Ultraviolet-B radiation co-opts defense signaling pathways

Infestation Differences and Control of the Clearwing Moth (Synanthedon myopaeformis Borkh.) in Apple Orchards, Egypt

<b>Background and Objective:</b> The clearwing moth (<i>Synanthedon myopaeformis</i> Borkh.) is considered one of the most dangerous pests infesting apple orchards in Egypt, causing the destruction of apple trees and a significant reduction in tree vigour and yield. Therefore, controlling it has become an important issue. The present study aimed to investigate the differences in <i>S. myopaeformis</i> infestation in apple orchards in different geographical regions and the infestation rates to different agricultural operations. <b>Materials and Methods:</b> The infestation percentages of <i>S. myopaeformis</i> in different geographical regions under varying agricultural operations were determined. Additionally, the amounts of chemical components (e.g., phenols, proline and total carbohydrates and proteins) in the stems and leaves of both infested and healthy apple trees were determined. Finally, the efficacy of seven different materials of varying types to control the borer was tested by painting the trunks of the apple trees. <b>Results:</b> The highest numbers of <i>S. myopaeformis</i> were recorded in July at locations with varying agricultural operations: Melig (25.88 adults), Kafer-Aleim (20.30 adults) and Al-Khatatba (13.02 adults). The emergence percentages recorded at the abovementioned locations were 44.55, 33.40 and 22.05%, respectively. The infested trees contained high amounts of proline and phenols in both stems and leaves, whereas total carbohydrate and protein contents were low. The highest reduction in the emergent population was recorded with the use of the pesticide Ranous (82.16%) and the application of used motor oil on the trunk (78.15%). <b>Conclusion:</b> The proposed treatment of infested apple trees is painting the trunks during the summer months, when <i>S. myopaeformis</i> activity is high, with motor oil and the pesticide Ranous. The accumulation of phenol and proline in the infested trees indicated a type of direct defense mechanism against <i>S. myopaeformis</i> attacks.

Current status and future challenges in implementing and upscaling vertical farming systems

Vertical farming can produce food in a climate-resilient manner, potentially emitting zero pesticides and fertilizers, and with lower land and water use than conventional agriculture. Vertical farming systems (VFS) can meet daily consumer demands for nutritious fresh products, forming a part of resilient food systems-particularly in and around densely populated areas. VFS currently produce a limited range of crops including fruits, vegetables and herbs, but successful implementation of vertical farming as part of mainstream agriculture will require improvements in profitability, energy efficiency, public policy and consumer acceptance. Here we discuss VFS as multi-layer indoor crop cultivation systems, exploring state-of-the-art vertical farming and future challenges in the fields of plant growth, product quality, automation, robotics, system control and environmental sustainability and how research and development, socio-economic and policy-related institutions must work together to ensure successful upscaling of VFS to future food systems.

Epigenetic and transcriptional responses underlying mangrove adaptation to UV-B

Tropical plants have adapted to strong solar ultraviolet (UV) radiation. Here we compare molecular responses of two tropical mangroves Avecennia marina and Rhizophora apiculata to high-dose UV-B. Whole-genome bisulfate sequencing indicates that high UV-B induced comparable hyper- or hypo-methylation in three sequence contexts (CG, CHG, and CHH, where H refers to A, T, or C) in A. marina but mainly CHG hypomethylation in R. apiculata. RNA and small RNA sequencing reveals UV-B induced relaxation of transposable element (TE) silencing together with up-regulation of TE-adjacent genes in R. apiculata but not in A. marina. Despite conserved upregulation of flavonoid biosynthesis and downregulation of photosynthesis genes caused by high UV-B, A. marina specifically upregulated ABC transporter and ubiquinone biosynthesis genes that are known to be protective against UV-B-induced damage. Our results point to divergent responses underlying plant UV-B adaptation at both the epigenetic and transcriptional level.

Plant Defense Responses to Biotic Stress and Its Interplay With Fluctuating Dark/Light Conditions

Plants are subjected to a plethora of environmental cues that cause extreme losses to crop productivity. Due to fluctuating environmental conditions, plants encounter difficulties in attaining full genetic potential for growth and reproduction. One such environmental condition is the recurrent attack on plants by herbivores and microbial pathogens. To surmount such attacks, plants have developed a complex array of defense mechanisms. The defense mechanism can be either preformed, where toxic secondary metabolites are stored; or can be inducible, where defense is activated upon detection of an attack. Plants sense biotic stress conditions, activate the regulatory or transcriptional machinery, and eventually generate an appropriate response. Plant defense against pathogen attack is well understood, but the interplay and impact of different signals to generate defense responses against biotic stress still remain elusive. The impact of light and dark signals on biotic stress response is one such area to comprehend. Light and dark alterations not only regulate defense mechanisms impacting plant development and biochemistry but also bestow resistance against invading pathogens. The interaction between plant defense and dark/light environment activates a signaling cascade. This signaling cascade acts as a connecting link between perception of biotic stress, dark/light environment, and generation of an appropriate physiological or biochemical response. The present review highlights molecular responses arising from dark/light fluctuations vis-à-vis elicitation of defense mechanisms in plants.

Molecular Mechanism Underlying Mechanical Wounding-Induced Flavonoid Accumulation in Dalbergia odorifera T. Chen, an Endangered Tree That Produces Chinese Rosewood

Dalbergia odorifera, a critically endangered tree species, produces heartwood containing a vast variety of flavonoids. This heartwood, also known as Chinese rosewood, has high economic and medicinal value, but its formation takes several decades. In this study, we showed that discolored wood induced by pruning displays similar color, structure, and flavonoids content to those of natural heartwood, suggesting that wounding is an efficient method for inducing flavonoid production in D. odorifera. Transcriptome analysis was performed to investigate the mechanism underlying wounding-induced flavonoids production in D. odorifera heartwood. Wounding upregulated the expression of 90 unigenes, which covered 19 gene families of the phenylpropanoid and flavonoid pathways, including PAL, C4H, 4CL, CHS, CHI, 6DCS, F3’5’H, F3H, FMO, GT, PMAT, CHOMT, IFS, HI4’OMT, HID, IOMT, I2’H, IFR, and I3’H. Furthermore, 47 upregulated unigenes were mapped to the biosynthesis pathways for five signal molecules (ET, JA, ABA, ROS, and SA). Exogenous application of these signal molecules resulted in the accumulation of flavonoids in cell suspensions of D. odorifera, supporting their role in wounding-induced flavonoid production. Insights from this study will help develop new methods for rapidly inducing the formation of heartwood with enhanced medicinal value.

