Leaf miner-induced changes in leaf transmittance cause variations in insect respiration rates

Stomata at the crossroad of molecular interaction between biotic and abiotic stress responses in plants

Increasing global food production is threatened by harsh environmental conditions along with biotic stresses, requiring massive new research into integrated stress resistance in plants. Stomata play a pivotal role in response to many biotic and abiotic stresses, but their orchestrated interactions at the molecular, physiological, and biochemical levels were less investigated. Here, we reviewed the influence of drought, pathogen, and insect herbivory on stomata to provide a comprehensive overview in the context of stomatal regulation. We also summarized the molecular mechanisms of stomatal response triggered by these stresses. To further investigate the effect of stomata-herbivore interaction at a transcriptional level, integrated transcriptome studies from different plant species attacked by different pests revealed evidence of the crosstalk between abiotic and biotic stress. Comprehensive understanding of the involvement of stomata in some plant-herbivore interactions may be an essential step towards herbivores' manipulation of plants, which provides insights for the development of integrated pest management strategies. Moreover, we proposed that stomata can function as important modulators of plant response to stress combination, representing an exciting frontier of plant science with a broad and precise view of plant biotic interactions.

Towards understanding Cameraria ohridella (Lepidoptera: Gracillariidae) development: effects of microhabitat variability in naturally growing horse-chestnut tree canopy

Dwelling intensity of horse-chestnut miner (Cameraria ohridella) larvae in various leaves insolation and temperature was measured to determine whether this pest's development follows a predictable pattern or depends more on local microenvironment conditions. Mines growing on leaves of mature host plants (Aesculus hippocastanum L.) in their natural conditions were photographed for two consecutive generations of the pest and in two separated vegetation periods. Apart from meteorological data obtained from the nearest station, the temperature of intact and mined parts of sun-exposed and shaded leaf blades was measured at various daytimes throughout the experiment. Obtained sets of digital data were analysed and combined to model mine area growth as a function of degree-days sum by adopting of Verhulst logistic equation. We showed the predictive potential of our model based on experimental data, and it may be useful in the scheduling of pest control measures in natural conditions. Our analyses also revealed that despite significant differences in microenvironment conditions depending on mines' insolation, the horse-chestnut miner larvae could partially compensate for them and complete their development at similar endpoints expressed as the cumulative sum of degree-days. We conclude that computer-aided analysis of photographic documentation of leaf-miner larval growth followed by mathematical modelling offers a noninvasive, reliable, and inexpensive alternative for monitoring local leaf-miners populations.

Extended phenotypes: buffers or amplifiers of climate change?

Historic approaches to understanding biological responses to climate change have viewed climate as something external that happens to organisms. Organisms, however, at least partially influence their own climate experience by moving within local mosaics of microclimates. Such behaviors are increasingly being incorporated into models of species distributions and climate sensitivity. Less attention has focused on how organisms alter microclimates via extended phenotypes: phenotypes that extend beyond the organismal surface, including structures that are induced or built. We argue that predicting the consequences of climate change for organismal performance and fitness will depend on understanding the expression and consequences of extended phenotypes, the microclimatic niches they generate, and the power of plasticity and evolution to shape those niches.

Hypoxia and hypercarbia in endophagous insects: Larval position in the plant gas exchange network is key

Gas composition is an important component of any micro-environment. Insects, as the vast majority of living organisms, depend on O2 and CO2 concentrations in the air they breathe. Low O2 (hypoxia), and high CO2 (hypercarbia) levels can have a dramatic effect. For phytophagous insects that live within plant tissues (endophagous lifestyle), gas is exchanged between ambient air and the atmosphere within the insect habitat. The insect larva contributes to the modification of this environment by expiring CO2. Yet, knowledge on the gas exchange network in endophagous insects remains sparse. Our study identified mechanisms that modulate gas composition in the habitat of endophagous insects. Our aim was to show that the mere position of the insect larva within plant tissues could be used as a proxy for estimating risk of occurrence of hypoxia and hypercarbia, despite the widely diverse life history traits of these organisms. We developed a conceptual framework for a gas diffusion network determining gas composition in endophagous insect habitats. We applied this framework to mines, galls and insect tunnels (borers) by integrating the numerous obstacles along O2 and CO2 pathways. The nature and the direction of gas transfers depended on the physical structure of the insect habitat, the photosynthesis activity as well as stomatal behavior in plant tissues. We identified the insect larva position within the gas diffusion network as a predictor of risk exposure to hypoxia and hypercarbia. We ranked endophagous insect habitats in terms of risk of exposure to hypoxia and/or hypercarbia, from the more to the less risky as cambium mines>borer tunnels≫galls>bark mines>mines in aquatic plants>upper and lower surface mines. Furthermore, we showed that the photosynthetically active tissues likely assimilate larval CO2 produced. In addition, temperature of the microhabitat and atmospheric CO2 alter gas composition in the insect habitat. We predict that (i) hypoxia indirectly favors the evolution of cold-tolerant gallers, which do not perform well at high temperatures, and (ii) normoxia (ambient O2 level) in mines allows miners to develop at high temperatures. Little is known, however, about physiological and morphological adaptations to hypoxia and hypercarbia in endophagous insects. Endophagy strongly constrains the diffusion processes with cascading consequences on the evolutionary ecology of endophagous insects.

