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.     

Insect Attraction versus Plant Defense: Young Leaves High in Glucosinolates Stimulate Oviposition by a Specialist Herbivore despite Poor Larval Survival due to High Saponin Content

Glucosinolates are plant secondary metabolites used in plant defense. For insects specialized on Brassicaceae, such as the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), glucosinolates act as “fingerprints” that are essential in host plant recognition. Some plants in the genus Barbarea (Brassicaceae) contain, besides glucosinolates, saponins that act as feeding deterrents for P. xylostella larvae, preventing their survival on the plant. Two-choice oviposition tests were conducted to study the preference of P. xylostella among Barbarea leaves of different size within the same plant. P. xylostella laid more eggs per leaf area on younger leaves compared to older ones. Higher concentrations of glucosinolates and saponins were found in younger leaves than in older ones. In 4-week-old plants, saponins were present in true leaves, while cotyledons contained little or no saponins. When analyzing the whole foliage of the plant, the content of glucosinolates and saponins also varied significantly in comparisons among plants that were 4, 8, and 12 weeks old. In Barbarea plants and leaves of different ages, there was a positive correlation between glucosinolate and saponin levels. This research shows that, in Barbarea plants, ontogenetical changes in glucosinolate and saponin content affect both attraction and resistance to P. xylostella. Co-occurrence of a high content of glucosinolates and saponins in the Barbarea leaves that are most valuable for the plant, but are also the most attractive to P. xylostella, provides protection against this specialist herbivore, which oviposition behavior on Barbarea seems to be an evolutionary mistake.     

Myzus persicae (green peach aphid) feeding on Arabidopsis induces the formation of a deterrent indole glucosinolate

Cruciferous plants produce a wide variety of glucosinolates as a protection against herbivores and pathogens. However, very little is known about the importance of individual glucosinolates in plant defense and the regulation of their production in response to herbivory. When Myzus persicae (green peach aphid) feeds on Arabidopsis aliphatic glucosinolates pass through the aphid gut intact, but indole glucosinolates are mostly degraded. Although aphid feeding causes an overall decrease in Arabidopsis glucosinolate content, the production of 4-methoxyindol-3-ylmethylglucosinolate is induced. This altered glucosinolate profile is not a systemic plant response, but is limited to the area in which aphids are feeding. Aphid feeding on detached leaves causes a similar change in the glucosinolate profile, demonstrating that glucosinolate transport is not required for the observed changes. Salicylate-mediated signaling has been implicated in other plant responses to aphid feeding. However, analysis of eds5, pad4, npr1 and NahG transgenic Arabidopsis, which are compromised in this pathway, demonstrated that aphid-induced changes in the indole glucosinolate profile were unaffected. The addition of purified indol-3-ylmethylglucosinolate to the petioles of cyp79B2 cyp79B3 mutant leaves, which do not produce indole glucosinolates, showed that this glucosinolate serves as a precursor for the aphid-induced synthesis of 4-methoxyindol-3-ylmethylglucosinolate. In artificial diets, 4-methoxyindol-3-ylmethylglucosinolate is a significantly greater aphid deterrent in the absence of myrosinase than its metabolic precursor indol-3-ylmethylglucosinolate. Together, these results demonstrate that, in response to aphid feeding, Arabidopsis plants convert one indole glucosinolate to another that provides a greater defensive benefit.     

Involvement of a glucosinolate (sinigrin) in the regulation of water transport in Brassica oleracea grown under salt stress

Members of the Brassicaceae are known for their contents of nutrients and health‐promoting phytochemicals, including glucosinolates. The concentrations of these chemopreventive compounds (glucosinolate‐degradation products, the bioactive isothiocyanates) may be modified under salinity. In this work, the effect of the aliphatic glucosinolate sinigrin (2‐propenyl‐glucosinolate) on plant water balance, involving aquaporins, was explored under salt stress. For this purpose, water uptake and its transport through the plasma membrane were determined in plants after NaCl addition, when sinigrin was also supplied. We found higher hydraulic conductance (L₀) and water permeability (P_f) and increased abundance of PIP2 aquaporins after the direct administration of sinigrin, showing the ability of the roots to promote cellular water transport across the plasma membrane in spite of the stress conditions imposed. The higher content of the allyl‐isothiocyanate and the absence of sinigrin in the plant tissues suggest that the isothiocyanate is related to water balance; in fact, a direct effect of this nitro‐sulphate compound on water uptake is proposed. This work provides the first evidence that the addition of a glucosinolate can regulate aquaporins and water transport: this effect and the mechanism(s) involved merit further investigation.     