No evidence of a protective or cumulative negative effect of UV-B on growth inhibition induced by gamma radiation in Scots pine (Pinus sylvestris) seedlings

Exposure to ambient UV-B radiation may prime protective responses towards various stressors in plants, though information about interactive effects of UV-B and gamma radiation is scarce. Here, we aimed to test whether UV-B exposure could prime acclimatisation mechanisms contributing to tolerance to low-moderate gamma radiation levels in Scots pine seedlings, and concurrently whether simultaneous UV-B and gamma exposure may have an additive adverse effect on seedlings that had previously not encountered either of these stressors. Responses to simultaneous UV-B (0.35 W m-2) and gamma radiation (10.2-125 mGy h-1) for 6 days with or without UV-B pre-exposure (0.35 W m-2, 4 days) were studied across various levels of organisation, as compared to effects of either radiation type. In contrast to UV-B, and regardless of UV-B presence, gamma radiation at ≥42.9 mGy h-1 caused increased formation of reactive oxygen species and reduced shoot length, and reduced root length at 125 mGy h-1. In all experiments there was a gamma dose rate-dependent increase in DNA damage at ≥10.8 mGy h-1, generally with additional UV-B-induced damage. Gamma-induced growth inhibition and gamma- and UV-B-induced DNA damage were still visible 44 days post-irradiation, even at 20.7 mGy h-1, probably due to genomic instability, but this was reversed after 8 months. In conclusion, there was no evidence of a protective effect of UV-B on gamma-induced growth inhibition and DNA damage in Scots pine, and no additive adverse effect of gamma and UV-B radiation on growth in spite of the additional UV-B-induced DNA damage.

Stress and defense responses in plant secondary metabolites production

In the growth condition(s) of plants, numerous secondary metabolites (SMs) are produced by them to serve variety of cellular functions essential for physiological processes, and recent increasing evidences have implicated stress and defense response signaling in their production. The type and concentration(s) of secondary molecule(s) produced by a plant are determined by the species, genotype, physiology, developmental stage and environmental factors during growth. This suggests the physiological adaptive responses employed by various plant taxonomic groups in coping with the stress and defensive stimuli. The past recent decades had witnessed renewed interest to study abiotic factors that influence secondary metabolism during in vitro and in vivo growth of plants. Application of molecular biology tools and techniques are facilitating understanding the signaling processes and pathways involved in the SMs production at subcellular, cellular, organ and whole plant systems during in vivo and in vitro growth, with application in metabolic engineering of biosynthetic pathways intermediates.

An ultraviolet B condition that affects growth and defense in Arabidopsis

Ultraviolet B light (UV-B, 280-315 nm) is the shortest wavelength of the solar spectrum reaching the surface of the Earth. It has profound effects on plants, ranging from growth regulation to severe metabolic changes. Low level UV-B mainly causes photomorphogenic effects while higher levels can induce stress, yet these effects tend to overlap. Here we identified a condition that allows growth reduction without obvious detrimental stress in wild type Arabidopsis rosette plants. This condition was used to study the effects of a daily UV-B dose on plant characteristics of UV-B adapted plants in detail. Exploration of the transcriptome of developing leaves indicated downregulation of genes involved in stomata formation by UV-B, while at the same time genes involved in photoprotective pigment biosynthesis were upregulated. These findings correspond with a decreased stomatal density and increased UV-B absorbing pigments. Gene ontology analysis revealed upregulation of defense related genes and meta-analysis showed substantial overlap of the UV-B regulated transcriptome with transcriptomes of salicylate and jasmonate treated as well as herbivore exposed plants. Feeding experiments showed that caterpillars of Spodoptera littoralis are directly affected by UV-B, while performance of the aphid Myzus persicae is diminished by a plant mediated process.

UVA, UVB and UVC Light Enhances the Biosynthesis of Phenolic Antioxidants in Fresh-Cut Carrot through a Synergistic Effect with Wounding

Previously, we found that phenolic content and antioxidant capacity (AOX) in carrots increased with wounding intensity. It was also reported that UV radiation may trigger the phenylpropanoid metabolism in plant tissues. Here, we determined the combined effect of wounding intensity and UV radiation on phenolic compounds, AOX, and the phenylalanine ammonia-lyase (PAL) activity of carrots. Accordingly, phenolic content, AOX, and PAL activity increased in cut carrots with the duration of UVC radiation, whereas whole carrots showed no increase. Carrot pies showed a higher increase compared to slices and shreds. Phenolics, AOX, and PAL activity also increased in cut carrots exposed to UVA or UVB. The major phenolics were chlorogenic acid and its isomers, ferulic acid, and isocoumarin. The type of UV radiation affected phenolic profiles. Chlorogenic acid was induced by all UV radiations but mostly by UVB and UVC, ferulic acid was induced by all UV lights to comparable levels, while isocoumarin and 4,5-diCQA was induced mainly by UVB and UVC compared to UVA. In general, total phenolics correlated linearly with AOX for all treatments. A reactive oxygen species (ROS) mediated hypothetical mechanism explaining the synergistic effect of wounding and different UV radiation stresses on phenolics accumulation in plants is herein proposed.

Hormone-controlled UV-B responses in plants

Ultraviolet B (UV-B) light is a portion of solar radiation that has significant effects on the development and metabolism of plants. Effects of UV-B on plants can be classified into photomorphogenic effects and stress effects. These effects largely rely on the control of, and interactions with, hormonal pathways. The fairly recent discovery of the UV-B-specific photoreceptor UV RESISTANCE LOCUS 8 (UVR8) allowed evaluation of the role of downstream hormones, leading to the identification of connections with auxin and gibberellin. Moreover, a substantial overlap between UVR8 and phytochrome responses has been shown, suggesting that part of the responses caused by UVR8 are under PHYTOCHROME INTERACTING FACTOR control. UV-B effects can also be independent of UVR8, and affect different hormonal pathways. UV-B affects hormonal pathways in various ways: photochemically, affecting biosynthesis, transport, and/or signaling. This review concludes that the effects of UV-B on hormonal regulation can be roughly divided in two: inhibition of growth-promoting hormones; and the enhancement of environmental stress-induced defense hormones.

Role of phytochromes A and B in the regulation of cell death and acclimatory responses to UV stress in Arabidopsis thaliana

Plants coordinate their responses to various biotic and abiotic stresses in order to optimize their developmental and acclimatory programmes. The ultimate response to an excessive amount of stress is local induction of cell death mechanisms. The death of certain cells can help to maintain tissue homeostasis and enable nutrient remobilization, thus increasing the survival chances of the whole organism in unfavourable environmental conditions. UV radiation is one of the environmental factors that negatively affects the photosynthetic process and triggers cell death. The aim of this work was to evaluate a possible role of the red/far-red light photoreceptors phytochrome A (phyA) and phytochrome B (phyB) and their interrelations during acclimatory responses to UV stress. We showed that UV-C treatment caused a disturbance in photosystem II and a deregulation of photosynthetic pigment content and antioxidant enzymes activities, followed by increased cell mortality rate in phyB and phyAB null mutants. We also propose a regulatory role of phyA and phyB in CO2 assimilation, non-photochemical quenching, reactive oxygen species accumulation and salicylic acid content. Taken together, our results suggest a novel role of phytochromes as putative regulators of cell death and acclimatory responses to UV.

