Healthy and unhealthy plants: The effect of stress on the metabolism of Brassicaceae

Brassicaceae plants are one of the most popular vegetables consumed all over the world and considered to be a good source of bioactive phytochemicals. Additionally, Brassica species and varieties are increasingly becoming a research model in plant science, as a consequence of the importance of their primary and secondary metabolites. Plant interaction with environmental stress factors including animals and insects herbivory, pathogens, metal ions, light, among others, is known to lead to the activation of various defense mechanisms resulting in a qualitative and/or quantitative change in plant metabolite production. Pre-harvest and/or post-harvest conditions are also known to affect this, since plants produce signaling molecules (e.g. salicylic acid, jasmonic acid, etc.) that cause a direct or indirect activation of metabolic pathways. That ultimately affects the production of phytochemicals, such as carbohydrates (sucrose and glucose), amino acids, phenolics (phenylpropanoids and flavonoids) and glucosinolates. These phytochemicals have diverse applications due to their antimicrobial, antioxidant and anti-carcinogenic properties, but on the other hand these compounds or their breakdown products can act as anti-nutritional factors in diet. In this review we report a wide range of the stress-induced metabolic responses in the Brassica plants commonly used for human consumption.     

Jasmonates and Related Compounds in Plant-Insect Interactions

Herbivore attack elicits defense responses in host plants by a complex chain of events that starts with the introduction of herbivore-specific elicitors into the wounds at the feeding or oviposition site, their recognition by the plant, and activation of several signaling cascades that trigger defense responses that finally increase resistance. Oxylipin signaling plays a central role in the activation of these herbivore-induced responses. Wounding activates some but not all of these defense responses, but herbivore attack frequently amplifies the oxylipin responses well beyond that elicited by wounding alone, suggesting recognition of herbivore attack. In addition to their signaling role within the plant, oxylipins can also directly influence the performance of herbivores or attract natural enemies to feeding herbivores. Here we review the literature on the regulation and function of herbivore-specific oxylipin signaling and the direct effects of oxylipins on herbivore performance. Online publication: 13 January 2005     

Jasmonate-dependent plant defenses mediate soybean thrips and soybean aphid performance on soybean

Insect herbivores from different feeding guilds induce different signaling pathways in plants. In this study, we examined the effects of salicylic acid (SA)- and jasmonic acid (JA)-mediated defenses on performance of insect herbivores from two different feeding guilds: cell-content feeders, soybean thrips and phloem feeders, soybean aphids. We used a combination of RT-qPCR analysis and elicitor-induced plant resistance to determine induction of SA and JA signaling pathways and the impact on herbivore performance. In the early interaction between the host plant and the two herbivores, SA and JA signaling seems to occur simultaneously. But overall, soybean thrips induced JA-related marker genes, whereas soybean aphids increased SA and ABA-related marker genes over a 24-h period. Populations of both soybean thrips and soybean aphids were reduced (47 and 25 %, respectively) in methyl jasmonate (MeJA)-pretreated soybean plants. SA treatment has no effect on either herbivore performance. A combination pretreatment of SA and MeJA did not impact soybean thrips population but reduced soybean aphid numbers which was comparable with MeJA treatment. Our data suggest that SA–JA antagonism could be responsible for the effect of hormone pretreatment on thrips performance, but not on aphid performance. By linking plant defense gene expression and elicitor-induced resistance, we were able to pinpoint the role for JA signaling pathway in resistance to two herbivores from different feeding guilds.     

Exposure of Unwounded Plants to Chemical Cues Associated with Herbivores Leads to Exposure-Dependent Changes in Subsequent Herbivore Attack

Although chemical predator cues often lead to changes in the anti-predator behavior of animal prey, it is not clear whether non-volatile herbivore kairomones (i.e. incidental chemical cues produced by herbivore movement or metabolism but not produced by an attack) trigger the induction of defense in plants prior to attack. I found that unwounded plants (Brassica nigra) that were regularly exposed to kairomones from snails (mucus and feces produced during movement of Helix aspersa) subsequently experienced reduced rates of attack by snails, unlike unwounded plants that received only one initial early exposure to snail kairomones. A follow-up experiment found that mucus alone did not affect snail feeding on previously harvested B. oleracea leaves, suggesting that changes in herbivory on B. nigra were due to changes in plant quality. The finding that chemicals associated with herbivores leads to changes in palatability of unwounded plants suggests that plants eavesdrop on components of non-volatile kairomones of their snail herbivores. Moreover, this work shows that the nature of plant exposure matters, supporting the conclusion that plants that have not been attacked or wounded nonetheless tailor their use of defenses based on incidental chemical information associated with herbivores and the timing with which cues of potential attack are encountered.     