Evolution of specifier proteins in glucosinolate-containing plants

ABSTRACT: BACKGROUND: The glucosinolate-myrosinase system is an activated chemical defense system found in plants of the Brassicales order. Glucosinolates are stored separately from their hydrolytic enzymes, the myrosinases, in plant tissues. Upon tissue damage, e.g. by herbivory, glucosinolates and myrosinases get mixed and glucosinolates are broken down to an array of biologically active compounds of which isothiocyanates are toxic to a wide range of organisms. Specifier proteins occur in some, but not all glucosinolate-containing plants and promote the formation of biologically active non-isothiocyanate products upon myrosinase-catalyzed glucosinolate breakdown. RESULTS: Based on a phytochemical screening among representatives of the Brassicales order, we selected candidate species for identification of specifier protein cDNAs. We identified ten specifier proteins from a range of species of the Brassicaceae and assigned each of them to one of the three specifier protein types (NSP, nitrile-specifier protein, ESP, epithiospecifier protein, TFP, thiocyanate-forming protein) after heterologous expression in Escherichia coli. Together with nine known specifier proteins and three putative specifier proteins found in databases, we subjected the newly identified specifier proteins to phylogenetic analyses. Specifier proteins formed three major clusters, named AtNSP5-cluster, AtNSP1-cluster, and ESP/TFP cluster. Within the ESP/TFP cluster, but not within the AtNSP1 cluster, specifier proteins grouped according to the Brassicaceae lineage they were identified from. Non-synonymous vs. synonymous substitution rate ratios suggested purifying selection to act on specifier protein genes. CONCLUSIONS: Among specifier proteins, NSPs represent the ancestral activity. The data support a monophyletic origin of ESPs from NSPs. The split between NSPs and ESPs/TFPs happened before the appearance of lineage I and expanded lineage II of the Brassicaceae. TFP activity evolved from ESPs at least twice independently in different Brassicaceae lineages. The ability to form non-isothiocyanate products by specifier protein activity may provide plants with a selective advantage. The evolution of specifier proteins in the Brassicaceae demonstrates the plasticity of secondary metabolism within an activated plant defense system.     

Cytochromes P450 in the biosynthesis of glucosinolates and indole alkaloids

Characteristic of cruciferous plants is the synthesis of nitrogen- and sulfur-rich compounds, such as glucosinolates and indole alkaloids. The intact glucosinolates have limited biological activity, but give rise to an array of bio-active breakdown products when hydrolysed by endogenous β-thioglucosidases (myrosinases) upon tissue disruption. Both glucosinolates and indole alkaloids constitute an important part of the defence of plants against herbivores and pathogens, with the difference that a basal level of glucosinolates is ever-present in the plant whereas indole alkaloids are true phytoalexins that are de novo synthesised upon pathogen attack. With the completion of the genome sequence of the model plant, Arabidopsis thaliana, which is a crucifer, many genes involved in the biosynthesis of glucosinolates and indole alkaloids have been identified and cytochromes P450 are key players in these pathways. In the present review, we will focus on the cytochromes P450 in the biosynthesis of both groups of compounds. Their functional roles and regulation will be discussed.     

