Cyanogenic glycosides: synthesis, physiology, and phenotypic plasticity

Accumulation Pattern of Amygdalin and Prunasin and Its Correlation with Fruit and Kernel Agronomic Characteristics during Apricot (Prunus armeniaca L.) Kernel Development

To reveal the accumulation pattern of cyanogenic glycosides (amygdalin and prunasin) in bitter apricot kernels to further understand the metabolic mechanisms underlying differential accumulation during kernel development and ripening and explore the association between cyanogenic glycoside accumulation and the physical, chemical and biochemical indexes of fruits and kernels during fruit and kernel development, dynamic changes in physical characteristics (weight, moisture content, linear dimensions, derived parameters) and chemical and biochemical parameters (oil, amygdalin and prunasin contents, β-glucosidase activity) of fruits and kernels from ten apricot (Prunus armeniaca L.) cultivars were systematically studied at 10 day intervals, from 20 days after flowering (DAF) until maturity. High variability in most of physical, chemical and biochemical parameters was found among the evaluated apricot cultivars and at different ripening stages. Kernel oil accumulation showed similar sigmoid patterns. Amygdalin and prunasin levels were undetectable in the sweet kernel cultivars throughout kernel development. During the early stages of apricot fruit development (before 50 DAF), the prunasin level in bitter kernels first increased, then decreased markedly; while the amygdalin level was present in quite small amounts and significantly lower than the prunasin level. From 50 to 70 DAF, prunasin further declined to zero; while amygdalin increased linearly and was significantly higher than the prunasin level, then decreased or increased slowly until full maturity. The cyanogenic glycoside accumulation pattern indicated a shift from a prunasin-dominated to an amygdalin-dominated state during bitter apricot kernel development and ripening. β-glucosidase catabolic enzyme activity was high during kernel development and ripening in all tested apricot cultivars, indicating that β-glucosidase was not important for amygdalin accumulation. Correlation analysis showed a positive correlation of kernel amygdalin content with fruit dimension parameters, kernel oil content and β-glucosidase activity, but no or a weak positive correlation with kernel dimension parameters. Principal component analysis (PCA) showed that the variance accumulation contribution rate of the first three principal components totaled 84.56%, and not only revealed differences in amygdalin and prunasin contents and β-glucosidase activity among cultivars, but also distinguished different developmental stages. The results can help us understand the metabolic mechanisms underlying differential cyanogenic glycoside accumulation in apricot kernels and provide a useful reference for breeding high- or low-amygdalin-content apricot cultivars and the agronomic management, intensive processing and exploitation of bitter apricot kernels.

High sink strength prevents photosynthetic down-regulation in cassava grown at elevated CO2 concentration

Cassava has the potential to alleviate food insecurity in many tropical regions, yet few breeding efforts to increase yield have been made. Improved photosynthetic efficiency in cassava has the potential to increase yields, but cassava roots must have sufficient sink strength to prevent carbohydrates from accumulating in leaf tissue and suppressing photosynthesis. Here, we grew eight farmer-preferred African cassava cultivars under free-air CO2 enrichment (FACE) to evaluate the sink strength of cassava roots when photosynthesis increases due to elevated CO2 concentrations ([CO2]). Relative to the ambient treatments, elevated [CO2] treatments increased fresh (+27%) and dry (+37%) root biomass, which was driven by an increase in photosynthesis (+31%) and the absence of photosynthetic down-regulation over the growing season. Moreover, intrinsic water use efficiency improved under elevated [CO2] conditions, while leaf protein content and leaf and root cyanide concentrations were not affected. Overall, these results suggest that higher cassava yields can be expected as atmospheric [CO2] increases over the coming decades. However, there were cultivar differences in the partitioning of resources to roots versus above-grown biomass; thus, the particular responses of each cultivar must be considered when selecting candidates for improvement.

Large-scale genome-wide association study, using historical data, identifies conserved genetic architecture of cyanogenic glucoside content in cassava (Manihot esculenta Crantz) root

Manihot esculenta (cassava) is a root crop originating from South America that is a major staple in the tropics, including in marginal environments. This study focused on South American and African germplasm and investigated the genetic architecture of hydrogen cyanide (HCN), a major component of root quality. HCN, representing total cyanogenic glucosides, is a plant defense component against herbivory but is also toxic for human consumption. We genotyped 3354 landraces and modern breeding lines originating from 26 Brazilian states and 1389 individuals were phenotypically characterized across multi-year trials for HCN. All plant material was subjected to high-density genotyping using genotyping by sequencing. We performed genome-wide association mapping to characterize the genetic architecture and gene mapping of HCN. Field experiments revealed strong broad- and narrow-sense trait heritability (0.82 and 0.41, respectively). Two major loci were identified, encoding for an ATPase and a MATE protein, and contributing up to 7 and 30% of the HCN concentration in roots, respectively. We developed diagnostic markers for breeding applications, validated trait architecture consistency in African germplasm and investigated further evidence for the domestication of sweet and bitter cassava. Fine genomic characterization revealed: (i) the major role played by vacuolar transporters in regulating HCN content; (ii) the co-domestication of sweet and bitter cassava major alleles are dependent upon geographical zone; and (iii) the major loci allele for high HCN in M. esculenta Crantz seems to originate from its ancestor, M. esculenta subsp. flabellifolia. Taken together, these findings expand our insights into cyanogenic glucosides in cassava roots and its glycosylated derivatives in plants.

