Role of Secondary Metabolites and Brassinosteroids in Plant Defense Against Environmental Stresses

Being sessile, plants are subjected to a diverse array of environmental stresses during their life span. Exposure of plants to environmental stresses results in the generation of reactive oxygen species (ROS). These activated oxygen species tend to oxidize various cellular biomolecules like proteins, nucleic acids, and lipids, a process that challenges the core existence of the cell. To prevent the accumulation of these ROS and to sustain their own survival, plants have developed an intricate antioxidative defence system. The antioxidative defence system comprises various enzymatic and nonenzymatic molecules, produced to counter the adverse effect of environmental stresses. A sizable number of these molecules belong to the category of compounds called secondary metabolites. Secondary metabolites are organic compounds that are not directly involved in the growth and development of plants but perform specialized functions under a given set of conditions. Absence of secondary metabolites results in long-term impairment of the plant’s survivability. Such compounds generally include pigments, phenolics, and so on. Plant phenolic compounds such as flavonoids and lignin precursors have been reported to accumulate in response to various biotic and abiotic stresses and are regarded as crucial defence compounds that can scavenge harmful ROS. Another important category of plant metabolites, called brassinosteroids, exhibit stress regulatory and growth-promoting activity and are classified as phytohormones. Elucidation of the physiological and molecular effects of secondary metabolites and brassinosteroids have catapulted them as highly promising and environment-friendly natural substances, suitable for wider application in plant protection and crop yield promotion. The present review focuses on our current understanding of how plants respond to the generation of excessive ROS and the role of secondary metabolites and brassinosteroids in countering the adverse effects of environmental stresses.     

On the study of plant defence and herbivory using comparative approaches: how important are secondary plant compounds

Species comparisons are a cornerstone of biology and there is a long tradition of using the comparative framework to study the ecology and evolution of plant defensive traits. Early comparative studies led to the hypothesis that plant chemistry plays a central role in plant defence, and the evolution of plant secondary chemistry in response to insect herbivory remains a classic example of coevolution. However, recent comparative work has disagreed with this paradigm, reporting little connection between plant secondary chemicals and herbivory across distantly related plant taxa. One conclusion of this new work is that the importance of secondary chemistry in plant defence may have been generally overstated in earlier research. Here, we attempt to reconcile these contradicting viewpoints on the role of plant chemistry in defence by critically evaluating the use and interpretation of species correlations as a means to study defence–herbivory relationships. We conclude that the notion that plant primary metabolites (e.g. leaf nitrogen content) are the principal determinants of herbivory (or the target of natural selection by herbivores) is not likely to be correct. Despite the inference of recent community‐wide studies of herbivory, strong evidence remains for a prime role of secondary compounds in plant defence against herbivores.     

Plant defence, an evolutionary dilemma: contrasting effects of (specialist and generalist) herbivores and natural enemies

Conclusions Contrasting effects of generalist and specialist herbivores can explain why all plants have not evolved high levels of defence. Maintenance of variation in concentration of defence substances can be explained by a shifting balance between natural selection for defence against herbivory by specialists and generalists. Generalist natural enemies will shift the optimal defence curve to lower concentrations of defences. Physiological costs of production of defence substances and selection by specialist herbivores of plant phenotypes with higher levels of defence compounds for sequestration are no essential elements of this model. They may, however, adjust the predicted optimum defence function and contribute to maintenance of variation of concentrations of defence substances.     

The development of butterfly settling posture: the role of predators, climate, hostplant-habitat and phylogeny

A model is described that explains the evolution of adult butterfly settling posture. The role of climate and the activity of butterflies is fundamental in determining the effectiveness of primary and secondary visual defence systems based on wing colour and pattern. Both dorsal (open winged) and lateral (closed wing) postures can be regarded as evolutionarily conservative; common postures are found within almost all sub-families. Lateral postures tend to be associated with cool temperate and arctic climates. Conditions that are marginal for activity place an emphasis on the development of underside primary defence, uncompromised by secondary visual defences. In turn, any primary defensive function of the upper surface is reduced but secondary defence may be emphasized. There is interaction between segregation of functions on different wing surfaces and the development of lateral posture. The use of predictable hostplant-habitat structures reduces the role of visual communication in mate-location and facilitates the adoption of a lateral resting posture and a cryptic underside wing pattern. When hostplant-habitat structures are unpredictable but a lateral posture is adopted, primary defence is more important than visual communication for the resting insect. Adjustments of wing morphology facilitate postural conservatism, allowing species to retain particular postures in new hostplant-habitats and climatic zones.     