Jasmonate-dependent defense signaling in plant tissues

Depending on the stress type, plants activate various signal transduction pathways inducing the optimum defense process. This review is devoted to jasmonate (JA) dependent signaling involved in plant defense against biotic and abiotic stresses, including those determined by wounding, necrotrophic pathogens, pests, and herbivores. The sequence of major events of JA signaling is discussed. It is noted that JA signaling in plants is incorporated into a complex signaling network.     

The role of natural plant products in modulating the immune system: An adaptable approach for combating disease in grazing animals

Plants provide herbivores with an array of chemicals with the potential to improve health and well-being. For instance, phytochemicals, known as secondary compounds, which protect plants from consumers and pests can adversely affect cellular and metabolic processes in herbivores, but at low doses and in appropriate mixtures, they can have beneficial effects on animal nutrition and health, though the latter has not been explored in great breadth or depth. In this review, we summarize the potential impact of natural plant products on immunomodulation and other therapeutic effects in herbivores. Development of preventative strategies to help animals resist disease would be a more economical, ecological and socially effective long-term healthcare strategy than treating diseases. In this realm, immunomodulation promoted by forages emerges as an interesting alternative and complement to chemotherapy. The challenge for feeding systems will be to incorporate mixes of plants with bioactive properties in ways that enhance health without compromising animal production and well-being. A solution to this challenge may involve developing management programs that acknowledge the ability of animals to learn about the beneficial effects of diverse foods.     

植物与植食性昆虫化学互作研究进展

植物与植食性昆虫之间存在着复杂的化学相互作用。一方面,当遭受植食性昆虫为害时,植物能识别植食性昆虫相关分子模式,触发早期信号事件和激素信号转导途径,并由此引起转录组与代谢组重组、直接和间接防御化合物含量升高,最后提高对植食性昆虫的抗性。另一方面,植食性昆虫也能识别植物的防御反应,并能通过分泌效应子、选贮、解毒以及降低敏感性等反防御措施抑制或适应植物的化学防御。深入剖析植物与植食性昆虫的化学互作,不仅可在理论上丰富对昆虫与植物互作关系的理解,而且可在实践上为作物害虫防控新技术的开发提供重要的理论与技术指导。     

Plant volatiles as method of communication

Plants emit volatile compounds that can act as a communication method to insects, neighboring plants and pathogens. Plants respond to leaf and root damage by herbivores and pathogens by emitting these compounds. The volatile compounds can deter the herbivores or pathogens directly or indirectly by attracting their natural enemies to kill them. The simultaneous damage of plants by herbivores and pathogens can influence plant defense. The induced plant volatiles can also make neighboring plants ready for defense or induce defense in parts distant from the damaged area of the same plant. Belowground root herbivory can alter the defense response to aboveground leaf herbivory. In addition, most plants normally emit volatile compounds from their flowers that directly attract foraging mutualistic insects for nectar, which in turn perform the very important function of pollination for subsequent reproduction. The volatile compounds emitted from the floral and vegetative parts of plants belong to three main classes of compounds: terpenoids, phenylpropanoids/benzenoids, and C6-aldehydes (green-leaf volatiles). The volatile phytohormones methyl salicylate and methyl jasmonate serve as important signaling molecules for communication purposes, and interact with each other to optimize the plant defense response. Here we discuss and integrate the current knowledge on all types of communication between plants and insects, neighboring plants and pathogens that are mediated through plant volatiles.     