Chemical Diversity of Plant Cyanogenic Glycosides: An Overview of Reported Natural Products

Cyanogenic glycosides are an important and widespread class of plant natural products, which are however structurally less diverse than many other classes of natural products. So far, 112 naturally occurring cyanogenic glycosides have been described in the phytochemical literature. Currently, these unique compounds have been reported from more than 2500 plant species. Natural cyanogenic glycosides show variations regarding both the aglycone and the sugar part of the molecules. The predominant sugar moiety is glucose but many substitution patterns of this glucose moiety exist in nature. Regarding the aglycone moiety, four different basic classes can be distinguished, aliphatic, cyclic, aromatic, and heterocyclic aglycones. Our overview covers all cyanogenic glycosides isolated from plants and includes 33 compounds with a non-cyclic aglycone, 20 cyclopentane derivatives, 55 natural products with an aromatic aglycone, and four dihydropyridone derivatives. In the following sections, we will provide an overview about the chemical diversity known so far and mention the first source from which the respective compounds had been isolated. This review will serve as a first reference for researchers trying to find new cyanogenic glycosides and highlights some gaps in the knowledge about the exact structures of already described compounds.

Planning of Urban Green Spaces: An Ecological Perspective on Human Benefits

In the context of urban land-use growth and the consequent impacts on the environment, green spaces provide ecosystem services for human health. The ecosystem services concept synthesises human–environmental interactions through a series of combined components of biodiversity and abiotic elements, linking ecological processes and functions. The concept of green infrastructure (GI) in the urban context emphasises the quality and quantity of urban and peri-urban green spaces and natural areas. In dense urban contexts, the applications of GI are limited and not applied to the potential urban spaces such as roofs and gardens. Often, roofs are characterised by impermeable paved surfaces with negative effects on human well-being, whereas garden designs do not consider social needs and environmental interactions. The role of urban stressors or the urban context as a driving force or pressure of urban green space is not always well understood and employed in the planning of green spaces. This is partly due to a knowledge gap between different science disciplines that operate on different scales, from single processes of the plants (which focus on plant responses to environmental stresses affecting human well-being) to urban ecosystems (which focus on the biodiversity and urban space planning–human well-being relationship). This can create a paradox, as green spaces that are not adequately designed might not produce the expected effects. In this paper, an overview of benefits and limitations of applying the ecosystem services approach when designing green spaces is presented. The focus is on the main urban ecosystem services provided by green roofs and community gardens such as GI that can represent strategies to provide ecological and social multifunctionality to waterproofed surfaces connected to the buildings and low-exploited gardens being the main areas that affect dense urban settlements, and thus, increasing the ecosystem services in the urban environment, such as reducing the Urban Heat Island, as well as flooding events. Specifically, the paper highlights (i) feedback between ecological processes and functions that support ecosystem services, (ii) urban environmental stresses in relation to disservices that these can create for human well-being and (iii) key issues that should be considered in the planning and design of urban ecosystem services. Such a new vision of urban ecosystem services highlights the need to look at GI as an active part of the urban space design in the built environment.

Identification of the Tyrosine- and Phenylalanine-Derived Soluble Metabolomes of Sorghum

The synthesis of small organic molecules, known as specialized or secondary metabolites, is one mechanism by which plants resist and tolerate biotic and abiotic stress. Many specialized metabolites are derived from the aromatic amino acids phenylalanine (Phe) and tyrosine (Tyr). In addition, the improved characterization of compounds derived from these amino acids could inform strategies for developing crops with greater resilience and improved traits for the biorefinery. Sorghum and other grasses possess phenylalanine ammonia-lyase (PAL) enzymes that generate cinnamic acid from Phe and bifunctional phenylalanine/tyrosine ammonia-lyase (PTAL) enzymes that generate cinnamic acid and p-coumaric acid from Phe and Tyr, respectively. Cinnamic acid can, in turn, be converted into p-coumaric acid by cinnamate 4-hydroxylase. Thus, Phe and Tyr are both precursors of common downstream products. Not all derivatives of Phe and Tyr are shared, however, and each can act as a precursor for unique metabolites. In this study, ¹³C isotopic-labeled precursors and the recently developed Precursor of Origin Determination in Untargeted Metabolomics (PODIUM) mass spectrometry (MS) analytical pipeline were used to identify over 600 MS features derived from Phe and Tyr in sorghum. These features comprised 20% of the MS signal collected by reverse-phase chromatography and detected through negative-ionization. Ninety percent of the labeled mass features were derived from both Phe and Tyr, although the proportional contribution of each precursor varied. In addition, the relative incorporation of Phe and Tyr varied between metabolites and tissues, suggesting the existence of multiple pools of p-coumaric acid that are fed by the two amino acids. Furthermore, Phe incorporation was greater for many known hydroxycinnamate esters and flavonoid glycosides. In contrast, mass features derived exclusively from Tyr were the most abundant in every tissue. The Phe- and Tyr-derived metabolite library was also utilized to retrospectively annotate soluble MS features in two brown midrib mutants (bmr6 and bmr12) identifying several MS features that change significantly in each mutant.

