Insect feeding, plant responses

More recently it has become apparent that plants respond dynamically at a metabolic level to insect feeding, and that these induced responses can be of sufficient diversity and intensity to confer resistance (6-11). Many of these responses in the plant s chemical composition are identical to those induced as a result of attack by microorganisms (9.12-14). For example, insect feeding... 

Another pathway is the fatty acid hydroperoxidase lyase (HPL) pathway. This pathway produces C6-aldehydes and C12-oxo acids. 2 C6-volatiles, including ( )-2-hexenal, (Z)-3-hexenal, hexanal, as well as their corresponding alcohols or esters, are produced from mechanically wounded plant tissue.197,203 C6-aldehydes are also formed during hypersensitive response to infection by bacterial pathogens, after insect feeding, and after exogenous application ofJA.204,205 As already shown in this chapter, some of C6-volatiles are known to be attractants of parasitoids and predators. 

Besides SA, jasmonic acid (JA) and its methyl ester are involved in ISR as signalling moleeules. JA and its methyl ester have been mainly considered as mediators of plant responses triggered by wounding and insect feeding, but their involvement in resistanee against pathogens has also been proven. For example, methyl jasmonate was used in parsley eell suspension cultures... 

Solanum lycopersicum. In response to insect feeding, tomato plants (S. lyco-persicum L. sub nom. Lycopersicon esculentum Mill.) were found to release locally and systemically elevated levels of the monoterpenes a-pinene, P-pinene, 2-carene, and P-phellandrene as well as different sesquiterpenes (Sect. 7.3) (Farag and Pare 2002). 

Plants respond to the mechanical or insect herbivore damage of their tissues." During herbivorous attacks, some plants emit a specific blend of volatiles, which may result in defense responses retarding development of the herbivores or attraction of herbivore enemies to feed upon them. In lima bean leaves, the spider mite-induced volatiles, as well as infestation and artificial wounding, activate the ethylene and JA signaling pathways. ... 

The selection or avoidance of potential host plants by phytophagous Insects Is guided by a complex combination of physical and chemical stimuli. Color, shape and olfactory cues may play a role In the Initial orientation, whereas acceptance or rejection of a plant depends on texture as well as chemical stimulants or deterrents. Initiation of feeding Is stimulated or deterred by the presence or absence of specific chemicals or groups of chemicals, many of which have been Identified. The selection of a suitable plant for ovlposltlon Is also crucial for survival of the progeny of most herbivorous Insects, but the chemical factors Involved are known In relatively few cases. Ovlposltlon stimulants and deterrents often appear to be quite different from the chemicals that elicit or Inhibit feeding responses of larvae. 

Different insect species posses different gustatory receptor cells, their response spectra being adapted to the perception of chemical components distributed in their host plant species (9 ). Taste perception in P. brassicae larvae forms a representative example for phytophagous insects, which are able to discriminate a number of compounds like sugars, amino acids, salts, and secondary plant substances acting as feeding inhibitors or feeding incitants (3,6,, 10). 

The apparent selective activation of genes responsible for induced odor production and the committed function of the resulting enzymes may allow for a precise fine tuning between insect-derived elicitors and the responses of the plant. Thus, plants have the potential to adapt their signals specifically to the insect that feeds on a plant. Several studies present evidence for such specificity. 

To investigate food preferences by insects or other herbivores, such as deer (Rautio et al. 2008), and the compounds responsible for their choices, these compounds can be added to their diet and tested in feeding bioassays. Here we will add a mixture of phenolic compounds, known as tannic acid, to the diet of homworm caterpillars. The tobacco homworm, Manduca sexta, normally feeds on Solanaceae such as tomato or potato plants. For more on the natural history of this insect, consult the information sheet prepared by the biological supply company that ships these caterpillars. 

BPH settling response on tillers of the susceptible VTN1V variety sprayed with the steam distillate extracts of resistant varieties showed the same pattern of response as when actual resistant plants were used. This indicated that treatment of the susceptible variety with steam distillate extracts of resistant varieties conferred resistance at least temporarily. The low amount of honeydew excreted by BPH females on the tillers of TNI plants treated with the extract of ARC 6650 resistant or Ptb 33 variety confirmed that the insect was unable to settle down for sustained feeding (Figure 3). Nymphs caged on similarly treated TNI plants were unable to settle on them and suffered high mortality. Thus, restlessness of BPH nymphs and adults on resistant plants could be attributed to exposure to the plant volatiles which have a repellent or toxic effect on the insect. 

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