Effects of Glucosinolates and Flavonoids on Colonization of the Roots of Brassica napus by Azorhizobium caulinodans ORS571

Plants of Brassica napus were assessed quantitatively for their susceptibility to lateral root crack colonization by Azorhizobium caulinodans ORS571(pXLGD4) (a rhizobial strain carrying the lacZ reporter gene) and for the concentration of glucosinolates in their roots by high-pressure liquid chromatography (HPLC). High- and low-glucosinolate-seed (HGS and LGS) varieties exhibited a relatively low and high percentage of colonized lateral roots, respectively. HPLC showed that roots of HGS plants contained a higher concentration of glucosinolates than roots of LGS plants. One LGS variety showing fewer colonized lateral roots than other LGS varieties contained a higher concentration of glucosinolates than other LGS plants. Inoculated HGS plants treated with the flavonoid naringenin showed significantly more colonization than untreated HGS plants. This increase was not mediated by a naringenin-induced lowering of the glucosinolate content of HGS plant roots, nor did naringenin induce bacterial resistance to glucosinolates or increase the growth of bacteria. The erucic acid content of seed did not appear to influence colonization by azorhizobia. Frequently, leaf assays are used to study glucosinolates and plant defense; this study provides data on glucosinolates and bacterial colonization in roots and describes a bacterial reporter gene assay tailored easily to the study of ecologically important phytochemicals that influence bacterial colonization. These data also form a basis for future assessments of the benefits to oilseed rape plants of interaction with plant growth-promoting bacteria, especially diazotrophic bacteria potentially able to extend the benefits of nitrogen fixation to nonlegumes.     

Effects of glucosinolates and flavonoids on colonization of the roots of Brassica napus by Azorhizobium caulinodans ORS571

Plants of Brassica napus were assessed quantitatively for their susceptibility to lateral root crack colonization by Azorhizobium caulinodans ORS571(pXLGD4) (a rhizobial strain carrying the lacZ reporter gene) and for the concentration of glucosinolates in their roots by high-pressure liquid chromatography (HPLC). High- and low-glucosinolate-seed (HGS and LGS) varieties exhibited a relatively low and high percentage of colonized lateral roots, respectively. HPLC showed that roots of HGS plants contained a higher concentration of glucosinolates than roots of LGS plants. One LGS variety showing fewer colonized lateral roots than other LGS varieties contained a higher concentration of glucosinolates than other LGS plants. Inoculated HGS plants treated with the flavonoid naringenin showed significantly more colonization than untreated HGS plants. This increase was not mediated by a naringenin-induced lowering of the glucosinolate content of HGS plant roots, nor did naringenin induce bacterial resistance to glucosinolates or increase the growth of bacteria. The erucic acid content of seed did not appear to influence colonization by azorhizobia. Frequently, leaf assays are used to study glucosinolates and plant defense; this study provides data on glucosinolates and bacterial colonization in roots and describes a bacterial reporter gene assay tailored easily to the study of ecologically important phytochemicals that influence bacterial colonization. These data also form a basis for future assessments of the benefits to oilseed rape plants of interaction with plant growth-promoting bacteria, especially diazotrophic bacteria potentially able to extend the benefits of nitrogen fixation to nonlegumes.     

芥子油苷在十字花科植物与昆虫相互关系中的作用

芥子油苷(glucosinolate, GS)是十字花科植物重要的次生代谢物,对调节十字花科植物与昆虫间的关系起着重要作用.由于GS及其代谢产物具有一定的毒性,因此它是十字花科植物抵御广食性植食昆虫攻击的有力手段.而寡食十字花科植物的昆虫由于具备多种GS应对机制,因此可通过GS这一十字花科植物特有的次生代谢物进行寄主选择.被植食性昆虫摄入的GS不仅可以影响天敌的生长发育,而且还对天敌有一定的驱避作用.而虫害后十字花科植物释放的GS代谢产物又可作为天敌的寄主选择信息.本文结合该领域的一些最新研究成果,从GS对植食性昆虫的毒性、寡食性害虫的寄主选择、植食性昆虫对GS的适应机制、虫害对GS-黒芥子酶系统的诱导,以及GS对天敌的影响等方面对GS如何影响植物与昆虫间的相互关系进行了综述,并就今后该领域的研究方向、研究方法提出建议.     