Allocation of Resources to Cyanogenic Glucosides Does Not Incur a Growth Sacrifice in Sorghum bicolor (L.) Moench

In plants, the production of secondary metabolites is considered to be at the expense of primary growth. Sorghum produces a cyanogenic glycoside (dhurrin) that is believed to act as its chemical defence. Studies have shown that acyanogenic plants are smaller in size compared to the wildtype. This study aimed to investigate whether the small plant size is due to delayed germination or due to the lack of dhurrin derived nitrogen. A novel plant system consisting of totally cyanide deficient class 1 (tcd1) and adult cyanide deficient 1 (acdc1) mutant lines was employed. The data for germination, plant height and developmental stage during seedling development and final plant reproductive fitness was recorded. The possible role of phytohormones in recovering the wildtype phenotype, especially in developmentally acyanogenic acdc1 line, was also investigated. The data on plant growth have shown that the lack of dhurrin is disadvantageous to growth, but only at the early developmental stage. The tcd1 plants also took longer to mature probably due to delayed flowering. None of the tested hormones were able to recover the wildtype phenotype. We conclude that the generation of dhurrin is advantageous for plant growth, especially at critical growth stages like germinating seed by providing a ready source of reduced nitrogen.

Development and validation of eight cyanogenic glucosides via ultra-high-performance liquid chromatography-tandem mass spectrometry in agri-food

An ultra-high-performance liquid chromatography-triple quadrupole tandem mass spectrometry (UHPLC-QqQ-MS/MS) method was established and validated for the simultaneous quantification of eight cyanogenic glucosides (CNGs) in agri-food. The eight CNGs were linamarin, lotaustralin, linustatin, neolinustatin, taxiphyllin, amygdalin, dhurrin and prunasin. CNGs were extracted with aqueous methanol and cleaned via solid-phase extraction. Analytes were separated with a C₁₈ column via gradient elution. MS/MS analysis was performed with electrospray ionisation in positive mode. Quantification was performed in multiple reaction monitoring mode. Satisfactory validation results were obtained in terms of linearity, sensitivity, precision and accuracy, matrix effect and stability. The method was applied in typical cyanogenic agri-food. CNGs in cassava, linseed, bamboo, sorghum, apricot, almond and lima bean were analyzed.

Prediction of Dhurrin Metabolism by Transcriptome and Metabolome Analyses in Sorghum

Sorghum (Sorghum bicolor (L.)) Moench is an important food for humans and feed for livestock. Sorghum contains dhurrin which can be degraded into toxic hydrogen cyanide. Here, we report the expression patterns of 14 candidate genes related to dhurrin ((S)-4-Hydroxymandelnitrile-β-D-glucopyranoside) metabolism and the effects of the gene expression on specific metabolite content in selected sorghum accessions. Dhurrin-related metabolism is vigorous in the early stages of development of sorghum. The dhurrin contents of most accessions tested were in the range of approximately 6-22 μg mg-1 fresh leaf tissue throughout growth. The p-hydroxybenzaldehyde (pHB) contents were high at seedling stages, but almost nonexistent at adult stages. The contents of p-hydroxyphenylacetic acid (pHPAAc) were relatively low throughout growth compared to those of dhurrin or pHB. Generally, the expression of the candidate genes was higher at seedling stage than at other stages and decreased gradually as plants grew. In addition, we identified significant SNPs, and six of them were potentially associated with non-synonymous changes in CAS1. Our results may provide the basis for choosing breeding materials to regulate cyanide contents in sorghum varieties to prevent HCN toxicity of livestock or to promote drought tolerance or pathogen resistance.

A flavin-dependent monooxygenase catalyzes the initial step in cyanogenic glycoside synthesis in ferns

Cyanogenic glycosides form part of a binary plant defense system that, upon catabolism, detonates a toxic hydrogen cyanide bomb. In seed plants, the initial step of cyanogenic glycoside biosynthesis-the conversion of an amino acid to the corresponding aldoxime-is catalyzed by a cytochrome P450 from the CYP79 family. An evolutionary conundrum arises, as no CYP79s have been identified in ferns, despite cyanogenic glycoside occurrence in several fern species. Here, we report that a flavin-dependent monooxygenase (fern oxime synthase; FOS1), catalyzes the first step of cyanogenic glycoside biosynthesis in two fern species (Phlebodium aureum and Pteridium aquilinum), demonstrating convergent evolution of biosynthesis across the plant kingdom. The FOS1 sequence from the two species is near identical (98%), despite diversifying 140 MYA. Recombinant FOS1 was isolated as a catalytic active dimer, and in planta, catalyzes formation of an N-hydroxylated primary amino acid; a class of metabolite not previously observed in plants.