Investigations of phagocytosis concerning the immunological defence mechanism of Mytilus edulis using a sublethal luminescent bacterial assay (Photobacterium phosphoreum).

1. A simple method for the determination of phagocytosis activity using mussel hemocytes by measuring the bioluminescence is presented. 2. The immunological defence activity based on phagocytosis is measured and quantified by a luminescent bacterial assay with Photobacterium phosphoreum. 3. The measuring system allows us to establish the stress of the immunological defence mechanism of organisms exposed to chemicals and polluted rivers or sewage. Results with reference substances and the phagocytosis indices of exposed mussels from Norwegian aquaculture plants compared to those of mussels from the German Wadden Sea are given as examples.     

Modification of the complement induced by PGE2 and gonadotropins

The Complement is one of the major effectors of the humoral aspecific immune system building up a defence mechanism of the organism. As it is known that some hormonal substances like gonadotropin (hCG) and some hormone-like substances (PGE2) influence the entire immunitary system, we wanted to see if they had specific action on the Complement. The measurement of CH50 was carried out using Mayer's method, derived by Ferrazzi and modified by us. Fractions C3 and C4 were measured by means of immunochemistry using Beckman nephelometer. The treatment with hCG (1,000 U + 10 Lf tetanus toxoid) caused an increase in the CH50 and in the fraction C3, while the fraction C4 was not modified. The treatment with PGE2 (0.25 microgram/rat/die) caused a higher increase of CH50 and C3 fraction. It seems possible to acknowledge C3 involvement in the variation of the Complement's haemolytic activity and this could confirm the intervention of the "alternative pathway". The notable increase in the activity of the Complement induced by hCG and PGE2 could indicate an alternative mechanism of activation of the aspecific humoral immunity in the defence of the organism in all those physio-pathological situations where these substances cause a state of depression of cellular mediated activity.     

Developments in aspects of ecological phytochemistry: the role of cis-jasmone in inducible defence systems in plants

The challenges and opportunities for protecting agricultural production of food and other materials will be met through exploiting the induction of defence pathways in plants to control pests, diseases and weeds. These approaches will involve processes that can be activated by application of natural products, patented in terms of this use, to "switch on" defence pathways. Already, a number of secondary metabolite defence compounds are known for which the pathways are conveniently clustered genomically, e.g. the benzoxazinoids (hydroxamic acids) and the avenacins. For the former, it is shown that the small molecular weight lipophilic activator cis-jasmone can induce production of these compounds and certain genes within the pathway. Numerous groups around the world work on inducible defence systems. The science is rapidly expanding and involves studying the interacting components of defence pathways and the switching mechanisms activated by small molecular weight lipophilic compounds. Examples are described of how plant breeding can exploit these systems and how heterologous gene expression will eventually give rise to a new range of GM crops for food and energy, without the need for external application of synthetic pesticides.     

Induced resistance during the interaction pathogen x plant and the use of resistance inducers

Plants react to aggressions through different defence responses. Mechanical barriers consist in the increase of production and deposition of substances capable of containing pathogen invasion. Chemical barriers consist in the increase of concentration or activity of defence proteins and synthesis of phenolic compounds and phytoalexins. Elicitor substances have been widely used in plant disease control showing impressive results and a low impact to the environment and man. This review contains information about plant defence mechanisms and shows the use of inducers of resistance in the control of pathogens and prospects of advance towards sustainable agriculture.     

The price of safety: food deprivation in early life influences the efficacy of chemical defence in an aposematic moth

Aposematism is the combination of a primary signal with a secondary defence that predators must learn to associate with one another. However, variation in the level of defence, both within and between species, is very common. As secondary defences influence individual fitness, this variation in quality and quantity requires an evolutionary explanation, particularly as it may or may not correlate with variation in primary signals. The costs of defence production are expected to play a considerable role in generating this variation, yet studies of the cost of chemical defence have focused on species that sequester their defences, while studies in species that produce them de novo are scarce. Here we examine effects of resource availability on the production of chemical defences in female wood tiger moths Arctia plantaginis. This species produces defensive fluids when attacked, and advertises this protection with bright colours on its hindwings. Furthermore, the chemicals in these fluids are produced de novo. In order to establish if the production of this chemical protection is costly, and thus resource‐limited, we manipulated resource availability (i.e. food) during larval development and measured its subsequent effects on adult chemical defence. We presented defensive fluids from female moths to wild blue tits, in the absence of any visual cues, to determine their effectiveness against avian predators. Our results demonstrate that the defensive fluids produced by female moths repel birds, and that these defences require resource investment to produce. We found that moths that were resource‐limited during development had less effective chemical defences, despite producing the same volume of defensive fluids as the control moths. As a reduction in available resources negatively influences the effectiveness of the chemical defence, resource availability may explain the variation seen among individuals.     

