Ozone alters the feeding behavior of the leaf beetle Agelastica coerulea (Coleoptera: Chrysomelidae) into leaves of Japanese white birch (Betula platyphylla var. japonica)

Secondary metabolites responses of plants exposed to ozone: an update

Tropospheric ozone (O3) is a secondary pollutant that causes oxidative stress in plants due to the generation of excess reactive oxygen species (ROS). Phenylpropanoid metabolism is induced as a usual response to stress in plants, and induction of key enzyme activities and accumulation of secondary metabolites occur, upon O3 exposure to provide resistance or tolerance. The phenylpropanoid, isoprenoid, and alkaloid pathways are the major secondary metabolic pathways from which plant defense metabolites emerge. Chronic exposure to O3 significantly accelerates the direction of carbon flows toward secondary metabolic pathways, resulting in a resource shift in favor of the synthesis of secondary products. Furthermore, since different cellular compartments have different levels of ROS sensitivity and metabolite sets, intracellular compartmentation of secondary antioxidative metabolites may play a role in O3-induced ROS detoxification. Plants' responses to resource partitioning often result in a trade-off between growth and defense under O3 stress. These metabolic adjustments help the plants to cope with the stress as well as for achieving new homeostasis. In this review, we discuss secondary metabolic pathways in response to O3 in plant species including crops, trees, and medicinal plants; and how the presence of this stressor affects their role as ROS scavengers and structural defense. Furthermore, we discussed how O3 affects key physiological traits in plants, foliar chemistry, and volatile emission, which affects plant-plant competition (allelopathy), and plant-insect interactions, along with an emphasis on soil dynamics, which affect the composition of soil communities via changing root exudation, litter decomposition, and other related processes.

Elevated ozone alters long-chain fatty acids in leaves of Japanese white birch saplings

Long-chain fatty acids (LCFAs) in leaves have attracted attention as nutritious phytochemicals and olfactory signals that influence the behavior and growth of herbivorous insects. In recognition of the negative effects of increasing tropospheric ozone (O₃) levels on plants, LCFAs can be altered through peroxidation by O₃. However, how elevated O₃ changes the amount and composition of LCFAs in field-grown plants is still unknown. We investigated palmitic, stearic, oleic, linoleic, linolenic LCFAs in the two leaf types (spring and summer) and two stages (early and late stage after expansion) of Japanese white birch (Betula platyphylla var. japonica) after a multi-year O₃ exposure on the field. Summer leaves exhibited a distinct composition of LCFAs under elevated O₃ at the early stage, whereas both stages of spring leaves did not exhibit significant changes in LCFAs composition by elevated O₃. In the spring leaves, the amounts of saturated LCFAs significantly increased at the early stage, however, the amount of total, palmitic, and linoleic acids at the late stage were significantly decreased by elevated O₃. Summer leaves had a lower amount of all LCFAs at both leaf stages. Regarding the early stage of summer leaves, the lower amount of LCFAs under elevated O₃ was possibly due to O₃-suppressed photosynthesis in the current spring leaves. Furthermore, the decrease ratio of spring leaves over time was significantly increased by elevated O₃ in all LCFAs, whereas summer leaves did not exhibit such an effect. These findings suggest that further studies should be conducted to reveal the biological functions of LCFAs under elevated O₃, considering the leaf type- and stage-dependent changes of LCFAs.

Do changes in Lactuca sativa metabolic performance, induced by mycorrhizal symbionts and leaf UV-B irradiation, play a role towards tolerance to a polyphagous insect pest?

The increased ultraviolet radiation (UV) due to the altered stratospheric ozone leads to multiple plant physiological and biochemical adaptations, likely affecting their interaction with other organisms, such as pests and pathogens. Arbuscular mycorrhizal fungi (AMF) and UV-B treatment can be used as eco-friendly techniques to protect crops from pests by activating plant mechanisms of resistance. In this study, we investigated plant (Lactuca sativa) response to UV-B exposure and Funneliformis mosseae (IMA1) inoculation as well as the role of a major insect pest, Spodoptera littoralis. Lettuce plants exposed to UV-B were heavier and taller than non-irradiated ones. A considerable enrichment in phenolic, flavonoid, anthocyanin, and carotenoid contents and antioxidant capacity, along with redder and more homogenous leaf color, were also observed in UV-B-treated but not in AMF-inoculated plants. Biometric and biochemical data did not differ between AMF and non-AMF plants. AMF-inoculated plants showed hyphae, arbuscules, vesicles, and spores in their roots. AMF colonization levels were not affected by UV-B irradiation. No changes in S. littoralis-feeding behavior towards treated and untreated plants were observed, suggesting the ability of this generalist herbivore to overcome the plant chemical defenses boosted by UV-B exposure. The results of this multi-factorial study shed light on how polyphagous insect pests can cope with multiple plant physiological and biochemical adaptations following biotic and abiotic preconditioning.