Nitric oxide is involved in integration of UV-B absorbing compounds among parts of clonal plants under a heterogeneous UV-B environment

In nature, ultraviolet-B (UV-B) radiation is highly heterogeneous, both spatially and temporally. Plants exposed to UV-B radiation produce UV-B absorbing compounds that function as a protective filter. For clonal plants under heterogeneous UV-B radiation conditions, integration among ramets can allow irradiated ramets to benefit un-irradiated ramets by causing them to increase their UV-B absorbing compounds content. In this study, we evaluated integration between pairs of clonal ramets of Glechoma longituba under heterogeneous or homogeneous UV-B conditions. We determined the levels of UV-B absorbing compounds, nitric oxide (NO) and hydrogen peroxide (H₂ O₂ ) and measured the activity of phenylalanine ammonia-lyase (PAL) in connected ramet pairs under homogeneous or heterogeneous UV-B conditions. Under heterogeneous UV-B conditions, the UV-B absorbing compounds content increased in leaves of irradiated and un-irradiated ramets, but not in the connecting stolons. The NO content increased in irradiated and un-irradiated leaves and stolons, but the H₂ O₂ content did not. Application of NO synthesis inhibitors and an NO blocker to irradiated ramets blocked the increase in UV-B absorbing compounds and PAL activity in un-irradiated ramets. These results suggested that NO is involved in the integration process for UV-B absorbing compounds among ramets. Our findings suggested that a UV-B-induced increase in NO transmits a signal to un-irradiated ramets via the stolon, leading to an increase in PAL activity and UV-B absorbing compounds content. The internal translocation of signal enables members of clonal networks to function as a whole unit and to mount an efficient defensive response to localized UV-B radiation.

Responses of wheat (Triticum aestivum) and turnip (Brassica rapa) to the combined exposure of carbaryl and ultraviolet radiation

The increase of ultraviolet (UV) radiation reaching the Earth's surface as a result of increased ozone layer depletion has affected crop production systems and, in combination with pesticides used in agricultural activities, can lead to greater risks to the environment. The impact of UV radiation and carbaryl singly and in combination on Triticum aestivum (wheat) and Brassica rapa (turnip) was studied. The combined exposure was analyzed using the MixTox tool and was based on the conceptual model of independent action, where possible deviations to synergism or antagonism and dose-ratio or dose-level response pattern were also considered. Compared with the control, carbaryl and UV radiation individually led to reductions in growth, fresh and dry weight, and water content for both species. Combined treatment of UV and carbaryl was more deleterious compared with single exposure. For T. aestivum length, no interaction between the 2 stressors was found (independent action), and a dose-level deviation was the best description for the weight parameters. For B. rapa, dose-ratio deviations from the conceptual model were found when length and dry weight were analyzed, and a higher than expected effect on the fresh weight (synergism) occurred with combined exposure.

Soybean resistance to stink bugs (Nezara viridula and Piezodorus guildinii) increases with exposure to solar UV-B radiation and correlates with isoflavonoid content in pods under field conditions

Solar UV-B radiation (280-315 nm) has a significant influence on trophic relationships in natural and managed ecosystems, affecting plant-insect interactions. We explored the effects of ambient UV-B radiation on the levels of herbivory by stink bugs (Nezara viridula and Piezodorus guildinii) in field-grown soybean crops. The experiments included two levels of UV-B radiation (ambient and attenuated UV-B) and four soybean cultivars known to differ in their content of soluble leaf phenolics. Ambient UV-B radiation increased the accumulation of the isoflavonoids daidzin and genistin in the pods of all cultivars. Soybean crops grown under attenuated UV-B had higher numbers of unfilled pods and damaged seeds than crops grown under ambient UV-B radiation. Binary choice experiments with soybean branches demonstrated that stink bugs preferred branches of the attenuated UV-B treatment. We found a positive correlation between percentage of undamaged seeds and the contents of daidzin and genistin in pods. Our results suggest that constitutive and UV-B-induced isoflavonoids increase plant resistance to stink bugs under field conditions.

Oxylipins and plant abiotic stress resistance

Oxylipins are signaling molecules formed enzymatically or spontaneously from unsaturated fatty acids in all aerobic organisms. Oxylipins regulate growth, development, and responses to environmental stimuli of organisms. The oxylipin biosynthesis pathway in plants includes a few parallel branches named after first enzyme of the corresponding branch as allene oxide synthase, hydroperoxide lyase, divinyl ether synthase, peroxygenase, epoxy alcohol synthase, and others in which various biologically active metabolites are produced. Oxylipins can be formed non-enzymatically as a result of oxygenation of fatty acids by free radicals and reactive oxygen species. Spontaneously formed oxylipins are called phytoprostanes. The role of oxylipins in biotic stress responses has been described in many published works. The role of oxylipins in plant adaptation to abiotic stress conditions is less studied; there is also obvious lack of available data compilation and analysis in this area of research. In this work we analyze data on oxylipins functions in plant adaptation to abiotic stress conditions, such as wounding, suboptimal light and temperature, dehydration and osmotic stress, and effects of ozone and heavy metals. Modern research articles elucidating the molecular mechanisms of oxylipins action by the methods of biochemistry, molecular biology, and genetics are reviewed here. Data on the role of oxylipins in stress signal transduction, stress-inducible gene expression regulation, and interaction of these metabolites with other signal transduction pathways in cells are described. In this review the general oxylipin-mediated mechanisms that help plants to adjust to a broad spectrum of stress factors are considered, followed by analysis of more specific responses regulated by oxylipins only under certain stress conditions. New approaches to improvement of plant resistance to abiotic stresses based on the induction of oxylipin-mediated processes are discussed.

Transcriptional profiling in dioecious plant Populus cathayana reveals potential and sex-related molecular adaptations to solar UV-B radiation

There is increasing evidence confirming that dioecious plants have evolved a series of sex-related physiological mechanisms allowing acclimation to environmental stresses, but few studies have attempted to thoroughly uncover the potential molecular regulation underlying these responses. Recently, we found in the dioecious plant, Populus cathayana, that males possess a greater tolerance to enhanced UV-B radiation than do females. To continue this work further, comparative transcriptome analyses were carried out. A total of 362 and 296 differentially expressed genes (DEGs) were detected in males and females, respectively. Similarly, as shown in previous studies, these DEGs were involved in a set of conserved functions and pathways related to UV-B stress, regardless of the sex. On the other hand, sex-related responses via transcriptome remodeling were detected, as changes in sex-related gene expression occurred in some pathways. For example, many DEGs involved in amino acid metabolism were mainly upregulated in males but downregulated in females. In some pathways, DEGs were expressed predominantly or exclusively in one sex. This may directly contribute to sex-related physiological responses. Taken together, our results not only provide strong evidence that males and females have adopted sex-related molecular strategies in response to solar UV-B radiation but also suggest many new insights, in particular into molecular regulation, via sex-biased gene expression. We conclude that these results can further enhance the sustainable exploitation and management of P. cathayana plants.