Function of Host and Fungal Metabolites in Resistance Response of Banana and Plantain in the Black Sigatoka Disease Pathosystem (Musa spp. –Mycosphaerella fijiensis)

The host–pathogen interactions of Musa spp. and Mycosphaerella fijiensis were investigated in order to determine the function of secondary metabolites within the pathosystem of the Black Sigatoka disease. The pentaketide metabolites flaviolin, 2-hydroxyjuglone, juglone and 2,4,8-trihydroxytetralone (2,4,8-THT) of the pathogen were identified. The concentration of 2,4,8-THT was significantly increased by application of the synthetic compound tricyclazole and by natural activators extracted from the intercellular space of leaf tissue of resistant Musa cultivars. When inoculated host plants were treated with tricyclazole, extensive necrosis of both susceptible and resistant Musa cultivar leaves were observed. Plant defence mechanisms of resistant Musa cultivars were first detected as an activation of phenylalanine–ammonia lyase and the subsequent accumulation of post-infectional substances which blocked fungal growth. These results indicated the bivalent importance of 2,4,8-THT for host-specific reactions, depending on its concentration at different stages of pathogenesis. Early activation of fungal 2,4,8-THT metabolism by resistant Musa cultivars caused necrotic micro-lesions and elicitation of post-infectional defence reactions leading to incompatibility between pathogen and host plant; growth of the fungus on susceptible cultivars caused necrotizing doses of 2,4,8-THT only after the establishment of a compatible interaction and development of typical symptoms at late stages of pathogenesis.     

外源抗虫蛋白与内源抗虫因子的交互作用

以次生代谢物质萜烯类、黄酮类以及单宁等为基础的自身化学抗虫性一直是植物化学防御的核心。随着基因重组技术的发展,许多作物获得了一种新的化学防御形式,即以表达外源基因产物来进行防御。外源抗虫蛋白与内源抗虫物质的协调性问题,在利用外源基因工程改良植物抗虫性时是非常重要的,同时也是转基因作物安全性和生态学评价的重要方面。转基因植物中外源与内源抗虫系统间的协调性的研究取得了一些成果,但尚未引起人们足够的重视。综述了离体条件下和在转基因植物体内,外源抗虫蛋白Bt和GNA等与植物次生代谢物质以及各抗虫蛋白之间交互作用的研究进展,并探讨了研究各抗虫因子交互作用的意义。     

Plant-mediated effects in the Brassicaceae on the performance and behaviour of parasitoids

Direct and indirect plant defences are well studied, particularly in the Brassicaceae. Glucosinolates (GS) are secondary plant compounds characteristic in this plant family. They play an important role in defence against herbivores and pathogens. Insect herbivores that are specialists on brassicaceous plant species have evolved adaptations to excrete or detoxify GS. Other insect herbivores may even sequester GS and employ them as defence against their own antagonists, such as predators. Moreover, high levels of GS in the food plants of non-sequestering herbivores can negatively affect the growth and survival of their parasitoids. In addition to allelochemicals, plants produce volatile chemicals when damaged by herbivores. These herbivore induced plant volatiles (HIPV) have been demonstrated to play an important role in foraging behaviour of insect parasitoids. In addition, biosynthetic pathways involved in the production of HIPV are being unraveled using the model plant Arabidopsis thialiana. However, the majority of studies investigating the attractiveness of HIPV to parasitoids are based on experiments mainly using crop plant species in which defence traits may have changed through artificial selection. Field studies with both cultivated and wild crucifers, the latter in which defence traits are intact, are necessary to reveal the relative importance of direct and indirect plant defence strategies on parasitoid and plant fitness. Future research should also consider the potential conflict between direct and indirect plant defences when studying the evolution of plant defences against insect herbivory.     

Control of sexual behaviour pattern 'secondary defence' in the female grasshopper, Chorthippus curtipennis

The sexual behaviour of Chorthippus curtipennis females consists of ‘primary defence’, passive and active copulatory readiness, ‘secondary defence’, and egg-laying behaviour. Mechanical or chemical stimulation of the spermatheca by the spermatophore does not cause the release of ‘secondary defence‘; chemical stimuli deriving from the ovaries can also be excluded. ‘Secondary defence’ is triggered by the pressure of natural or artificial eggs against the walls of the lateral oviducts.     