Plant-insect communication in urban forests: Similarities of plant volatile compositions among tree species (host vs. non-host trees) for alder leaf beetle Agelastica coerulea

Behavior of insects, such as pollination and grazing, is usually determined by biogenic volatile organic compounds (BVOCs). However, particularly in O₃-polluted urban forests, the BVOCs-based plant-insect communication can be disrupted by the reaction of O₃ with leaf-emitted BVOCs, such as between Japanese white birch (Betula platyphylla var. japonica) and a leaf beetle (Agelastica coerulea). To understand plant-insect communication in O₃-polluted environments, it is necessary to identify chemical species of BVOCs that contribute to attractiveness toward insects but are diminished by elevated O₃. In this study, we conducted olfactory response tests and gas chromatography mass spectrometry (GC-MS) analyses to clarify whether there is a similarity of BVOC components among Betulaceae host trees that can explain the attraction of the stenophagous insect A. coerulea. The olfactory response tests indicated that Betulaceae host trees attract A. coerulea via leaf-emitted BVOCs, while there was no preference of the leaf beetles to non-host trees (Sorbus commixta and Morus bombycis). However, GC-MS analyses indicated that the composition of BVOC blends considerably differed among Betulaceae host trees, although alders (Alnus hirsuta and A. japonica) had a similar composition of BVOC blend in each season (June and September) during which the adult leaf beetle is active. A distinct characteristic of the emission from B. platyphylla was that 2-carene and limonene, which are O₃-reactive species, were emitted with a high monoterpene ratio irrespective of the season. Thus, these volatiles and the blend could be expected to lead the disrupted communication found between B. platyphylla and A. coerulea under elevated O₃ in previous field studies. In addition, our results indicated that A. coerulea is attracted to more than one blend within Betulaceae host trees, suggesting that grazing damages can be affected by different host preferences and O₃ reactivity with specific BVOCs in the field. BVOCs-based plant-insect interactions should be further studied in multi-species communities to better understand plant-insect communication in O₃-polluted environments.

In the tripartite combination ozone-poplar-Chrysomela populi, the pollutant alters the plant-insect interaction via primary metabolites of foliage

Ozone (O₃)-induced metabolic changes in leaves are relevant and may have several ecological significances. Here, variations in foliar chemistry of two poplar clones (Populus deltoides × maximowiczii, Eridano, and P. × euramericana, I-214) under a chronic O₃ treatment (80 ppb, 5 h d^-1 for 10 consecutive days) were investigated. The aim was to elucidate if leaf age and/or O₃-sensitivity (considering Eridano and I-214 as O₃-sensitive and O₃-resistant, respectively) can affect suitability of poplar foliage for Chrysomela populi L. (Coleoptera Chrysomelidae), in terms of palatability. Comparing controls, only low amino acid (AA) contents were reported in Eridano [about 3- and 4-fold in mature and young leaves (ML and YL, respectively)], and all the investigated primary metabolites [i.e. water soluble carbohydrates (WSC), proteins (Prot) and AA] were higher in YL than in ML of I-214 (+23, +54 and + 20%, respectively). Ozone increased WSC only in YL of Eridano (+24%, i.e. highest values among samples; O₃ effects are always reported comparing O₃-treated plants with the related controls). A concomitant decrease of Prot was observed in both ML and YL of Eridano, while only in YL of I-214 (-41, -45 and -51%, respectively). In addition, O₃ decreased AA in YL of Eridano and in ML of I-214 (-40 and -14%, respectively). Comparing plants maintained under charcoal-filtered air, total ascorbate (Asc) was lower in Eridano in both ML and YL (around -22%), and abscisic acid (ABA) was similar between clones; furthermore, higher levels of Asc were reported in YL than in ML of Eridano (+19%). Ozone increased Asc and ABA (about 2- and 3-fold, respectively) in both ML and YL of Eridano, as well as ABA in YL of I-214 (about 2-fold). Comparing leaves maintained under charcoal-filtered air, the choice feeding test showed that the 2nd instar larvae preferred YL, and the quantity of YL consumed was 9 and 4-fold higher than ML in Eridano and I-214, respectively. Comparing leaves exposed to O₃-treatment, a significant feeding preference for YL disks was also observed, regardless of the clone. The no-choice feeding test showed that larval growth was slightly higher on untreated YL than on untreated ML (+19 and + 10% in Eridano and I-214, respectively). The body mass of larvae fed with O₃-treated YL was also significantly higher than that of larvae fed with untreated YL (3- and 2-fold in Eridano and I-214). This study highlights that realistic O₃ concentrations can significantly impact the host/insect interactions, a phenomenon dependent on leaf age and O₃-sensitivity of the host.