Biosynthèse des glucosinolates indoliques et rôle écologique de leurs modifications secondaires

Indole glucosinolates are plant secondary metabolites derived from the amino acid tryptophan. They are part of a large group of sulfur-containing molecules almost exclusively found among Brassicales, which include the mustard family (Brassicaceae) with many edible plant species of major nutritional importance. These compounds mediate numerous interactions between these plants and their natural enemies and are therefore of major biological and economical interest. This literature review aims at taking stock of recent advances of our knowledge about the biosynthetic pathways of indole glucosinolates, but also about the defense strategies and ecological processes involving these metabolites.     

Nutritional and phytochemical value of Brassica crops from the agri‐food perspective

Brassica foods are among the top 10 economic crops in the world (i.e. broccoli, kale, cauliflower and Chinese cabbage). These vegetables have been identified as important components of a healthy diet because of their high levels of nutrients and health‐promoting phytochemicals (i.e. phenolics, glucosinolates, vitamins and minerals). Epidemiological studies have shown that increased consumption of Brassica foods is strongly associated with a reduced risk of degenerative diseases, cancer, cardiovascular disease and immune dysfunction. Nevertheless, the nutritional content and profile in Brassica vegetables have been reported to vary considerably during the growth period due to agronomical factors including light, temperature, water availability and soil fertility among others. Moreover, the conditions of postharvest processing and cooking are also important factors on food quality. A better understanding of specific preharvest and postharvest conditions is essential to improve cultivars with value‐added nutritional quality. Thus, in this article are going to be addressed the effects of the most common crop management strategies and processes on the variation of nutritive compounds present within Brassica from the agri‐food perspective.     

The ‘mustard oil bomb’: not so easy to assemble?! Localization, expression and distribution of the components of the myrosinase enzyme system

Glucosinolates are plant secondary metabolites that are hydrolysed by the action of myrosinases into various products (isothiocyanates, thiocyanates, epithionitriles, nitriles, oxazolidines). Massive hydrolysis of glucosinolates occurs only upon tissue damage but there is also evidence indicating metabolism of glucosinolates in intact plant tissues. It was originally believed that the glucosinolate–myrosinase system in intact plants was stable due to a spatial separation of the components. This has been coined as the ‘mustard oil bomb’ theory. Proteins that form complexes with myrosinases have been described: myrosinase-binding proteins (MBPs) and myrosinase-associated proteins (MyAPs/ESM). The roles of these proteins and their biological relevance are not yet completely known. Other proteins of the myrosinase enzyme system are the epithiospecifier protein (ESP) and the thiocyanate-forming protein (TFP) that divert the glucosinolate hydrolysis from isothiocyanate production to nitrile/epithionitrile or thiocyanate production. Some glucosinolate hydrolysis products act as plant defence compounds against insects and pathogens or have beneficial health effects on humans. In this review, we survey and critically assess the available information concerning the localization, both at the tissular/cellular and subcellular level, of the different components of the myrosinase enzyme system. Data from the model plant Arabidopsis thaliana is compared to that from other glucosinolate-producing Brassicaceae in order to show common as well as divergent features of the ‘mustard oil bomb’ among these species.     

芥子油苷在植物-生物环境关系中的作用

芥子油苷是一类含氮、含硫的植物次生代谢物质,主要分布于白花菜目的十字花科植物。芥子油苷及其降解产物具有多种生化活性,近年来人们更多地关注芥子油苷代谢与其它物质代谢途径的相互联系以及与植物生存环境的相互作用。介绍了芥子油苷及其分布、芥子油苷-黑芥子酶系统以及由芥子油苷介导的植物对昆虫、病原体的防御作用和植物与植物之间的化感作用的研究概况。     

Genetic control of natural variation in Arabidopsis glucosinolate accumulation

Glucosinolates are biologically active secondary metabolites of the Brassicaceae and related plant families that influence plant/insect interactions. Specific glucosinolates can act as feeding deterrents or stimulants, depending upon the insect species. Hence, natural selection might favor the presence of diverse glucosinolate profiles within a given species. We determined quantitative and qualitative variation in glucosinolates in the leaves and seeds of 39 Arabidopsis ecotypes. We identified 34 different glucosinolates, of which the majority are chain-elongated compounds derived from methionine. Polymorphism at only five loci was sufficient to generate 14 qualitatitvely different leaf glucosinolate profiles. Thus, there appears to be a modular genetic system regulating glucosinolate profiles in Arabidopsis. This system allows the rapid generation of new glucosinolate combinations in response to changing herbivory or other selective pressures. In addition to the qualitative variation in glucosinolate profiles, we found a nearly 20-fold difference in the quantity of total aliphatic glucosinolates and were able to identify a single locus that controls nearly three-quarters of this variation.     