不同水稻品种对虫害胁迫的生理响应

以褐飞虱-水稻为模式,研究虫害胁迫下植物的化学防御生理生态特征,测定了可溶性糖含量、叶绿素含量、光合速率、蒸腾速率、气孔导度以及细胞间隙二氧化碳浓度、防御酶的时间变化特征.结果表明,经虫害诱导,不同品种水稻可溶性糖含量降低;同时,光合速率以及叶绿素含量也相应下降;蒸腾速率、气孔导度和细胞间隙二氧化碳浓度的变化不显著。虫害对水稻叶片多酚氧化酶、过氧化物酶和脂氧合酶均具有诱导作用.这种诱导作用具有时间效应,且不同品种诱导作用不一致.     

Altered drug metabolism in parasitic diseases

While the immune system represents the main line of host defence against parasite infections, mixed function oxidase (MFO) systems (Box 1) offer the main line of defence against drugs and other biologically active substances. But, as this review shows, many parasites can exert a profound effect on the host MFO system by altering the microsomal drug-metabolizing enzymes and electron transport carriers such as cytochrome P-450. This can markedly affect the host's ability to metabolize biologically active compounds, often with adverse physiological, pharmacological and toxicological consequences. In mammals, drug metabolism occurs predominantly in the liver, and to a lesser extent in the spleen, lungs, kidneys, intestine and cerebral tissues. Thus those parasites that occupy sites in these tissues - such as amoebae, Fasciola, schistosomes and malaria - tend to be those with greatest effects on the host's ability to metabolize drugs. The effects can modify the host response to substances unrelated to the infection, and to drugs which may be administered under a chemotherapeutic regime.     

Automimicry and the evolution of discrete prey defences

We consider the neglected question of how secondary defences of prey animals evolve if they are discontinuous in nature, being either present or absent, or expressible over a limited number of levels. We present a novel computer model that evaluates the conditions in which defended mutant prey may (1) fail to rise above nontrivial levels within a population, (2) reach values close to fixation, or (3) find some evolutionarily stable strategy (ESS) frequency between these two situations. Undefended prey that coexist with defended conspecifics are known as automimics. One finding is that automimicry can be an ESS over a range of conditions, but especially when prey are relatively cryptic and secondary defences are very effective at deterring predation. Evolutionarily stable automimicry emerges from the interplay between the direct benefits of costly defences in surviving individual attacks by predators and frequency-dependent benefits conferred on all prey, from a reduction in the rate of attack on all identical-looking prey. When, in contrast, secondary defences have continuous variation, the result is effectively a monomorphic state of defence across the population. Thus the degree and kind of variation that a defence takes has a profound effect on its initial evolution. We discuss the interesting possibility that mixed ESSs may help explain some examples of variation in prey secondary defences.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 393–402.     

外源抗虫蛋白与内源抗虫因子的交互作用

以次生代谢物质萜烯类、黄酮类以及单宁等为基础的自身化学抗虫性一直是植物化学防御的核心.随着基因重组技术的发展,许多作物获得了一种新的化学防御形式,即以表达外源基因产物来进行防御.外源抗虫蛋白与内源抗虫物质的协调性问题,在利用外源基因工程改良植物抗虫性时是非常重要的,同时也是转基因作物安全性和生态学评价的重要方面.转基因植物中外源与内源抗虫系统间的协调性的研究取得了一些成果,但尚未引起人们足够的重视.综述了离体条件下和在转基因植物体内,外源抗虫蛋白Bt和GNA等与植物次生代谢物质以及各抗虫蛋白之间交互作用的研究进展,并探讨了研究各抗虫因子交互作用的意义.     

Investigations of phagocytosis concerning the immunological defence mechanism of Mytilus edulis using a sublethal luminescent bacterial assay (Photobacterium phosphoreum)

• 1.1. A simple method for the determination of phagocytosis activity using mussel hemocytes by measuring the bioluminescence is presented.
• 2.2. The immunological defence activity based on phagocytosis is measured and quantified by a luminescent bacterial assay with Photobacterium phosphoreum.
• 3.3. The measuring system allows us to establish the stress of the immunological defence mechanism of organisms exposed to chemicals and polluted rivers or sewage. Results with reference substances and the phagocytosis indices of exposed mussels from Norwegian aquaculture plants compared to those of mussels from the German Wadden Sea are given as examples.