Stone Fruit as Biofactories of Phytochemicals With Potential Roles in Human Nutrition and Health

Phytochemicals or secondary metabolites present in fruit are key components contributing to sensory attributes like aroma, taste, and color. In addition, these compounds improve human nutrition and health. Stone fruits are an important source of an array of secondary metabolites that may reduce the risk of different diseases. The first part of this review is dedicated to the description of the main secondary organic compounds found in plants which include (a) phenolic compounds, (b) terpenoids/isoprenoids, and (c) nitrogen or sulfur containing compounds, and their principal biosynthetic pathways and their regulation in stone fruit. Then, the type and levels of bioactive compounds in different stone fruits of the Rosaceae family such as peach (Prunus persica), plum (P. domestica, P. salicina and P. cerasifera), sweet cherries (P. avium), almond kernels (P. dulcis, syn. P. amygdalus), and apricot (P. armeniaca) are presented. The last part of this review encompasses pre- and postharvest treatments affecting the phytochemical composition in stone fruit. Appropriate management of these factors during pre- and postharvest handling, along with further characterization of phytochemicals and the regulation of their synthesis in different cultivars, could help to increase the levels of these compounds, leading to the future improvement of stone fruit not only to enhance organoleptic characteristics but also to benefit human health.

Unique and highly specific cyanogenic glycoside localization in stigmatic cells and pollen in the genus Lomatia (Proteaceae)

BACKGROUND AND AIMS

Floral chemical defence strategies remain understudied despite the significance of flowers to plant fitness, and the fact that many flowers contain secondary metabolites that confer resistance to herbivores. Optimal defence and apparency theories predict that the most apparent plant parts and/or those most important to fitness should be most defended. To test whether within-flower distributions of chemical defence are consistent with these theories we used cyanogenic glycosides (CNglycs), which are constitutive defence metabolites that deter herbivores by releasing hydrogen cyanide upon hydrolysis.

METHODS

We used cyanogenic florets of the genus Lomatia to investigate at what scale there may be strategic allocation of CNglycs in flowers, what their localization reveals about function, and whether levels of floral CNglycs differ between eight congeneric species across a climatic gradient. Within-flower distributions of CNglycs during development were quantified, CNglycs were identified and their localization was visualized in cryosectioned florets using matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI).

KEY RESULTS

Florets of all congeneric species studied were cyanogenic, and concentrations differed between species. Within florets there was substantial variation in CNglyc concentrations, with extremely high concentrations (up to 14.6 mg CN g-1 d. wt) in pollen and loose, specialized surface cells on the pollen presenter, among the highest concentrations reported in plant tissues. Two tyrosine-derived CNglycs, the monoglycoside dhurrin and diglycoside proteacin, were identified. MALDI-MSI revealed their varying ratios in different floral tissues; proteacin was primarily localized to anthers and ovules, and dhurrin to specialized cells on the pollen presenter. The mix of transient specialized cells and pollen of L. fraxinifolia was ~11 % dhurrin and ~1.1 % proteacin by mass.

CONCLUSIONS

Tissue-specific distributions of two CNglycs and substantial variation in their concentrations within florets suggests their allocation is under strong selection. Localized, high CNglyc concentrations in transient cells challenge the predictions of defence theories, and highlight the importance of fine-scale metabolite visualization, and the need for further investigation into the ecological and metabolic roles of CNglycs in floral tissues.

Biosynthesis of cyanogenic glucosides in Phaseolus lunatus and the evolution of oxime-based defenses

Lima bean, Phaseolus lunatus, is a crop legume that produces the cyanogenic glucosides linamarin and lotaustralin. In the legumes Lotus japonicus and Trifolium repens, the biosynthesis of these two α-hydroxynitrile glucosides involves cytochrome P450 enzymes of the CYP79 and CYP736 families and a UDP-glucosyltransferase. Here, we identify CYP79D71 as the first enzyme of the pathway in P. lunatus, producing oximes from valine and isoleucine. A second CYP79 family member, CYP79D72, was shown to catalyze the formation of leucine-derived oximes, which act as volatile defense compounds in Phaseolus spp. The organization of the biosynthetic genes for cyanogenic glucosides in a gene cluster aided their identification in L. japonicus. In the available genome sequence of P. vulgaris, the gene orthologous to CYP79D71 is adjacent to a member of the CYP83 family. Although P. vulgaris is not cyanogenic, it does produce oximes as volatile defense compounds. We cloned the genes encoding two CYP83s (CYP83E46 and CYP83E47) and a UDP-glucosyltransferase (UGT85K31) from P. lunatus, and these genes combined form a complete biosynthetic pathway for linamarin and lotaustralin in Lima bean. Within the genus Phaseolus, the occurrence of linamarin and lotaustralin as functional chemical defense compounds appears restricted to species belonging to the closely related Polystachios and Lunatus groups. A preexisting ability to produce volatile oximes and nitriles likely facilitated evolution of cyanogenesis within the Phaseolus genus.