Ozone disrupts the communication between plants and insects in urban and suburban areas: an updated insight on plant volatiles

Plant-insect interactions are basic components of biodiversity conservation. To attain the international Sustainable Development Goals (SDGs), the interactions in urban and in suburban systems should be better understood to maintain the health of green infrastructure. The role of ground-level ozone (O₃) as an environmental stress disrupting interaction webs is presented. Ozone mixing ratios in suburbs are usually higher than in the center of cities and may reduce photosynthetic productivity at a relatively higher degree. Consequently, carbon-based defense capacities of plants may be suppressed by elevated O₃ more in the suburbs. However, contrary to this expectation, grazing damages by leaf beetles have been severe in some urban centers in comparison with the suburbs. To explain differences in grazing damages between urban areas and suburbs, the disruption of atmospheric communication signals by elevated O₃ via changes in plant-regulated biogenic volatile organic compounds and long-chain fatty acids are considered. The ecological roles of plant volatiles and the effects of O₃ from both a chemical and a biological perspective are presented. Ozone-disrupted plant volatiles should be considered to explain herbivory phenomena in urban and suburban systems.

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Ozone affects plant, insect, and soil microbial communities: A threat to terrestrial ecosystems and biodiversity

Elevated tropospheric ozone concentrations induce adverse effects in plants. We reviewed how ozone affects (i) the composition and diversity of plant communities by affecting key physiological traits; (ii) foliar chemistry and the emission of volatiles, thereby affecting plant-plant competition, plant-insect interactions, and the composition of insect communities; and (iii) plant-soil-microbe interactions and the composition of soil communities by disrupting plant litterfall and altering root exudation, soil enzymatic activities, decomposition, and nutrient cycling. The community composition of soil microbes is consequently changed, and alpha diversity is often reduced. The effects depend on the environment and vary across space and time. We suggest that Atlantic islands in the Northern Hemisphere, the Mediterranean Basin, equatorial Africa, Ethiopia, the Indian coastline, the Himalayan region, southern Asia, and Japan have high endemic richness at high ozone risk by 2100.

Does Ozone Alter the Attractiveness of Japanese White Birch Leaves to the Leaf Beetle Agelastica coerulea via Changes in Biogenic Volatile Organic Compounds (BVOCs): An Examination with the Y-Tube Test

Elevated ground-level ozone (O3) reduced C-based defense chemicals; however, severe grazing damages were found in leaves grown in the low O3 condition of a free air O3-concentration enrichment (O3-FACE) system. To explain this phenomenon, this study investigates the role of BVOCs (biogenic volatile organic compounds) as signaling compounds for insect herbivores. BVOCs act as scents for herbivore insects to locate host plants, while some BVOCs show high reactivity to O3, inducing changes in the composition of BVOCs in atmospheres with elevated O3. To assess the aforementioned phenomenon, profiles of BVOCs emitted from birch (Betula platyphylla var. japonica Hara) leaves were analyzed ex situ, and Y-tube insect preference tests were conducted in vitro to study the insect olfactory response. The assays were conducted in June and August or September, according to the life cycle of the adult alder leaf beetle Agelastica coerulea Baly (Coleoptera: Chrysomelidae). The Y-tube tests revealed that the leaf beetles were attracted to BVOCs, and O3 per se had neither an attractant nor a repellent effect. BVOCs became less attractant when mixed with highly concentrated O3 (>80 ppb). About 20% of the total BVOCs emitted were highly O3-reactive compounds, such as β-ocimene. The results suggest that BVOCs emitted from the birch leaves can be altered by elevated O3, thus potentially reducing the attractiveness of leaves to herbivorous insects searching for food.

