraspecific competition hypothesis proposes that juveniles only survive to maturity in patches lacking adults. We mapped the natural distribution of ten juvenile and ten mature patches and used transects to assess juvenile and adult density in 2 y. We found that patches cycled annually from juvenile to mixed, supporting the intraspecific competition hypothesis. We looked for biotic (interspecific competition) and abiotic (shading) factors tha...

Spatial and Temporal Segregation of Juvenile and Mature Garlic Mustard Plants (Alliaria petiolata) in a Central Pennsylvania Woodland

We show with a model that variation in environmental stress between generations facilitates the evolution of stress resistance through assortative mating. Stress induces delayed maturation of susceptible phenotypes, segregating their fertile period from resistant phenotypes. Assortment of mates enhances the responsiveness of populations to natural selection by inflating genetic variance. Thus, positive selection and inflated genetic variance in stressful environments can cause a strong evolutionary increase in resistance. By contrast, benign environments do not segregate phenotypes, and the random mating among phenotypes deflates genetic variance, leading to a weaker response to selection against resistance, assuming that resistance is costly. When environments vary randomly from benign to stressful, populations respond asymmetrically to negative and positive selection. This asymmetry (1) accelerates fixation of a resistance allele if resistance is generally favoured (stressful generations more frequent) but delays the loss of the allele if it is generally disfavoured (benign generations more frequent), and (2) it can push a resistance allele to fixation even when long-term costs modestly exceed benefits. When resistance alleles pleiotropically delay mating, stress-induced random mating has complementary effects. Serial autocorrelation in the stressor amplifies these effects. These results suggest a novel mechanism for the persistence of resistance polymorphisms.

Juliette Winterer

Papers

Diamondback moth compensatory consumption of protease inhibitor-transformed plants.

Spatial and Temporal Segregation of Juvenile and Mature Garlic Mustard Plants (Alliaria petiolata) in a Central Pennsylvania Woodland

Stress‐induced assortative mating and the evolution of stress resistance