Behavioral archives link the chemistry and clonal structure of trembling aspen to the food choice of North American porcupine

Understanding the links among plant genotype, plant chemistry, and food selection by vertebrate herbivores is critical to assess the role of herbivores in the evolution of plant secondary chemistry. Some specialized vertebrate herbivores have been shown to select plants differentially according to plant genotype, but examples from generalists, which constitute the vast majority of vertebrate herbivores, are few, especially in natural conditions. We examined the relationship between the North American porcupine (Erethizon dorsatum), a generalist mammalian herbivore, and clonal trembling aspen (Populus tremuloides), a preferred food source of porcupines. We determined preference for certain aspen trees through visual examination of porcupine climbing scars left on tree bark, and through a controlled feeding experiment. We used genetic and biochemical analyses to link the behavioral archives (climbing scars) left by porcupines on aspen trunks to the clonal structure and chemical composition of trees. We show that two phenolic glycosides (tremulacin and salicortin), which are under a high degree of genetic control and thus vary in concentration across clones, are the chemical variables that most influence (deter) feeding choices by porcupines. Using behavioral archives left by a wild herbivore on a natural stand of plants thus allowed us to demonstrate that a generalist vertebrate herbivore can choose plants according to their clonal structure and genetically based chemical composition. Our results contribute to extending previous findings obtained with generalist herbivores studied in controlled conditions, and with specialist herbivores studied in the field. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pathogen resistance of transgenic tobacco plants producing caffeine

Caffeine (1,3,7-trimethylxanthine) is a typical purine alkaloid, and produced by a variety of plants such as coffee and tea. Its physiological function, however, is not completely understood, but chemical defense against pathogens and herbivores, and allelopathic effects against competing plant species have been proposed. Previously, we constructed transgenic tobacco plants, which produced caffeine up to 5 microg per gram fresh weight of leaves, and showed them to repel caterpillars of tobacco cutworms (Spodoptera litura). In the present study, we found that these transgenic plants constitutively expressed defense-related genes encoding pathogenesis-related (PR)-1a and proteinase inhibitor II under non-stressed conditions. We also found that they were highly resistant against pathogens, tobacco mosaic virus and Pseudomonas syringae. Expression of PR-1a and PR-2 was higher in transgenic plants than in wild-type plants during infection. Exogenously applied caffeine to wild-type tobacco leaves exhibited the similar resistant activity. These results suggested that caffeine stimulated endogenous defense system of host plants through directly or indirectly activating gene expression. This assumption is essentially consistent with the idea of chemical defense, in which caffeine may act as one of signaling molecules to activate defense response. It is thus conceivable that the effect of caffeine is bifunctional; direct interference with pest metabolic pathways, and activation of host defense systems.     

TROPHIC INTERACTIONS IN THE SEA: AN ECOLOGICAL ROLE FOR CLIMATE RELEVANT VOLATILES?1

When attacked by herbivores, land plants can produce a variety of volatile compounds that attract carnivorous mutualists. Plants and carnivores can benefit from this symbiotic relationship, because the induced defensive interaction increases foraging success of the carnivores, while reducing the grazing pressure exerted by the herbivores on the plants. Here, we examine whether aquatic phytoplankton use volatile chemical cues in analogous tritrophic interactions. Marine algae produce several classes of biogenic gases such as non‐methane hydrocarbons, organohalogens, ammonia and methylamines, and dimethylsulfide. The grazing‐induced release of marine biogenic volatiles is poorly understood, however, and its effect on the chemical ecology of plankton and the foraging behavior of predators is essentially unknown. We outline grazing‐induced defenses in algae and highlight the biogenic production of volatiles when phytoplankton are attacked by herbivores. The role of chemical signaling in marine ecology presents several possible avenues for future research, and we believe that progress in this area will result in better understanding of species competition, bloom development, and the structuring of food webs in the sea. This has further implications for biogeochemical cycles, because several key compounds are emitted that influence the chemistry of the atmosphere and global climate.     