Leaf defense capacity of Japanese elm (Ulmus davidiana var. japonica) seedlings subjected to a nitrogen loading and insect herbivore dynamics in a free air ozone-enriched environment

Japanese elm (Ulmus davidiana var. japonica) is a native species in cool-temperate forests in Japan. We investigated growth, physiological reactions, and leaf defense capacity of Japanese elm seedlings under nitrogen (N) loading (45.3 kg N ha^-1 year^-1) and seasonal insect dynamics in a free-air ozone (O₃)-enriched environment (about 54.5 nmol O₃ mol^-1) over a growing season. Higher leaf N content and lower condensed tannin content in the presence of N loading and lower condensed tannin content in elevated O₃ were observed, suggesting that both N loading and elevated O₃ decreased the leaf defense capacity and that N loading further enhanced the leaf quality as food resource of insect herbivores. Two major herbivores were observed on the plants, elm leaf beetle (Pyrrhalta maculicollis) and elm sawfly (Arge captiva). The peak number of observed insects was decreased by N loading. Visible foliar injury caused by N loading might directly induce the reduction of number of the observed elm sawfly individuals. While elevated O₃ slightly suppressed the chemical defense capacity, significantly lower number of elm leaf beetle was observed in elevated O₃. We conclude that N loading and elevated O₃ can alter not only the leaf defense capacity of Japanese elm seedlings but also the dynamics of elm leaf beetle and sawfly herbivores.

Plant-mediated effects of ozone on herbivores depend on exposure duration and temperature

Abiotic stress by elevated tropospheric ozone and temperature can alter plants’ metabolism, growth, and nutritional value and modify the life cycle of their herbivores. We investigated how the duration of exposure of Sinapis arvensis plants to high ozone and temperature levels affect the life cycle of the large cabbage white, Pieris brassicae. Plants were exposed to ozone-clean (control) or ozone-enriched conditions (120 ppb) for either 1 or 5 days and were afterwards kept in a greenhouse with variable temperature conditions. When given the choice, P. brassicae butterflies laid 49% fewer eggs on ozone-exposed than on control plants when the exposure lasted for 5 days, but showed no preference when exposure lasted for 1 day. The caterpillars took longer to hatch on ozone-exposed plants and at lower ambient temperatures. The ozone treatment had a positive effect on the survival of the eggs. Ozone decreased the growth of caterpillars reared at higher temperatures on plants exposed for 5 days, but not on plants exposed for 1 day. Overall, longer exposure of the plants to ozone and higher temperatures affected the life cycle of the herbivore more strongly. With global warming, the indirect impacts of ozone on herbivores are likely to become more common.

Effects of ozone and ammonium sulfate on cauliflower: Emphasis on the interaction between plants and insect herbivores

Ammonium sulfate [(NH₄)₂SO₄] deposition and elevated ozone (O₃) concentrations may negatively affect plants and trophic interactions. This study aimed to evaluate for the first time the interactive effects of high (NH₄)₂SO₄ load and elevated O₃ levels on cauliflower (Brassica oleracea L.) under field conditions. Cauliflower seedlings were treated with 0 (AS0) or 50 (AS50) kg ha^-1 (NH₄)₂SO₄ and exposed to ambient (AOZ, ≈20 ppb) or elevated (EOZ, ≈55 ppb) O₃ for about one month, in a Free Air O₃ Concentration Enrichment (FACE) system. The oligophagous diamondback moth (Plutella xylostella Linnaeus, 1758) showed a clear preference towards the seedlings treated with AS50, which intensively grazed. Plant-herbivore interactions were driven by (NH₄)₂SO₄ availability, rather than O₃, via increased nitrogen content in the leaves. Further laboratory bioassays were followed to confirm the validity of these observations using polyphagous Eri silkmoth larvae (Samia ricini) as a biological model in a standardized experimental setup. Choice assays, where larvae could select leaves among leaf samples from the different experimental conditions, and no-choice assays, where larvae could graze leaves from just one experimental condition, were conducted. In the choice assay, the larvae preferred AS50-treated leaves, in agreement with the field observations with diamondback moth. In the no-choice assay, larval body mass growth was inhibited when fed with leaves treated with EOZ and/or AS50. Larvae fed with AS50-treated leaves displayed increased mortality. These observations coincide with higher NO₃ and Zn content in AS50-treated leaves. This study shows that plant-herbivore interactions can be driven by (NH₄)₂SO₄ availability, independently of O₃, and suggests that high N deposition may have severe health implications in animals consuming such plant tissues. Key message: Plant-herbivore interactions are driven by high (NH₄)₂SO₄ availability, independently of O₃.