Thiosulfinate Tolerance Is a Virulence Strategy of an Atypical Bacterial Pathogen of Onion

Unlike most characterized bacterial plant pathogens, the broad-host-range plant pathogen Pantoea ananatis lacks both the virulence-associated type III and type II secretion systems. In the absence of these typical pathogenicity factors, P. ananatis induces necrotic symptoms and extensive cell death in onion tissue dependent on the HiVir proposed secondary metabolite synthesis gene cluster. Onion (Allium. cepa L), garlic (A. sativum L.), and other members of the Allium genus produce volatile antimicrobial thiosulfinates upon cellular damage. However, the roles of endogenous thiosulfinate production in host-bacterial pathogen interactions have not been described. We found a strong correlation between the genetic requirements for P. ananatis to colonize necrotized onion tissue and its capacity for tolerance to the thiosulfinate "allicin" based on the presence of an eleven-gene, plasmid-borne, virulence cluster of sulfur redox genes. We have designated them "alt" genes for allicin tolerance. We show that allicin and onion thiosulfinates restrict bacterial growth with similar kinetics. The alt gene cluster is sufficient to confer allicin tolerance and protects the glutathione pool during allicin treatment. Independent alt genes make partial phenotypic contributions indicating that they function as a collective cohort to manage thiol stress. Our work implicates endogenous onion thiosulfinates produced during cellular damage as major mediators of interactions with bacteria. The P. ananatis-onion pathosystem can be modeled as a chemical arms race of pathogen attack, host chemical counterattack, and pathogen defense.

Metabolic profiles of six African cultivars of cassava (Manihot esculenta Crantz) highlight bottlenecks of root yield

Cassava is an important staple crop in sub-Saharan Africa, due to its high productivity even on nutrient poor soils. The metabolic characteristics underlying this high productivity are poorly understood including the mode of photosynthesis, reasons for the high rate of photosynthesis, the extent of source/sink limitation, the impact of environment, and the extent of variation between cultivars. Six commercial African cassava cultivars were grown in a greenhouse in Erlangen, Germany, and in the field in Ibadan, Nigeria. Source leaves, sink leaves, stems and storage roots were harvested during storage root bulking and analyzed for sugars, organic acids, amino acids, phosphorylated intermediates, minerals, starch, protein, activities of enzymes in central metabolism and yield traits. High ratios of RuBisCO:phosphoenolpyruvate carboxylase activity support a C₃ mode of photosynthesis. The high rate of photosynthesis is likely to be attributed to high activities of enzymes in the Calvin-Benson cycle and pathways for sucrose and starch synthesis. Nevertheless, source limitation is indicated because root yield traits correlated with metabolic traits in leaves rather than in the stem or storage roots. This situation was especially so in greenhouse-grown plants, where irradiance will have been low. In the field, plants produced more storage roots. This was associated with higher AGPase activity and lower sucrose in the roots, indicating that feedforward loops enhanced sink capacity in the high light and low nitrogen environment in the field. Overall, these results indicated that carbon assimilation rate, the K battery, root starch synthesis, trehalose, and chlorogenic acid accumulation are potential target traits for genetic improvement.

Overexpressed β-cyanoalanine synthase functions with alternative oxidase to improve tobacco resistance to salt stress by alleviating oxidative damage

β-Cyanoalanine synthase (β-CAS) is an enzyme involved in cyanide detoxification. However, little information is available regarding the effects of β-CAS activity changes on plant resistance to environmental stress. Here, we found that β-CAS overexpression (CAS-OE) improves the resistance of tobacco plants to salt stress, whereas plants with β-CAS silencing suffer more oxidative damage than wild-type plants. Notably, blocking respiration by the alternative oxidase (AOX) pathway significantly aggravates stress injury and impairs the salt stress tolerance mediated by CAS-OE. These findings present novel insights into the synergistic effect between β-CAS and AOX in protecting plants from salt stress, where β-CAS plays a vital role in restraining cyanide accumulation, and AOX helps to alleviate the toxic effect of cyanide.

The domestication syndrome in vegetatively propagated field crops

BACKGROUND

Vegetatively propagated crops are globally significant in terms of current agricultural production, as well as for understanding the long-term history of early agriculture and plant domestication. Today, significant field crops include sugarcane (Saccharum officinarum), potato (Solanum tuberosum), manioc (Manihot esculenta), bananas and plantains (Musa cvs), sweet potato (Ipomoea batatas), yams (Dioscorea spp.) and taro (Colocasia esculenta). In comparison with sexually reproduced crops, especially cereals and legumes, the domestication syndrome in vegetatively propagated field crops is poorly defined.

AIMS AND SCOPE

Here, a range of phenotypic traits potentially comprising a syndrome associated with early domestication of vegetatively propagated field crops is proposed, including: mode of reproduction, yield of edible portion, ease of harvesting, defensive adaptations, timing of production and plant architecture. The archaeobotanical visibility of these syndrome traits is considered with a view to the reconstruction of the geographical and historical pathways of domestication for vegetatively propagated field crops in the past.

CONCLUSIONS

Although convergent phenotypic traits are identified, none of them are ubiquitous and some are divergent. In contrast to cereals and legumes, several traits seem to represent varying degrees of plastic response to growth environment and practices of cultivation, as opposed to solely morphogenetic 'fixation'.