Pulse-Driven Loss of Top-Down Control: The Critical-Rate Hypothesis

In systems ranging from lakes and woodlands to coral reefs, the long-term ecosystem state may often be determined largely by rare extreme events such as wet ENSO years, droughts, or disease outbreaks. Such events can flip these systems into a contrasting state that represents either an alternative attractor or a transient that is slow enough to persist even if the frequency of events that push the system to this state is low. Here we show that escape from herbivores is a mechanism that can play a role in several state shifts of this kind. This can happen if plants become less susceptible to herbivory as they grow. Using a model we show that, surprisingly, this mechanism can lead to a situation where a brief resource pulse for plants may invoke a persistent shift to a high biomass state whereas gradual enrichment to the same resource level is insufficient to allow such a change. This counterintuitive phenomenon occurs if the numerical response of herbivores is sufficiently slow to allow the plants to use the resource pulse to escape to a safe biomass at which herbivory is reduced. Our results imply that rates of environmental change can sometimes be more important than their magnitude. This has many ramifications. On the conceptual side, our findings suggest that key mechanisms that regulate long-term ecosystem dynamics are easily missed by the traditional focus of modelers on equilibria. A more practical corollary is that increased climatic variability may have more profound effects in some ecosystems than gradual change in conditions.     

The role of jasmonic acid and ethylene crosstalk in direct defense of Nicotiana attenuata plants against chewing herbivores

We examined performance of herbivores on plants lacking either jasmonate (JA, asLOX3) or ethylene (ET, mETR1) signaling or both (mETR1asLOX3). Plant defenses against Manduca sexta caterpillars were strongly impaired in JA-deficient asLOX3 plants; however, making asLOX3 plants ethylene insensitive did not further increase the performance of the larvae on a mETR1asLOX3 genetic cross. This result demonstrates the dominant role of JA over ET in the regulation of plant defenses against herbivores. However, ET-insensitivity combined with otherwise normal levels of JA in mETR1 plants promoted faster caterpillar growth, which correlated with reduced accumulation of the alkaloidal direct defense nicotine in mETR1 compared to WT plants. Our data points to an important accessory function of ET in the activation of JA-regulated plant defenses against herbivores at the level of alkaloid biosynthesis in the roots and/or accumulation in the leaves.Key words: herbivory, jasmonic acid and ethylene crosstalk, Nicotiana attenuata, nicotine, trypsin proteinase inhibitors (TPIs)     

Effects of phytochemicals on ionization radiation-mediated carcinogenesis and cancer therapy

Ionizing radiation (IR)-induced cellular damage is implicated in carcinogenesis as well as therapy of cancer. Advances in radiation therapy have led to the decrease in dosage and localizing the effects to the tumor; however, the development of radioresistance in cancer cells and radiation toxicity to normal tissues are still the major concerns. The development of radioresistance involves several mechanisms, including the activation of mitogenic and survival signaling, induction of DNA repair, and changes in redox signaling and epigenetic regulation. The current strategy of combining radiation with standard cytotoxic chemotherapeutic agents can potentially lead to unwanted side effects due to both agents. Thus agents are needed that could improve the efficacy of radiation killing of cancer cells and prevent the damage to normal cells and tissues caused by the direct and bystander effects of radiation, without have its own systemic toxicity. Chemopreventive phytochemicals, usually non-toxic agents with both cancer preventive and therapeutic activities, could rightly fit in this approach. In this regard, naturally occurring compounds, including curcumin, parthenolide, genistein, gossypol, ellagic acid, withaferin, plumbagin and resveratrol, have shown considerable potential. These agents suppress the radiation-induced activation of receptor tyrosine kinases and nuclear factor-κB signaling, can modify cell survival and DNA repair efficacy, and may potentiate ceramide signaling. These radiosensitizing and counter radioresistance mechanisms of phytochemicals in cancer cells are also associated with changes in epigenetic gene regulation. Because radioresistance involves multiple mechanisms, more studies are needed to discover novel phytochemicals having multiple mechanisms of radiosensitization and to overcome radioresistance of cancer cells. Pre-clinical studies are needed to address the appropriate dosage, timing, and duration of the application of phytochemicals with radiation to justify clinical trials. Nonetheless, some phytochemicals in combination with IR may play a significant role in enhancing the therapeutic index of cancer treatment.     