Interactive effects of changing stratospheric ozone and climate on tropospheric composition and air quality, and the consequences for human and ecosystem health

The composition of the air we breathe is determined by emissions, weather, and photochemical transformations induced by solar UV radiation. Photochemical reactions of many emitted chemical compounds can generate important (secondary) pollutants including ground-level ozone (O₃) and some particulate matter, known to be detrimental to human health and ecosystems. Poor air quality is the major environmental cause of premature deaths globally, and even a small decrease in air quality can translate into a large increase in the number of deaths. In many regions of the globe, changes in emissions of pollutants have caused significant changes in air quality. Short-term variability in the weather as well as long-term climatic trends can affect ground-level pollution through several mechanisms. These include large-scale changes in the transport of O₃ from the stratosphere to the troposphere, winds, clouds, and patterns of precipitation. Long-term trends in UV radiation, particularly related to the depletion and recovery of stratospheric ozone, are also expected to result in changes in air quality as well as the self-cleaning capacity of the global atmosphere. The increased use of substitutes for ozone-depleting substances, in response to the Montreal Protocol, does not currently pose a significant risk to the environment. This includes both the direct emissions of substitutes during use and their atmospheric degradation products (e.g. trifluoroacetic acid, TFA).

Enzyme activity modification in adult beetles (Agelastica coerulea) inhabiting birch trees in an ozone-enriched atmosphere

Tropospheric ozone (O₃) is a naturally occurring gas in the atmosphere. However, the concentration of O₃ increased in the twentieth century. Although the effects of O₃ on vegetation have been extensively studied since the 1950s, limited information exists regarding the effects of O₃ on insect herbivores. In particular, evidence is lacking regarding the effects of O₃ on the biology of insect herbivores. Agelastica coerulea Baly (1874) is a coleopteran species that grazes on Betulaceae plants. In this study, to investigate the effects of O₃ on A. coerulea biology for the first time, female adult insects were collected from Japanese white birch trees grown in a Free Air Controlled Exposure System (FACE) in Sapporo, Japan. These beetles inhabited trees exposed either to ambient or to elevated O₃ for 23 days. After collection, the enzyme activities in the beetles were measured. Elevated O₃ led to a greater total antioxidant activity and lower α- and β-esterase activities, a phenomenon that may suggest an increased resistance of the beetles to stress. Our results are further discussed with regard to biological and toxicological aspects. Collectively, our findings indicate that total antioxidants and α- and β-esterase activities can serve as effective O₃ biomarker systems in this beetle species. This adaptive response of the beetle, which was induced by moderate O₃ exposure, should be further tested across generations and for its protection against greater exposure.

Growth and nutrition of Agelastica coerulea (Coleoptera: Chrysomelidae) larvae changed when fed with leaves obtained from an O3-enriched atmosphere

A series of laboratory no-choice assays were performed to test changes in the feeding, growth, and nutrition of leaf beetle (Agelastica coerulea) larval instars on O₃-treated leaves of Japanese white birch (Betula platyphylla var. japonica). Larvae fed with O₃-treated leaves grew and developed significantly faster throughout their developmental cycle than the corresponding controls. The growth rate (GR) and consumption index (CI) were mostly decreased with age for both control and O₃-treated leaves. Efficiency of conversion of both ingested and digested food (ECI, ECD) showed an increase from the 2nd to the 4th instar, after which they decreased significantly and reached the lowest value in the last larval instars (7th). GR, CI, ECI, and ECD were greater and approximate digestibility (AD) was lower in larvae fed with O₃-treated leaves than those fed with control leaves. This indicated that the greater rate of growth on fumigated leaves was due primarily to a greater rate of consumption (i.e., O₃ increased the "acceptability" of the host more than "suitability") and efficiency in converting food into body mass. Overall, larval performance seemed to have improved when fed with O₃-treated leaves in these assays. This study suggests that insects may be more injurious to O₃-treated plants and warrants further investigations on birch-beetle interactions under field conditions.