Biochemical study of the effect of stress conditions on the mandelonitrile-associated salicylic acid biosynthesis in peach

Salicylic acid (SA) plays a central role in plant responses to environmental stresses. In a recent study, we suggested a third pathway for SA biosynthesis from mandelonitrile (MD) in peach plants. This pathway is an alternative to the phenylalanine ammonia-lyase pathway and links SA biosynthesis and cyanogenesis. In the present work, using biochemical approaches, we studied the effect of salt stress and Plum pox virus (PPV) infection on this proposed SA biosynthetic pathway from MD. Peach plants were submitted to salt stress and Plum pox virus (PPV) infection. We studied the levels of SA and its intermediates/precursors (phenylalanine, MD, amygdalin and benzoic acid) in in vitro shoots. Moreover, in peach seedlings, we analysed the content of H₂ O₂ -related enzymes, SA and the stress-related hormones abscisic acid and jasmonic acid. We showed that the contribution of this SA biosynthetic pathway from MD to the total SA pool does not seem to be important under the stress conditions assayed. Nevertheless, MD treatment not only affected the SA content, but also had a pleiotropic effect on abscisic acid and jasmonic acid levels. Furthermore, MD modulates the antioxidative metabolism via SA-dependent or -independent redox-related signalling pathways. Even though the proposed SA biosynthetic pathway seems to be functional under stress conditions, MD, and hence cyanogenic glycosides, may be operating more broadly than by influencing SA pathways and signalling. Thus, the physiological function of the proposed SA biosynthetic pathway remains to be elucidated.

Stabilization of dhurrin biosynthetic enzymes from Sorghum bicolor using a natural deep eutectic solvent

In recent years, ionic liquids and deep eutectic solvents (DESs) have gained increasing attention due to their ability to extract and solubilize metabolites and biopolymers in quantities far beyond their solubility in oil and water. The hypothesis that naturally occurring metabolites are able to form a natural deep eutectic solvent (NADES), thereby constituting a third intracellular phase in addition to the aqueous and lipid phases, has prompted researchers to study the role of NADES in living systems. As an excellent solvent for specialized metabolites, formation of NADES in response to dehydration of plant cells could provide an appropriate environment for the functional storage of enzymes during drought. Using the enzymes catalyzing the biosynthesis of the defense compound dhurrin as an experimental model system, we demonstrate that enzymes involved in this pathway exhibit increased stability in NADES compared with aqueous buffer solutions, and that enzyme activity is restored upon rehydration. Inspired by nature, application of NADES provides a biotechnological approach for long-term storage of entire biosynthetic pathways including membrane-anchored enzymes.

Albanitriles A-G: Antiprotozoal Polyacetylene Nitriles from a Mycale Marine Sponge

Seven new nitrile-bearing polyacetylenes, named albanitriles A-G, were isolated from a marine sponge of the Mycale genus (Order: Poecilosclerida, Family: Mycalidae) collected near Albany, Western Australia. Structural elucidation was achieved using a combination of high-resolution mass spectrometry and ultraviolet/visible, infrared, and nuclear magnetic resonance spectroscopy. The compounds were found to possess moderate activity against Giardia duodenalis when compared to a metronidazole positive control.

The Sweet Side of Plant-Specialized Metabolism

Glycosylation plays a major role in the structural diversification of plant natural products. It influences the properties of molecules by modifying the reactivity and solubility of the corresponding aglycones, so influencing cellular localization and bioactivity. Glycosylation of plant natural products is usually carried out by uridine diphosphate(UDP)-dependent glycosyltransferases (UGTs) belonging to the carbohydrate-active enzyme glycosyltransferase 1 (GT1) family. These enzymes transfer sugars from UDP-activated sugar moieties to small hydrophobic acceptor molecules. Plant GT1s generally show high specificity for their sugar donors and recognize a single UDP sugar as their substrate. In contrast, they are generally promiscuous with regard to acceptors, making them attractive biotechnological tools for small molecule glycodiversification. Although microbial hosts have traditionally been used for heterologous engineering of plant-derived glycosides, transient plant expression technology offers a potentially disruptive platform for rapid characterization of new plant glycosyltransferases and biosynthesis of complex glycosides.

The Interplay Between Water Limitation, Dhurrin, and Nitrate in the Low-Cyanogenic Sorghum Mutant adult cyanide deficient class 1

Sorghum bicolor (L.) Moench produces the nitrogen-containing natural product dhurrin that provides chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide gas. Drought can increase dhurrin in shoot tissues to concentrations toxic to livestock. As dhurrin is also a remobilizable store of reduced nitrogen and plays a role in stress mitigation, reductions in dhurrin may come at a cost to plant growth and stress tolerance. Here, we investigated the response to an extended period of water limitation in a unique EMS-mutant adult cyanide deficient class 1 (acdc1) that has a low dhurrin content in the leaves of mature plants. A mutant sibling line was included to assess the impact of unknown background mutations. Plants were grown under three watering regimes using a gravimetric platform, with growth parameters and dhurrin and nitrate concentrations assessed over four successive harvests. Tissue type was an important determinant of dhurrin and nitrate concentrations, with the response to water limitation differing between above and below ground tissues. Water limitation increased dhurrin concentration in the acdc1 shoots to the same extent as in wild-type plants and no growth advantage or disadvantage between the lines was observed. Lower dhurrin concentrations in the acdc1 leaf tissue when fully watered correlated with an increase in nitrate content in the shoot and roots of the mutant. In targeted breeding efforts to down-regulate dhurrin concentration, parallel effects on the level of stored nitrates should be considered in all vegetative tissues of this important forage crop to avoid potential toxic effects.