Light-mediated antibacterial activity of Lippia origanoides H.B.K. in vitro

An ethanol extract and different partition fractions obtained from Lippia origanoides H.B.K. were assayed for light-mediated activity against strains of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). Identical assays were conducted with and without exposure to UV-A (400-315 nm) light to test for light enhanced activity. The ethanol extract and dichloromethane fraction showed light-mediated activity against the S. aureus strain, but not against the E. coli strain. The dichloromethane fraction was more active than the ethanol extract. Naringenin did not display light-mediated activity against the tested bacteria, indicating that the light-mediated antimicrobial activity of the dichloromethane fraction is not due to its major component. The results represent the first report of light-mediated antimicrobial activity of Lippia origanoides and show that its phytochemicals could be used as light-mediated antimicrobial agents.

Impact of UV-A radiation on the performance of aphids and whiteflies and on the leaf chemistry of their host plants

Ultraviolet (UV) radiation directly regulates a multitude of herbivore life processes, in addition to indirectly affecting insect success via changes in plant chemistry and morphogenesis. Here we looked at plant and insect (aphid and whitefly) exposure to supplemental UV-A radiation in the glasshouse environment and investigated effects on insect population growth. Glasshouse grown peppers and eggplants were grown from seed inside cages covered by novel plastic filters, one transparent and the other opaque to UV-A radiation. At a 10-true leaf stage for peppers (53 days) and 4-true leaf stage for eggplants (34 days), plants were harvested for chemical analysis and infested by aphids and whiteflies, respectively. Clip-cages were used to introduce and monitor the insect fitness and populations of the pests studied. Insect pre-reproductive period, fecundity, fertility and intrinsic rate of natural increase were assessed. Crop growth was monitored weekly for 7 and 12 weeks throughout the crop cycle of peppers and eggplants, respectively. At the end of the insect fitness experiment, plants were harvested (68 days and 18-true leaf stage for peppers, and 104 days and 12-true leaf stage for eggplants) and leaves analysed for secondary metabolites, soluble carbohydrates, amino acids, total proteins and photosynthetic pigments. Our results demonstrate for the first time, that UV-A modulates plant chemistry with implications for insect pests. Both plant species responded directly to UV-A by producing shorter stems but this effect was only significant in pepper whilst UV-A did not affect the leaf area of either species. Importantly, in pepper, the UV-A treated plants contained higher contents of secondary metabolites, leaf soluble carbohydrates, free amino acids and total content of protein. Such changes in tissue chemistry may have indirectly promoted aphid performance. For eggplants, chlorophylls a and b, and carotenoid levels decreased with supplemental UV-A over the entire crop cycle but UV-A exposure did not affect leaf secondary metabolites. However, exposure to supplemental UV-A had a detrimental effect on whitefly development, fecundity and fertility presumably not mediated by plant cues as compounds implied in pest nutrition - proteins and sugars - were unaltered.

Predisposition in plant disease: exploiting the nexus in abiotic and biotic stress perception and response

Predisposition results from abiotic stresses occurring prior to infection that affect susceptibility of plants to disease. The environment is seldom optimal for plant growth, and even mild, episodic stresses can predispose plants to inoculum levels they would otherwise resist. Plant responses that are adaptive in the short term may conflict with those for resisting pathogens. Abiotic and biotic stress responses are coordinated by complex signaling networks involving phytohormones and reactive oxygen species (ROS). Abscisic acid (ABA) is a global regulator in stress response networks and an important phytohormone in plant-microbe interactions with systemic effects on resistance and susceptibility. However, extensive cross talk occurs among all the phytohormones during stress events, and the challenge is discerning those interactions that most influence disease. Identifying convergent points in the stress response circuitry is critically important in terms of understanding the fundamental biology that underscores the disease phenotype as well as translating research to improve stress tolerance and disease management in production systems.

Mass spectrometry of flavonoid vicenin-2, based sunlight barriers in Lychnophora species

Lychnophora salicifolia plants collected from four different places in Brazil (three states: Goias, Minas Gerais and Bahia) revealed a conserved accumulation of vicenin-2, a di-C-glycosyl flavonoid. Quantitative studies by UPLC-MS/MS showed high concentration of vicenin-2 in leaves from sixty specimens of six Lychnophora species. So the tissue distributions of vicenin-2 were evaluated in wild Lychnophora leaves (Asteraceae) by laser based imaging mass spectrometry (IMS) to propose its distributions and possible functions for the species analyzed. Mass spectrometric imaging revealed that vicenin-2, unlike other flavonoids, was produced at the top of the leaves. The combination of localization and UV absorption properties of vicenin-2 suggests that it could act as a UV light barrier to protect the plants, since plants are sessile organisms that have to protect themselves from harsh external conditions such as intense sunlight.

Jasmonic acid negatively regulates resistance to Tobacco mosaic virus in tobacco

Nicotiana tabacum (tobacco) cultivars possessing the N resistance gene to Tobacco mosaic virus (TMV) induce a hypersensitive response, which is accompanied by the production of phytohormones such as salicylic acid (SA) and jasmonic acid (JA), to enclose the invaded virus at the initial site of infection, which inhibits viral multiplication and spread. SA functions as a positive regulator of TMV resistance. However, the role of JA in TMV resistance has not been fully elucidated. Exogenously applied methyl jasmonate, a methyl ester of JA, reduced local resistance to TMV and permitted systemic viral movement. Furthermore, in contrast to a previous finding, we demonstrated that silencing of CORONATINE-INSENSITIVE 1 (COI1), a JA receptor, reduced viral accumulation in a tobacco cultivar possessing the N gene, as did that of allene oxide synthase, a JA biosynthetic enzyme. The reduction in viral accumulation in COI1-silenced tobacco plants was correlated with an increase in SA, and lowering SA levels by introducing an SA hydroxylase gene attenuated this reduction. Viral susceptibility did not change in a COI1-silenced tobacco cultivar lacking the N gene. These results suggest that JA signaling is not directly responsible for susceptibility to TMV, but is indirectly responsible for viral resistance through the partial inhibition of SA-mediated resistance conferred by the N gene, and that a balance between endogenous JA and SA levels is important for determining the degree of resistance.