How novel are the chemical weapons of garlic mustard in North American forest understories?

The Novel Weapons Hypothesis predicts that invasive plants excel in their new ranges because they produce novel metabolites to which native species possess little resistance. We examined the novelty of the phytochemistry of the Eurasian invader, Alliaria petiolata, in North America by comparing its phytochemical profile with those of closely related Brassicaceae native to North America. We examined the profile and/or concentrations of glucosinolates, alliarinoside, flavonoids, cyanide, and trypsin inhibitors in cauline leaves of field-collected A. petiolata, Arabis laevigata, Cardamine concatenata, C. bulbosa, and C. douglassii. Cyanide and the glucosinolates and flavonoids produced by A. petiolata were detected only in A. petiolata. Trypsin inhibitor activity was highest in A. laevigata, intermediate in the Cardamine species, and lowest in A. petiolata. The phytochemical profile of A. petiolata was distinct from those of four closely related and/or abundant Brassicaceaeous species native to North America, providing support for the Novel Weapons Hypothesis.     

The effects of nutrients and secondary compounds of Coffea arabica on the behavior and development of Coccus viridis

Phytochemicals may modify the food quality, reduce a plant's palatability to insects, or defend against pests. This work aimed to study 1) relationships between the nitrogen and potassium levels given to plants in nutritive solutions and the foliar phytochemical concentrations, 2) the effect of nutrients and secondary compounds of Coffea arabica on the behavior of Coccus viridis, and 3) tolerance of C. arabica to losses. Deficient, normal, and excessive nitrogen and potassium fertilization treatments were used. Each treatment had two plants (one infested and one noninfested plant). The contents of phytochemicals in the infested plants' leaves and their dry matter of roots, stems, and leaves as well as the total contents in noninfested plants, were determined. The adults and nymphs of C. viridis were counted for 60 d in all treatments. It was verified that elevated nitrogen and potassium levels in the nutritional solutions led to increased of nymphs and adults of C. viridis to the coffee plants over time. Potassium and nitrogen had both direct and indirect effects on C. viridis. The direct effect was because of the increase of the nitrogen content in the leaves. The indirect effect instead was because of reductions in the caffeine and chlorogenic acid contents in the leaves. This is the first study to show relationship nutrient levels of coffee phytochemicals in response to herbivory by scale insects. Caffeine and chlorogenic acid applied on coffee leaves stimulated the locomotory activity of the green scale, thus reducing their feeding compared with untreated leaves. The elevation of caffeine and chlorogenic acid levels in coffee leaves affect this generalist insect by stimulating the locomotion of crawlers.     

Synergistic effects of floral phytochemicals against a bumble bee parasite

Floral landscapes comprise diverse phytochemical combinations. Individual phytochemicals in floral nectar and pollen can reduce infection in bees and directly inhibit trypanosome parasites. However, gut parasites of generalist pollinators, which consume nectar and pollen from many plant species, are exposed to phytochemical combinations. Interactions between phytochemicals could augment or decrease effects of single compounds on parasites. Using a matrix of 36 phytochemical treatment combinations, we assessed the combined effects of two floral phytochemicals, eugenol and thymol, against four strains of the bumblebee gut trypanosome Crithidia bombi. Eugenol and thymol had synergistic effects against C. bombi growth across seven independent experiments, showing that the phytochemical combination can disproportionately inhibit parasites. The strength of synergistic effects varied across strains and experiments. Thus, the antiparasitic effects of individual compounds will depend on both the presence of other phytochemicals and parasite strain identity. The presence of synergistic phytochemical combinations could augment the antiparasitic activity of individual compounds for pollinators in diverse floral landscapes.