Divergent responses of above- and below-ground chemical defence to nitrogen and phosphorus supply in waratahs (Telopea speciosissima)

Plant nutrition can affect the allocation of resources to plant chemical defences, yet little is known about how phosphorus (P) supply, and relative nitrogen (N) and P supply, affect chemical defences, especially in species with intrinsically conservative nutrient use adapted to P-impoverished soils. Waratah (Telopea speciosissima (Sm.) R.Br.), like other Proteaceae, is adapted nutrient-poor soils. It was identified as having cyanogenic glycosides (CNglycs) throughout the plant. T. speciosissima seedlings were grown for 15 weeks under two N and P concentrations. CNglycs (N-based defence) and nutrients were quantified in above- and below-ground organs; foliar carbon (C)-based phenolics and tannins were also quantified. CNglyc concentrations in roots were on average 51-fold higher than in above-ground tissues and were affected by both N and P supply, whereas foliar CNglyc concentrations only responded to N supply. Leaves had high concentrations of C-based defences, which increased under low N, and were not correlated with N-based defences. Greater root chemical defence against herbivores and pathogens may be important in a non-mycorrhizal species that relies on basal resprouting following disturbance. The differing responses of secondary chemistry in above- and below-ground organs to P and N demonstrate the importance of broadening the predominantly foliar focus of plant defence studies.

Highly sensitive and straightforward methods for the detection of cyanide using profluorescent glutathionylcobalamin

The extreme toxicity of cyanide and its continued use in various industries have raised concerns over environmental contamination and, therefore, considerable attention has given to develop facile and sensitive methods of cyanide detection. In this study, we developed highly sensitive and straightforward methods of cyanide detection using eosin-labeled glutathionylcobalamin (E-GSCbl) containing fluorescent eosin-labeled glutathione (E-GSH) as the upper axial ligand to the cobalt. E-GSH fluorescence was strongly quenched in E-GSCbl. The E-GSH ligand of E-GSCbl was replaced specifically by cyanide, showing recovery of the E-GSH fluorescence. This profluorescent property of E-GSCbl enabled detection of cyanide in aqueous solutions, yielding a lower detection limit of 10 nM (0.26 μg L^-1). Moreover E-GSH exhibited strong luminescence under UV-light that was quenched in E-GSCbl, and this allowed naked-eye detection of cyanide at concentrations as low as 100 nM. This study demonstrates that profluorescent E-GSCbl is a highly sensitive cyanide chemosensor that can detect nanomolar concentrations of cyanide.

Down-Regulation of CYP79A1 Gene Through Antisense Approach Reduced the Cyanogenic Glycoside Dhurrin in [Sorghum bicolor (L.) Moench] to Improve Fodder Quality

A major limitation for the utilization of sorghum forage is the production of the cyanogenic glycoside dhurrin in its leaves and stem that may cause the death of cattle feeding on it at the pre-flowering stage. Therefore, we attempted to develop transgenic sorghum plants with reduced levels of hydrogen cyanide (HCN) by antisense mediated down-regulation of the expression of cytochrome P450 CYP79A1, the key enzyme of the dhurrin biosynthesis pathway. CYP79A1 cDNA was isolated and cloned in antisense orientation, driven by rice Act1 promoter. Shoot meristem explants of sorghum cultivar CSV 15 were transformed by the particle bombardment method and 27 transgenics showing the integration of transgene were developed. The biochemical assay for HCN in the transgenic sorghum plants confirmed significantly reduced HCN levels in transgenic plants and their progenies. The HCN content in the transgenics varied from 5.1 to 149.8 μg/g compared to 192.08 μg/g in the non-transformed control on dry weight basis. Progenies with reduced HCN content were advanced after each generation till T3. In T3 generation, progenies of two promising events were tested which produced highly reduced levels of HCN (mean of 62.9 and 76.2 μg/g, against the control mean of 221.4 μg/g). The reduction in the HCN levels of transgenics confirmed the usefulness of this approach for reducing HCN levels in forage sorghum plants. The study effectively demonstrated that the antisense CYP79A1 gene deployment was effective in producing sorghum plants with lower HCN content which are safer for cattle to feed on.

Signaling by hydrogen sulfide and cyanide through post-translational modification

Two cysteine metabolism-related molecules, hydrogen sulfide and hydrogen cyanide, which are considered toxic, have now been considered as signaling molecules. Hydrogen sulfide is produced in chloroplasts through the activity of sulfite reductase and in the cytosol and mitochondria by the action of sulfide-generating enzymes, and regulates/affects essential plant processes such as plant adaptation, development, photosynthesis, autophagy, and stomatal movement, where interplay with other signaling molecules occurs. The mechanism of action of sulfide, which modifies protein cysteine thiols to form persulfides, is related to its chemical features. This post-translational modification, called persulfidation, could play a protective role for thiols against oxidative damage. Hydrogen cyanide is produced during the biosynthesis of ethylene and camalexin in non-cyanogenic plants, and is detoxified by the action of sulfur-related enzymes. Cyanide functions include the breaking of seed dormancy, modifying the plant responses to biotic stress, and inhibition of root hair elongation. The mode of action of cyanide is under investigation, although it has recently been demonstrated to perform post-translational modification of protein cysteine thiols to form thiocyanate, a process called S-cyanylation. Therefore, the signaling roles of sulfide and most probably of cyanide are performed through the modification of specific cysteine residues, altering protein functions.