Altitudinal and seasonal changes of phenolic compounds in Buxus sempervirens leaves and cuticles

The variation in the leaf content of phenolic compounds has been related to the UV-B changes of the environment in which plants grow. In this context, we aimed to investigate: a) whether the seasonal and altitudinal changes in the content of phenolic compounds of Buxus sempervirens L. leaves and cuticles could be related to the natural fluctuations in UV-B levels and b) the possible use of specific phenolic compounds as biomarkers of ambient UV-B levels. To achieve these goals we sampled, every three months during one year, leaves of B. sempervirens along an altitudinal gradient. At the lowest and the highest altitudes, we also conducted a UV-exclusion experiment to discern whether the observed changes could be attributed to the natural variation in UV-B. Results show that total phenolic content of leaves was lower in June than in the other sampling dates, which suggests a leaf ontogenic rather than a UV-B effect on the leaf content of these compounds. Regarding the elevational gradient, the overall amount of phenolic acids and neolignan of entire leaves increased with altitude while the total amount of flavonoids in leaf cuticles decreased. However, the lack of a significant effect of our UV-exclusion treatment on the content of these compounds suggests that the observed variations along the altitudinal gradient would respond to other factors rather than to UV-B. Concomitantly, we did not find any phenolic compound in leaves or cuticles of B. sempervirens that could be considered as a biomarker of ambient UV-B levels.

Beneficial effects of solar UV-B radiation on soybean yield mediated by reduced insect herbivory under field conditions

Ultraviolet-B radiation (UV-B: 280-315 nm) has damaging effects on cellular components and macromolecules. In plants, natural levels of UV-B can reduce leaf area expansion and growth, which can lead to reduced productivity and yield. UV-B can also have important effects on herbivorous insects. Owing to the successful implementation of the Montreal Protocol, current models predict that clear-sky levels of UV-B radiation will decline during this century in response to ozone recovery. However, because of climate change and changes in land use practices, future trends in UV doses are difficult to predict. In the experiments reported here, we used an exclusion approach to study the effects of solar UV-B radiation on soybean crops, which are extensively grown in many areas of the world that may be affected by future variations in UV-B radiation. In a first experiment, performed under normal management practices (which included chemical pest control), we found that natural levels of UV-B radiation reduced soybean yield. In a second experiment, where no pesticides were applied, we found that solar UV-B significantly reduced insect herbivory and, surprisingly, caused a concomitant increase in crop yield. Our data support the idea that UV-B effects on agroecosystems are the result of complex interactions involving multiple trophic levels. A better understanding of the mechanisms that mediate the anti-herbivore effect of UV-B radiation may be used to design crop varieties with improved adaptation to the cropping systems that are likely to prevail in the coming decades in response to agricultural intensification.

Flavonoids: biosynthesis, biological functions, and biotechnological applications

Flavonoids are widely distributed secondary metabolites with different metabolic functions in plants. The elucidation of the biosynthetic pathways, as well as their regulation by MYB, basic helix-loop-helix (bHLH), and WD40-type transcription factors, has allowed metabolic engineering of plants through the manipulation of the different final products with valuable applications. The present review describes the regulation of flavonoid biosynthesis, as well as the biological functions of flavonoids in plants, such as in defense against UV-B radiation and pathogen infection, nodulation, and pollen fertility. In addition, we discuss different strategies and achievements through the genetic engineering of flavonoid biosynthesis with implication in the industry and the combinatorial biosynthesis in microorganisms by the reconstruction of the pathway to obtain high amounts of specific compounds.

Flavonoids: biosynthesis, biological functions, and biotechnological applications

Flavonoids are widely distributed secondary metabolites with different metabolic functions in plants. The elucidation of the biosynthetic pathways, as well as their regulation by MYB, basic helix-loop-helix (bHLH), and WD40-type transcription factors, has allowed metabolic engineering of plants through the manipulation of the different final products with valuable applications. The present review describes the regulation of flavonoid biosynthesis, as well as the biological functions of flavonoids in plants, such as in defense against UV-B radiation and pathogen infection, nodulation, and pollen fertility. In addition, we discuss different strategies and achievements through the genetic engineering of flavonoid biosynthesis with implication in the industry and the combinatorial biosynthesis in microorganisms by the reconstruction of the pathway to obtain high amounts of specific compounds.

Crosstalk between abiotic ultraviolet-B stress and biotic (flg22) stress signalling in Arabidopsis prevents flavonol accumulation in favor of pathogen defence compound production

Plants respond to both abiotic and biotic stresses with alterations in the expression of genes required to produce protective metabolites. Sometimes plants can be challenged with different stresses simultaneously and as they cannot evade from this situation, priorities have to be set to deal with the most urgent threat. The abiotic stress ultraviolet-B (UV-B) light induces the production of UV-protective flavonols in Arabidopsis Col-0 cell suspension cultures and this accumulation is attenuated by concurrent application of the bacterial elicitor flg22 (simulating biotic stress). This inhibition correlates with strong suppression of the flavonol biosynthesis genes. In parallel, flg22 induces the production of defence-related compounds, such as the phytoalexins, camalexin and scopoletin, as well as lignin, a structural barrier thought to restrict pathogen spread. This correlated positively with flg22-mediated expression of enzymes for lignin, scopoletin and camalexin production. As flavonols, lignin and scopoletin are all derived from phenylalanine, it appears that the plant focuses the metabolism on production of scopoletin and lignin at the expense of flavonol production. Furthermore, it appears that this crosstalk involves antagonistic regulation of two opposing MYB transcription factors, the positive regulator of the flavonol pathway MYB12 (UV-B-induced and flg22-suppressed) and the negative regulator MYB4 (UV-B- and flg22-induced).

Chromatin remodelling in plant light signalling

The structure and compaction of chromatin exerts a major regulatory influence on eukaryotic transcription. Changes in both histone composition and post-translational modifications of individual histone proteins can lead to remodelling of higher order chromatin structure. Chromatin remodelling regulates transcriptional activity through modifying gene accessibility, via DNA/histone interactions and the recruitment of non-histone proteins to DNA. Plant growth and development is regulated by the integration of multiple environmental signals. Of these, light is one of the most important. Chromatin remodelling processes have been identified in plants following a variety of different light treatments. These include the initiation of seedling de-etiolation, changes in photon irradiance and ultraviolet-B radiation exposure. In this review, we will summarize the roles of chromatin remodelling in plant photomorphogenesis and discuss these in the wider context of plant environmental adaptation.

Increased exposure to UV-B radiation during early development leads to enhanced photoprotection and improved long-term performance in Lactuca sativa

Plant responses to solar UV radiation are numerous and have often been considered from a perspective of negative outcomes for plant productivity. In this study, we used two experimental approaches consisting of: (1) field-based spectrally modifying filters in addition to (2) controlled indoor exposure to UV-B, to examine the effects of UV radiation on growth and photosynthetic performance of lettuce (Lactuca sativa L.) seedlings. Various aspects of growth were affected in plants grown under a UV-inclusive environment compared to a UV-depleted environment, including reductions in leaf expansion, increases in leaf thickness and the rate of net photosynthesis. After transplantation to a uniform field environment, lettuce plants initially propagated under the UV-inclusive environment exhibited higher harvestable yields than those from a UV-depleted environment. In controlled conditions, photosynthetic rates were higher in plants grown in the presence of UV-B radiation, and relative growth of plants pre-acclimatized to UV-B was also increased, in addition to higher maximum photochemical efficiency of photosystem II (PSII) (F(v) /F(m) ) following subsequent exposure to high photosynthetically active radiation (PAR) and temperature stress. Our findings are discussed within the context of sustainability in agriculture and the paradigm shift in photobiology which such beneficial responses to UV radiation could represent.

