Elevated CO 2 and temperature are altering the interactions between plants and insects with important implications for food security and natural ecosystems. Ecologically, the acceleration of plant phenology by warming is generating mismatches between plants and insect pollinators. Similarly, shifting the rate of plant development relative to insect development can amplify or minimize the consequences of herbivory. Warming also enables some insects to increase the number of generations per year, thus increasing damage to plant communities. The suitability of plant tissues as food for insects also is modulated by global change. Elevated CO 2 typically increases the concentration of leaf carbohydrates and in combination with elevated temperature decreases nitrogen (N) content. Together, these changes lower nutritional value, causing certain herbivores to consume more foliage to meet their nutritional needs. Whereas the responses of primary metabolites in plants to global change are reasonably well understood, how elevated CO 2 and temperature affect plant defensive compounds (allelochemicals) is considerably less predictable. Recent studies indicate that exposure to elevated CO 2 suppresses the plant defense hormone jasmonic acid (JA) while stimulating production of salicylic acid (SA). By differentially affecting defense compounds, these changes in plant hormones potentially increase susceptibility to chewing insects and enhance resistance to pathogens. Exposure to elevated temperature, in contrast, stimulates JA, ethylene (ET), and SA, enhancing defenses. A deeper understanding of how elevated CO 2 and temperature, singly and in combination, modulate plant hormones promises to increase our understanding of how these elements of global change will affect the positive and negative interactions between plants and insects.Chemoautotrophs notwithstanding, plants provide energy in the form of carbohydrates for all nonphotosynthetic organisms, including insects. Not long after the colonization of land by plants 510 million years ago, plants and insects have engaged in an evolutionary arms race that continues today-plants evolve mechanisms to minimize consumption by insects, and insects evolve mechanisms to circumvent these defenses. Rapid changes in Earth's atmosphere initiated by the human use of fossil fuels is resetting this complex coevolutionary relationship, not only between plants and herbivores but also between plants and their mutualistic partners, including pollinators. Insects have the potential to cause enormous reductions in crop yields and the productivity of natural ecosystems, as well as to provide irreplaceable pollination services that underpin much of the world's agriculture.The combustion of fossil fuels during the Industrial Revolution initiated a rapid rise in atmospheric CO 2 concentration that is accelerating today; preindustrial levels were approximately 280 mL L 21 and below 300 mL L 21 for the previous 20 million years (Pearson and Palmer, 2000). Today's atmosphere is approximately 397 mL L 21...
This review briefly examines the indirect effects of herbivory on photosynthesis, measured by gas exchange or chlorophyll fluorescence, and identifies four mechanisms contributing to the indirect suppression of photosynthesis in remaining leaf tissues: severed vasculature, altered sink demand, defence-induced autotoxicity, and defence-induced down-regulation of photosynthesis. We review the chlorophyll fluorescence and thermal imaging techniques used to gather layers of spatial data and discuss methods for compiling these layers to achieve greater insight into mechanisms contributing to the indirect suppression of photosynthesis. We also elaborate on a few herbivore-induced gene-regulating mechanisms which modulate photosynthesis and discuss the difficult nature of measuring spatial heterogeneity when combining fluorescence imaging and gas exchange technology. Although few studies have characterized herbivore-induced indirect effects on photosynthesis at the leaf level, an emerging literature suggests that the loss of photosynthetic capacity following herbivory may be greater than direct loss of photosynthetic tissues. Depending on the damage guild, ignoring the indirect suppression of photosynthesis by arthropods and other organisms may lead to an underestimate of their physiological and ecological impacts.
By changing the chemical composition of foliage, the increase in atmospheric CO(2) is fundamentally altering insect herbivory. The responses of folivorous insects to these changes is, however, highly variable. In this review we highlight emerging mechanisms by which increasing CO(2) alters the defense chemistry and signaling of plants. The response of allelochemicals affecting insect performance varies under elevated CO(2), and results suggest this is driven by changes in plant hormones. Increasing CO(2) suppresses the production of jasmonates and ethylene and increases the production of salicylic acid, and these differential responses of plant hormones affect specific secondary chemical pathways. In addition to changes in secondary chemistry, elevated CO(2) decreases rates of water loss from leaves, increases temperature and feeding rates, and alters nutritional content. New insights into the mechanistic responses of secondary chemistry to elevated CO(2) increase our ability to predict the ecological and evolutionary responses of plants attacked by insects.
Endoparasitism by gall-forming insects dramatically alters the plant phenotype by altering growth patterns and modifying plant organs in ways that appear to directly benefit the gall former. Because these morphological and physiological changes are linked to the presence of the insect, the induced phenotype is said to function as an extension of the parasite, albeit by unknown mechanisms. Here we report the gall-forming aphid-like parasite phylloxera, Daktulosphaira vitifoliae, induces stomata on the adaxial surface of grape leaves where stomata typically do not occur. We characterized the function of the phylloxera-induced stomata by tracing transport of assimilated carbon. Because induction of stomata suggests a significant manipulation of primary metabolism, we also characterized the gall transcriptome to infer the level of global reconfiguration of primary metabolism and the subsequent changes in downstream secondary metabolism. Phylloxera feeding induced stomata formation in proximity to the insect and promoted the assimilation and importation of carbon into the gall. Gene expression related to water, nutrient, and mineral transport; glycolysis; and fermentation increased in leaf-gall tissues. This shift from an autotrophic to a heterotrophic profile occurred concurrently with decreased gene expression for nonmevalonate and terpenoid synthesis and increased gene expression in shikimate and phenylpropanoid biosynthesis, secondary metabolite systems that alter defense status in grapes. These functional insect-induced stomata thus comprise part of an extended phenotype, whereby D. vitifoliae globally reprograms grape leaf development to alter patterns of primary metabolism, nutrient mobilization, and defense investment in favor of the galling habit.source-sink | Vitis | photosynthesis
Herbivory initiates a shift in plant metabolism from growth to defence that may reduce fitness in the absence of further herbivory. However, the defence-induced changes in carbon assimilation that precede this reallocation in resources remain largely undetermined. This study characterized the response of photosynthesis to herbivore induction of jasmonic acid (JA)-related defences in Nicotiana attenuata to increase understanding of these mechanisms. It was hypothesized that JA-induced defences would immediately reduce the component processes of photosynthesis upon attack and was predicted that wild-type plants would suffer greater reductions in photosynthesis than plants lacking JA-induced defences. Gas exchange, chlorophyll fluorescence, and thermal spatial patterns were measured together with the production of defence-related metabolites after attack and through recovery. Herbivore damage immediately reduced electron transport and gas exchange in wild-type plants, and gas exchange remained suppressed for several days after attack. The sustained reductions in gas exchange occurred concurrently with increased defence metabolites in wild-type plants, whereas plants lacking JA-induced defences suffered minimal suppression in photosynthesis and no increase in defence metabolite production. This suppression in photosynthesis occurred only after sustained defence signalling and defence chemical mobilization, whereas a short bout of feeding damage only transiently altered components of photosynthesis. It was identified that lipoxygenase signalling interacted with photosynthetic electron transport and that the resulting JA-related metabolites reduced photosynthesis. These data represent a metabolic cost to mounting a chemical defence against herbivory and link defence-signalling networks to the differential effects of herbivory on photosynthesis in remaining leaf tissues in a time-dependent manner.
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