An emerging understanding of mechanisms governing insect herbivory under elevated CO2

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.

Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts

The life cycles of many organisms are constrained by the seasonality of resources. This is particularly true for leaf-mining herbivorous insects that use deciduous leaves to fuel growth and reproduction even beyond leaf fall. Our results suggest that an intimate association with bacterial endosymbionts might be their way of coping with nutritional constraints to ensure successful development in an otherwise senescent environment. We show that the phytophagous leaf-mining moth Phyllonorycter blancardella (Lepidoptera) relies on bacterial endosymbionts, most likely Wolbachia, to manipulate the physiology of its host plant resulting in the 'green-island' phenotype--photosynthetically active green patches in otherwise senescent leaves--and to increase its fitness. Curing leaf-miners of their symbiotic partner resulted in the absence of green-island formation on leaves, increased compensatory larval feeding and higher insect mortality. Our results suggest that bacteria impact green-island induction through manipulation of cytokinin levels. This is the first time, to our knowledge, that insect bacterial endosymbionts have been associated with plant physiology.

Regional climate modulates the canopy mosaic of favourable and risky microclimates for insects

1. One major gap in our ability to predict the impacts of climate change is a quantitative analysis of temperatures experienced by organisms under natural conditions. We developed a framework to describe and quantify the impacts of local climate on the mosaic of microclimates and physiological states of insects within tree canopies. This approach was applied to a leaf mining moth feeding on apple leaf tissues. 2. Canopy geometry was explicitly considered by mapping the 3D position and orientation of more than 26 000 leaves in an apple tree. Four published models for canopy radiation interception, energy budget of leaves and mines, body temperature and developmental rate of the leaf miner were integrated. Model predictions were compared with actual microclimate temperatures. The biophysical model accurately predicted temperature within mines at different positions within the tree crown. 3. Field temperature measurements indicated that leaf and mine temperature patterns differ according to the regional climatic conditions (cloudy or sunny) and depending on their location within the canopy. Mines in the sun can be warmer than those in the shade by several degrees and the heterogeneity of mine temperature was incremented by 120%, compared with that of leaf temperature. 4. The integrated model was used to explore the impact of both warm and exceptionally hot climatic conditions recorded during a heat wave on the microclimate heterogeneity at canopy scale. During warm conditions, larvae in sunlight-exposed mines experienced nearly optimal growth conditions compared with those within shaded mines. The developmental rate was increased by almost 50% in the sunny microhabitat compared with the shaded location. Larvae, however, experienced optimal temperatures for their development inside shaded mines during extreme climatic conditions, whereas larvae in exposed mines were overheating, leading to major risks of mortality. 5. Tree canopies act as both magnifiers and reducers of the climatic regime experienced in open air outside canopies. Favourable and risky spots within the canopy do change as a function of the climatic conditions at the regional scale. The shifting nature of the mosaic of suitable and risky habitats may explain the observed uniform distribution of leaf miners within tree canopies.

Herbivory mitigation through increased water-use efficiency in a leaf-mining moth-apple tree relationship

Herbivory alters plant gas exchange but the effects depend on the type of leaf damage. In contrast to ectophagous insects, leaf miners, by living inside the leaf tissues, do not affect the integrity of the leaf surface. Thus, the effect of leaf miners on CO2 uptake and water-use efficiency by leaves remains unclear. We explored the impacts of the leaf-mining moth Phyllonorycter blancardella (Lepidoptera: Gracillariidae) on light responses of the apple leaf gas exchanges to determine the balance between the negative effects of reduced photosynthesis and potential positive impacts of increased water-use efficiency (WUE). Gas exchange in intact and mined leaf tissues was measured using an infrared gas analyser. The maximal assimilation rate was slightly reduced but the light response of net photosynthesis was not affected in mined leaf tissues. The transpiration rate was far more affected than the assimilation rate in the mine integument as a result of stomatal closure from moderate to high irradiance level. The WUE was about 200% higher in the mined leaf tissues than in intact leaf portions. Our results illustrate a novel mechanism by which plants might minimize losses from herbivore attacks; via trade-offs between the negative impacts on photosynthesis and the positive effects of increased WUE.