The interplay between light and jasmonate signalling during defence and development

During their evolution, plants have acquired diverse capabilities to sense their environment and modify their growth and development as required. The versatile utilization of solar radiation for photosynthesis as well as a signal to coordinate developmental responses to the environment is an excellent example of such a capability. Specific light quality inputs are converted to developmental outputs mainly through hormonal signalling pathways. Accordingly, extensive interactions between light and the signalling pathways of every known plant hormone have been uncovered in recent years. One such interaction that has received recent attention and forms the focus of this review occurs between light and the signalling pathway of the jasmonate hormone with roles in regulating plant defence and development. Here the recent research that revealed new mechanistic insights into how plants might integrate light and jasmonate signals to modify their growth and development, especially when defending themselves from either pests, pathogens, or encroaching neighbours, is discussed.

Expression profile of jasmonic acid-induced genes and the induced resistance against the root-knot nematode (Meloidogyne incognita) in tomato plants (Solanum lycopersicum) after foliar treatment with methyl jasmonate

We investigated what gene(s) in the plant roots have the positive role against repressing root-knot nematode (RKN) infection. We investigated the interaction between RKN infection and gene expression in the plant roots induced by methyl jasmonate (MeJA). We focused on the induced resistance response and the duration after foliar treatment with MeJA of 0.1, 0.5, 1.0, and 5.0mM at 1, 24, 48, and 72h prior to the inoculation of RKN. As a result, the foliar treatment with MeJA at 0.5mM or higher concentrations significantly reduced the infection of RKN in plants and the effect lasted for about 1 week. The repressing effect on RKN population declined to the lowest level in two weeks after MeJA treatment. The expression of proteinase inhibitors (PIs) and multicystatin (MC) were induced while the repressing effect on RKN was valid and a negative correlation was found between the expression of PIs or MC and RKN infection. In addition, when tomato plants no longer expressing MC and PIs were treated again with MeJA, the repressing effect revived. These phenomena appeared to be regardless of the existence of Mi-genes or isolate of RKN. Our results indicate that the expression level of MC and PIs may be effective as marker genes for estimating the induced resistance response against RKN infection.

Transcriptome changes in grapevine (Vitis vinifera L.) cv. Malbec leaves induced by ultraviolet-B radiation

BACKGROUND

Ultraviolet-B radiation (UV-B, 280-315 nm) is a natural component of sunlight, which has numerous regulatory effects on plant physiology. The nature of the response to UV-B is dependent on fluence rate, dose, duration and wavelength of the UV-B treatment. Some reports have analyzed the changes in gene expression caused by UV-B light on several plant species using microarray technology. However, there is no information on the transcriptome response triggered by UV-B in grapevine. In this paper we investigate the gene expression responses of leaves from in vitro cultured Vitis vinifera cv. Malbec plants subjected to the same dose of biologically effective UV-B radiation (4.75 kJ m-2 d-1) administered at two different fluence rates (16 h at ≅ 8.25 μW cm-2, 4 h at ≅ 33 μW cm-2) using a new custom made GrapeGen Affymetrix GeneChip®.

RESULTS

The number of genes modulated by high fluence rate UV-B doubled the number of genes modulated by low fluence UV-B. Their functional analyses revealed several functional categories commonly regulated by both UV-B treatments as well as categories more specifically modulated depending on UV-B fluence rate. General protective responses, namely the induction of pathways regulating synthesis of UV-B absorbing compounds such as the Phenylpropanoid pathway, the induction of different antioxidant defense systems and the activation of pathways commonly associated with pathogen defense and abiotic stress responses seem to play critical roles in grapevine responses against UV-B radiation. Furthermore, high fluence rate UV-B seemed to specifically modulate additional pathways and processes in order to protect grapevine plantlets against UV-B-induced oxidative stress, stop the cell cycle progression, and control protein degradation. On the other hand, low fluence rate UV-B regulated the expression of specific responses in the metabolism of auxin and abscisic acid as well as in the modification of cell walls that could be involved in UV-B acclimation-like processes.

CONCLUSION

Our results show the UV-B radiation effects on the leaf transcriptome of grapevine (Vitis vinifera cv. Malbec) plantlets. Functional categories commonly modulated under both UV-B treatments as well as transcripts specifically regulated in an UV-B-intensity dependent way were identified. While high fluence rate UV-B had regulatory effects mainly on defense or general multiple-stress responses pathways, low fluence rate UV-B promoted the expression of genes that could be involved in UV-B protection or the amelioration of the UV-B-induced damage. This study also provides an extensive list of genes regulating multiple metabolic pathways involved in the response of grapevine to UV-B that can be used for future researches.

Effects of UV-B radiation in morpho-genetic characters of Gnaphalium luteo-album

Genetic analyses of Gnaphalium's DNA showed a high degree of polymorphism in UV-B irradiated plants when compared to controls. Among the five tested primers the four ISSR primers selected for this analysis generated a total of 189 fragments. A high proportion of polymorphic bands ranging from 70% to 28% were found using these ISSR markers. Nei and Li similarity indexes [1] were used to evaluate genetic divergence among plants. The dendrograms obtained using these markers efficiently separate plants from different treatments. A linear relationship was observed between UV-B dose and percentage of dissimilarity which may be related to DNA damage caused by the different UV-B treatments.

Response of wild-type and high pigment-1 tomato fruit to UV-B depletion: flavonoid profiling and gene expression

The tomato high pigment-1 (hp-1) mutant is characterised by exaggerated photoresponsiveness and increased fruit pigmentation, and carries a mutation in the HP1/LeDDB1 gene, encoding the tomato homologue of the negative regulator of the light signal transduction DDB1a from Arabidopsis. Here, we investigated the molecular events underlying flavonoid accumulation in flesh and peel of wild-type and hp-1 fruits in presence or absence of UV-B light. In hp-1 peel, a twofold higher level of rutin and an earlier accumulation of flavonoids than in wild-type were observed, which correlated to the earlier activation of most flavonoid biosynthetic genes compared to wild-type. In hp-1 flesh, flavonoid content was up to 8.5-fold higher than in wild-type and correlated to the higher transcript level of flavonoid genes compared to wild-type. In both tissues, the expression of flavonoid genes was correlated with the anticipated and/or enhanced activation of the light signal transduction genes: LeCOP1LIKE, LeCOP1 and LeHY5. In wild-type, flavonoid content was severely reduced by UV-B depletion mostly in peel, whereas in hp-1 it was significantly increased in flesh. The activation of flavonoid and light signal transduction genes was UV-B dependent mostly at the mature green stage, whereas LeDDB1 expression was not regulated by UV-B.

