This review focuses on individual effects of major global change factors, such as elevated CO 2 , O 3 , UV light and temperature, on plant secondary chemistry. These secondary metabolites are well-known for their role in plant defense against insect herbivory. Global change effects on secondary chemicals appear to be plant species-specific and dependent on the chemical type. Even though plant chemical responses induced by these factors are highly variable, there seems to be some specificity in the response to different environmental stressors. For example, even though the production of phenolic compounds is enhanced by both elevated CO 2 and UV light levels, the latter appears to primarily increase the concentrations of flavonoids. Likewise, specific phenolic metabolites seem to be induced by O 3 but not by other factors, and an increase in volatile organic compounds has been particularly detected under elevated temperature. More information is needed regarding how global change factors influence inducibility of plant chemical defenses as well as how their indirect and direct effects impact insect performance and behavior, herbivory rates and pathogen attack. This knowledge is crucial to better understand how plants and their associated natural enemies will be affected in future changing environments. Climate change has been defined as "any change in climate over time, whether due to natural variability or as a result of human activity" (Intergovernmental Panel on Climate Change, IPCC 2007). However, this term is usually used in the context of global changes resulting from anthropogenic actions. Regardless of subtle differences in the usage of this term, there is a general consensus that current changes in climatic factors are primarily due to human-driven practices such as burning of fossil fuels and deforestation. These activities release significant amounts carbon into the atmosphere, which have caused a dramatic increase in carbon dioxide (CO 2 ) concentrations during the past 60 years. The IPCC has repeatedly reported that augmented emissions of greenhouse gases into the atmosphere (mainly elevated CO 2 followed by methane and ozone) are the major cause of climatic changes observed today. These changes include global warming, precipitation fluctuations, rising sea levels and other "extreme climatic events". Despite the amount of information that exists on global climate change, little is known regarding how these changes may affect natural ecosystems, particularly interactions among living organisms.Interactions of plants and insects are of major importance in most natural ecosystems since these two groups of organisms are extremely diverse and comprise almost 50% of all identified species on earth (Price 1997). The phytochemical coevolution theory suggests that secondary metabolites are likely the most important mediators of plant-insect interactions (Ehrlich and Raven 1964; Berenbaum 1983 Berenbaum , 1995 Cornell and Hawkins 2003). According to this theory, both plants and insect herbivores gene...

Glucosinolates are commonly found in Arabidopsis thaliana and its crucifer relatives, which are known for their role in defense against insect herbivory. In a common garden experiment, we assessed genotypic variation in glucosinolates in A. thaliana and evaluated the association between this chemistry and both plant damage and fitness. Specifically, glucosinolate concentrations were directly associated with damage levels and inversely associated with fitness. These results are contrary to the general expectation that enhanced chemical defense should result in decreased insect herbivory. As the measured insect community in this field trial was dominated by specialist herbivores, this positive relationship between glucosinolates and herbivory agrees with previous observations that glucosinolates (or their hydrolysis products) attract specialist insects. In addition, glucosinolate diversity in this common garden appeared to affect herbivore damage levels. For example, genotypes that contained alkenyl glucosinolates had higher mean damage levels than those that contained hydroxyalkyl glucosinolates. Results suggest that genotypic variation in glucosinolates may be a major factor in determining plant utilization patterns by insect herbivores in the field.

A general prediction of the specialist/generalist paradigm indicates that plant responses to insect herbivores may depend on the degree of ecological specialization of the insect attacker. However, results from a single greenhouse experiment evaluating the responses of the model plant Arabidopsis thaliana to three specialist (Plutella xylostella, Pieris rapae, and Brevicoryne brassicae) and three generalist (Trichoplusia ni, Spodoptera exigua, and Myzus persicae) insect species did not support the previous prediction. Using an ecological genomic approach, we assessed plant responses in terms of herbivore-induced changes in genome-wide gene expression, defense-related pathways, and concentrations of glucosinolates (i.e., secondary metabolites that are ubiquitously present in cruciferous plants). Our results showed that plant responses were not influenced by the degree of specialization of insect herbivores. In contrast, responses were more strongly shaped by insect taxa (i.e., aphid vs. lepidopteran species), likely due to their different feeding modes. Interestingly, similar patterns of plant responses were induced by the same insect herbivore species in terms of defense signaling (jasmonic acid pathway), aliphatic glucosinolate metabolism (at both the gene expression and phenotypic levels) and genome-wide responses. Furthermore, plant responses to insect herbivores belonging to the same taxon (i.e., four lepidopteran species) were not explained by herbivore specialization or phylogenetic history. Overall, this study suggests that different feeding modes of insect taxa as well as herbivore-specific plant responses, which may result from distinct ecological/evolutionary interactions between A. thaliana (or a close relative) and each of the lepidopteran species, may explain why observed responses deviate from those predicted by the specialist/generalist paradigm.

We experimentally demonstrate that elevated CO(2) can modify herbivory-induced plant chemical responses in terms of both total and individual glucosinolate concentrations. Overall, herbivory by larvae of diamondback moths (Plutella xylostella) resulted in no change in glucosinolate levels of the annual plant Arabidopsis thaliana under ambient CO(2) conditions. However, herbivory induced a significant 28-62% increase in glucosinolate contents at elevated CO(2). These inducible chemical responses were both genotype-specific and dependent on the individual glucosinolate considered. Elevated CO(2) can also affect structural defenses such as trichomes and insect-glucosinolate interactions. Insect performance was significantly influenced by specific glucosinolates, although only under CO(2) enrichment. This study can have implications for the evolution of inducible defenses and coevolutionary adaptations between plants and their associated herbivores in future changing environments.