Induced plant defense via volatile production is dependent on rhizobial symbiosis

Nitrogen-fixing rhizobia can substantially influence plant–herbivore interactions by altering plant chemical composition and food quality. However, the effects of rhizobia on plant volatiles, which serve as indirect and direct defenses against arthropod herbivores and as signals in defense-associated plant–plant and within-plant signaling, are still unstudied. We measured the release of jasmonic acid (JA)-induced volatiles of rhizobia-colonized and rhizobia-free lima bean plants (Fabaceae: Phaseolus lunatus L.) and tested effects of their respective bouquets of volatile organic compounds (VOCs) on a specialist insect herbivore (Mexican bean beetle; Coccinellidae: Epilachna varivestis Mulsant) in olfactometer choice trials. In a further experiment, we showed that VOC induction by JA reflects the plant responses to mechanical wounding and insect herbivory. Following induction with JA, rhizobia-colonized plants released significantly higher amounts of the shikimic acid-derived compounds, whereas the emission of compounds produced via the octadecanoid, mevalonate and non-mevalonate pathways was reduced. These changes affected the choice behavior of beetles as the preference of non-induced plants was much more pronounced for plants that were colonized by rhizobia. We showed that indole likely represents the causing agent for the observed repellent effects of jasmonic acid-induced VOCs of rhizobia-colonized lima bean plants. Our study demonstrates a rhizobia-triggered efficacy of induced plant defense via volatiles. Due to these findings, we interpret rhizobia as an integral part of legume defenses against herbivores.     

A porin-like protein from oral secretions of Spodoptera littoralis larvae induces defense-related early events in plant leaves

Insect herbivory on plants is a complex incident consisting of at least two different aspects, namely mechanical damage and chemical challenge, as feeding insects introduce oral secretions (OS) into the wounded tissue of the attacked plant. Mechanical wounding alone is sufficient to induce a set of defense-related reactions in host plants, but some early events such as membrane potential (Vm) changes and cytosolic Ca²⁺-elevations can be triggered only by herbivores suggesting that OS-derived molecules are involved in those processes. Following an assay-guided purification based on planar lipid bilayer membrane technique in combination with proteomic analysis, a porin-like protein (PLP) of most likely bacterial origin was determined from collected OS of Spodoptera littoralis larvae. PLP exhibited channel-forming activity. Further, early defense-related events in plant–insect interaction were evaluated by using a purified fraction and α-hemolysin (α-HL) as a commercial pore-forming compound. Both up-regulated the calmodulin-like CML42 in Arabidopsis thaliana, which only responds to oral secretion and not to wounding. An elevation of in vivo [Ca²⁺]_cyt was not observed. Because membrane channel formation is a widespread phenomenon in plant–insect interactions, this PLP might represent an example for microbial compounds from the insect gut which are initially involved in plant–insect interactions.     

Interactions between plants and herbivores: A review of plant defense

Ecologists have long ignored or underestimated the importance of plant–herbivore interactions owing to the diversities of herbivores, plant defensive strategies and ecological systems. In this review, we briefly discussed the categories of herbivores. Then we reviewed the major types of plant defenses against herbivores. Selective forces of herbivore pressures have led to the evolution of various defensive mechanisms in plants, which can be classified into (i) resistance traits that reduce the amount of damage received, including physical, chemical, and biotic traits; (ii) tolerance mechanisms that decrease the impact of herbivore damage, and (iii) escape strategies that reduce the probability of plants to be found by herbivores. These strategies have been studied at different levels from molecular genetics and genomics, to chemistry and physiology, to community and ecosystem ecology. We summarized the development of the methodology for studying plant defenses against herbivores. Particularly, 24 of those hypotheses and models, which are influential in the international community concerning the relationship between plants and herbivores, including the defensive mimicry hypothesis, the compensatory continuum hypothesis, the slow-growth-high-mortality hypothesis, etc, were introduced and grouped into four categories according to plant defense strategies in the present review. Finally, we also reviewed the research progress of plant–herbivore interactions in China, and discussed the perspectives of studies on plant–herbivore interactions.