Jasmonate-dependent and -independent pathways mediate specific effects of solar ultraviolet B radiation on leaf phenolics and antiherbivore defense

Ultraviolet B (UV-B) radiation, a very small fraction of the daylight spectrum, elicits changes in plant secondary metabolism that have large effects on plant-insect interactions. The signal transduction pathways that mediate these specific effects of solar UV-B are not known. We examined the role of jasmonate signaling by measuring responses to UV-B in wild-type and transgenic jasmonate-deficient Nicotiana attenuata plants in which a lipoxygenase gene (NaLOX3) was silenced (as-lox). In wild-type plants, UV-B failed to elicit the accumulation of jasmonic acid (JA) or the bioactive JA-isoleucine conjugate but amplified the response of jasmonate-inducible genes, such as trypsin proteinase inhibitor (TPI), to wounding and methyl jasmonate, and increased the accumulation of several phenylpropanoid derivatives. Some of these phenolic responses (accumulation of caffeoyl-polyamine conjugates) were completely lacking in as-lox plants, whereas others (accumulation of rutin and chlorogenic acid) were similar in both genotypes. In open field conditions, as-lox plants received more insect damage than wild-type plants, as expected, but the dramatic increase in resistance to herbivory elicited by UV-B exposure, which was highly significant in wild-type plants, did not occur in as-lox plants. We conclude that solar UV-B (1) uses jasmonate-dependent and -independent pathways in the elicitation of phenolic compounds, and (2) increases sensitivity to jasmonates, leading to enhanced expression of wound-response genes (TPI). The lack of UV-B-induced antiherbivore protection in as-lox plants suggests that jasmonate signaling plays a central role in the mechanisms by which solar UV-B increases resistance to insect herbivores in the field.

Metabolomic and physiological responses reveal multi-phasic acclimation of Arabidopsis thaliana to chronic UV radiation

Biochemical changes in vivo and pathway interactions were investigated using integrated physiological and metabolic responses of Arabidopsis thaliana L. to ultraviolet (UV) radiation (280-400 nm) at 9.96 kJ m(-2) d(-1) over the entire life cycle from seed to seed (8 weeks). Columbia-0 (Col-0) and a UV-B sensitive accession (fah-1) showed significant (P < 0.001) reductions in leaf growth after 6 weeks. Col-0 recovered growth after 8 weeks, with recovery corresponding to a switch from production of phenylpropanoids to flavonoids. fah-1 failed to recover, indicating that sinapate production is an essential component of recovery. Epidermal features show that UV radiation caused significant (P < 0.001) increases in trichome density, which may act as a structural defence response. Stomatal indices showed a significant (P < 0.0001) reduction in Col-0 and a significant (P < 0.001) increase in fah-1. Epidermal cell density was significantly increased under UV radiation on the abaxial leaf surface, suggesting that that a fully functioning phenylpropanoid pathway is a requirement for cell expansion and leaf development. Despite wild-type acclimation, the costs of adaptation lead to reduced plant fitness by decreasing flower numbers and total seed biomass. A multi-phasic acclimation to UV radiation and the induction of specific metabolites link stress-induced biochemical responses to enhanced acclimation.

UV-B action spectrum for UVR8-mediated HY5 transcript accumulation in Arabidopsis

Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is a UV-B-specific signaling component that mediates low fluence, photomorphogenic responses to UV-B. It is required for UV-B-induced expression of the gene encoding the ELONGATED HYPOCOTYL5 (HY5) transcription factor. HY5 is a key effector of responses mediated by UVR8. In mature leaf tissue, HY5 transcript accumulation occurred rapidly in response to a brief UV-B treatment and no induction was observed in a uvr8 mutant over a broad range of UV wavelengths. In response to monochromatic light, maximal transcript accumulation occurred in wild-type plants at wavelengths 280-300 nm. HY5 transcript accumulation showed reciprocity between the fluence rate and duration of UV-B exposure, and on this basis conditions were chosen to generate an action spectrum for the UVR8 signaling pathway. Dose-response curves were produced for a range of UV wavelengths using 20 min exposure to UV and harvesting tissue 2 h after the start of illumination. Experiments using mutants defective in sinapate ester and flavonoid biosynthesis indicated that the presence of UV-absorbing compounds did not affect the construction of an action spectrum under the conditions employed. The action spectrum for the induction of HY5 by the UVR8 pathway showed a main peak at 280 nm with a smaller peak at 300 nm. The data are discussed in relation to the proposed mechanisms of UV-B photoreception.

Independent responses to ultraviolet radiation and herbivore attack in broccoli

The plant responses to ultraviolet-B radiation (UV-B) and to insect herbivory are believed to be partially similar. In this study, responses to these factors were investigated in the crop species broccoli (Brassica oleracea L. convar. botrytis, Brassicaceae). Plants were first grown under three UV-B regimes (80%, 23%, and 4% transmittance of ambient UV-B) in greenhouses covered with either innovative materials (high and medium transmittance) or conventional glass (low transmittance). Half of the plants then remained under these conditions, but the other half were transferred to the field with ambient light and herbivore access for up to 3 d. The plant responses to distinct environmental conditions were examined by analysing the morphological and chemical parameters of plants kept inside and plants exposed in the field. Furthermore, suitability of field-exposed plants to naturally occurring insects was investigated in relation to UV-B pretreatment. High levels of UV-B radiation led to increased flavonoid concentrations, but to a lower biomass accumulation in broccoli. These patterns remained after outdoor exposure. However, UV-induced changes of plant traits did not alter attractiveness to herbivorous insects: thrips, whiteflies, and aphids attacked plants independently of UV-B pretreatment. A 3-fold increase of indolyl glucosinolate concentrations occurred in above-ground tissue of all the plants, most likely due to massive herbivore attack after 3 d of field exposure. The results show that plants respond with high specificity to different abiotic and biotic impacts, demonstrating the separate perception and processing of stress factors.

Signal Transduction in Responses to UV-B Radiation

UV-B radiation is a key environmental signal that initiates diverse responses in plants that affect metabolism, development, and viability. Many effects of UV-B involve the differential regulation of gene expression. The response to UV-B depends on the nature of the UV-B treatment, the extent of adaptation and acclimation to UV-B, and interaction with other environmental factors. Responses to UV-B are mediated by both nonspecific signaling pathways, involving DNA damage, reactive oxygen species, and wound/defense signaling molecules, and UV-B-specific pathways that mediate photomorphogenic responses to low levels of UV-B. Importantly, photomorphogenic signaling stimulates the expression of genes involved in UV-protection and hence promotes plant survival in UV-B. Photomorphogenic UV-B signaling is mediated by the UV-B-specific component UV RESISTANCE LOCUS8 (UVR8). Both UVR8 and CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1) are required for UV-B-induced expression of the ELONGATED HYPOCOTYL5 (HY5) transcription factor, which plays a central role in the regulation of genes involved in photomorphogenic UV-B responses.

UVR8 in Arabidopsis thaliana regulates multiple aspects of cellular differentiation during leaf development in response to ultraviolet B radiation

Responses specific to ultraviolet B (UV-B) wavelengths are still poorly understood, both in terms of initial signalling and effects on morphogenesis. Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is the only known UV-B specific signalling component, but the role of UVR8 in leaf morphogenesis is unknown. The regulatory effects of UVR8 on leaf morphogenesis at a range of supplementary UV-B doses were characterized, revealing both UVR8-dependent and independent responses to UV irradiation. Inhibition of epidermal cell division in response to UV-B is largely independent of UVR8. However, overall leaf growth under UV-B irradiation in wild-type plants is enhanced compared with a uvr8 mutant because of a UVR8-dependent compensatory increase of cell area in wild-type plants. UVR8 was also required for the regulation of endopolyploidy in response to UV-B, and the uvr8 mutant also has a lower density of stomata than the wild type in the presence of UV-B, indicating that UVR8 has a regulatory role in other developmental events. Our findings show that, in addition to regulating UV-protective gene expression responses, UVR8 is involved in controlling aspects of leaf growth and morphogenesis. This work extends our understanding of how UV-B response is orchestrated at the whole-plant level.

Ultraviolet radiation as a limiting factor in leaf expansion and development

Reductions in leaf growth are a commonly observed response to ultraviolet radiation, but the underlying mechanisms remain poorly defined. This study examined the response of leaves exposed to a UV environment across a range of organizational scales, including leaf expansion rate, epidermal cell size and number, biomechanical properties, leaf-water relations and activity of cell-wall peroxidases. Two experimental approaches were used; Lettuce (Lactuca sativa L.) plants were propagated under (a) supplementary UV-B (9 kJ m(-2) day(-1)) in controlled environment (CE) conditions, and (b) field conditions, where plants were placed under three horticultural films with differing UV transmissions. In both experiments, UV-B caused the greatest reductions in leaf expansion and final leaf size, with some reductions attributable to UV-A wavelengths. In supplementary UV-B conditions, adaxial cell size was reduced, while in field plants, both cell size and cell number were lower in an increased UV environment, as was the case with abaxial cells in CE plants. Although leaf turgor and leaf extensibility were not affected by UV wavelengths, breaking strain of leaf tissue was decreased under supplementary UV-B. Cell-wall peroxidase activity was increased in both supplementary UV conditions and in the field, where only a zero UV environment showed no upregulation of cell-wall peroxidase.

Global change effects on plant chemical defenses against insect herbivores

This review focuses on individual effects of major global change factors, such as elevated CO2, O3, 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 CO2 and UV light levels, the latter appears to primarily increase the concentrations of flavonoids. Likewise, specific phenolic metabolites seem to be induced by O3 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.

Ambient ultraviolet radiation induces protective responses in soybean but does not attenuate indirect defense

We investigated the effects of ambient ultraviolet (UV) radiation on (i) the performance and chemistry of soybean plants, (ii) the performance of Spodoptera frugiperda and (iii) the foraging behavior of the herbivore's natural enemy Cotesia marginiventris which exploits herbivore-induced plant volatiles (VOC) for host location. The accumulation of protective phenolics was faster in plants receiving ambient UV than in controls exposed to sun light lacking UV. Accordingly, isorhamnetin- and quercetin-based flavonoids were increased in UV exposed plants. No UV effects were found on the performance and feeding behavior of S. frugiperda. Herbivore-damaged plants emitted the same VOC when grown under ambient or attenuated UV for 5, 10 or 30 days. Consequently, C. marginiventris was attracted but did not discriminate between exposed and unexposed soybeans. In summary, ambient UV radiation affected soybean morphology and physiology but did not destabilize interactions between trophic levels.

Influence of ambient and enhanced ultraviolet-B radiation on the plant growth and physiological properties in two contrasting populations of Hippophae rhamnoides

Two contrasting sea buckthorn (Hippophae rhamnoides L.) populations from low and high altitude regions were employed to investigate the effects of prevailing and enhanced ultraviolet-B (UV-B) radiation on plant growth and physiological properties under a UV-B-enhanced/exclusion system. The experimental design included three UV-B regimes, including excluded (-UVB), near-ambient (NA) and enhanced UV-B (+UVB) radiation. Compared with the control (-UVB), NA caused the formation of smaller but thicker plant leaves in both sea buckthorn populations, paralleled with significant increments of carotenoids and UV-absorbing compounds as well as improved water economy. NA also induced more biomass partition from shoot to root, but CO(2) assimilation rate (A), photosynthetic area and biomass accumulation were unaffected. The low-altitude population seemed sensitive to +UVB, as indicated by the decreases in total biomass, A and ascorbic acid content (Asa, an antioxidant) compared with NA. However, little +UVB effect occurred on the high-altitude population, and we suggest that the higher tolerance of this population could be associated with its specific morphological and physiological characteristics, such as small but thick leaves and high-level of Asa content, as well as its greater physiological modification in response to NA, e.g., increases in protective compounds (carotenoids and UV-absorbing compounds) and improvement in water economy, in comparison to the low-altitude population, which form an effective adaptation strategy to enhanced UV-B stress.

Silencing RNA-directed RNA polymerase 2 increases the susceptibility of Nicotiana attenuata to UV in the field and in the glasshouse

RNA-directed RNA-polymerases (RdRs) are essential in small interfering RNA (siRNA) biogenesis and appear to be functionally specialized. We examined the consequences of silencing RdR2 in Nicotiana attenuata with a field release, and transcriptional, two-dimensional proteomic and metabolite analyses. NaRdR2-silenced plants (irRdR2) had large reductions (46% of wild type) in 22-24-nt small RNAs (smRNAs), and smaller reductions (35, 23 and 26% of wild type) in the 19-21, 25-27 and 28-30-nt smRNAs, respectively. When planted into their native habitats in the Great Basin Desert, irRdR2 plants had impaired growth and reproductive output, which were associated with reduced levels of leaf phenolics (rutin and 4'-chlorogenic acid) and MYB and PAL transcripts, but were unaffected in their herbivore resistance. These phenotypes were confirmed in glasshouse experiments, but only when irRdR2 plants were grown with UV-B radiation. irRdR2 plants had wild-type levels of elicited phytohormones and resistance to Manduca sexta attack, but when exposed to UV-B, had reduced growth, fitness, levels of MYB and PAL transcripts, and phenolics. Proteins related to protection against oxidative and physiological stresses, chromatin remodeling and transcription were also downregulated. Silencing the MYB gene by virus-induced gene silencing (VIGS) in wild-type plants reduced levels of PAL transcripts and phenolics, as it did in UV-exposed irRdR2 plants. Bioinformatic analysis revealed that genes involved in phenylpropanoid biosynthesis contained a large number of smRNA binding motives, suggesting that these genes are targets of smRNAs. We conclude that although NaRdR2 transcripts are upregulated in response to both UV-B and herbivore elicitation, the responses they regulate have been tailored to provide protection from UV-B radiation.