Chemical convergence between plants and insects: biosynthetic origins and functions of common secondary metabolites

Despite the phylogenetic distance between plants and insects, these two groups of organisms produce some secondary metabolites in common. Identical structures belonging to chemical classes such as the simple monoterpenes and sesquiterpenes, iridoid monoterpenes, cyanogenic glycosides, benzoic acid derivatives, benzoquinones and naphthoquinones are sometimes found in both plants and insects. In addition, very similar glucohydrolases involved in activating two-component defenses, such as glucosinolates and cyanogenic glycosides, occur in both plants and insects. Although this trend was first noted many years ago, researchers have long struggled to find convincing explanations for such co-occurrence. In some cases, identical compounds may be produced by plants to interfere with their function in insects. In others, plant and insect compounds may simply have parallel functions, probably in defense or attraction, and their co-occurrence is a coincidence. The biosynthetic origin of such co-occurring metabolites may be very different in insects as compared to plants. Plants and insects may have different pathways to the same metabolite, or similar sequences of intermediates, but different enzymes. Further knowledge of the ecological roles and biosynthetic pathways of secondary metabolites may shed more light on why plants and insects produce identical substances.

Sequestration and biosynthesis of cyanogenic glucosides in passion vine butterflies and consequences for the diversification of their host plants

The colorful heliconiine butterflies are distasteful to predators due to their content of defense compounds called cyanogenic glucosides (CNglcs), which they biosynthesize from aliphatic amino acids. Heliconiine larvae feed exclusively on Passiflora plants where ~30 kinds of CNglcs have been reported. Among them, some CNglcs derived from cyclopentenyl glycine can be sequestered by some Heliconius species. In order to understand the evolution of biosynthesis and sequestration of CNglcs in these butterflies and its consequences for their arms race with Passiflora plants, we analyzed the CNglc distribution in selected heliconiine and Passiflora species. Sequestration of cyclopentenyl CNglcs is not an exclusive trait of Heliconius, since these compounds were present in other heliconiines such as Philaethria, Dryas and Agraulis, and in more distantly related genera Cethosia and Euptoieta. Thus, it is likely that the ability to sequester cyclopentenyl CNglcs arose in an ancestor of the Heliconiinae subfamily. Biosynthesis of aliphatic CNglcs is widespread in these butterflies, although some species from the sara-sapho group seem to have lost this ability. The CNglc distribution within Passiflora suggests that they might have diversified their cyanogenic profile to escape heliconiine herbivory. This systematic analysis improves our understanding on the evolution of cyanogenesis in the heliconiine-Passiflora system.

Production of the cyanogenic glycoside dhurrin in yeast

Cyanogenic glycosides are defense compounds found in a wide range of plant species, including many crops. We demonstrate that the cyanogenic glucoside dhurrin, naturally found in sorghum, can be produced at high titers in Saccharomyces cerevisiae, constituting the first report of cyanogenic glycoside production in a microbe. Genetic modifications to increase the supply of the dhurrin precursor tyrosine enabled dhurrin production in excess of 80 mg/L. The dhurrin-producing yeast strain was used as a chassis to investigate previously uncharacterized enzymes identified close to the biosynthetic gene cluster containing the dhurrin pathway enzymes. This work shows the potential of heterologous expression in yeast to facilitate investigations of plant cyanogenic glycoside pathways.

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First production of cyanogenic glycosides in a microbe.

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Strategies for optimizing production of cyanogenic glycosides.

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Platform for rapidly characterizing the enzymes which constitute cyanogenic glycoside biosynthetic pathways.

Deletion of biosynthetic genes, specific SNP patterns and differences in transcript accumulation cause variation in hydroxynitrile glucoside content in barley cultivars

Barley (Hordeum vulgare L.) produces five leucine-derived hydroxynitrile glucosides, potentially involved in alleviating pathogen and environmental stresses. These compounds include the cyanogenic glucoside epiheterodendrin. The biosynthetic genes are clustered. Total hydroxynitrile glucoside contents were previously shown to vary from zero to more than 10,000 nmoles g-1 in different barley lines. To elucidate the cause of this variation, the biosynthetic genes from the high-level producer cv. Mentor, the medium-level producer cv. Pallas, and the zero-level producer cv. Emir were investigated. In cv. Emir, a major deletion in the genome spanning most of the hydroxynitrile glucoside biosynthetic gene cluster was identified and explains the complete absence of hydroxynitrile glucosides in this cultivar. The transcript levels of the biosynthetic genes were significantly higher in the high-level producer cv. Mentor compared to the medium-level producer cv. Pallas, indicating transcriptional regulation as a contributor to the variation in hydroxynitrile glucoside levels. A correlation between distinct single nucleotide polymorphism (SNP) patterns in the biosynthetic gene cluster and the hydroxynitrile glucoside levels in 227 barley lines was identified. It is remarkable that in spite of the demonstrated presence of a multitude of SNPs and differences in transcript levels, the ratio between the five hydroxynitrile glucosides is maintained across all the analysed barley lines. This implies the involvement of a stably assembled multienzyme complex.

Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels

In 2016, the EFSA Panel on Contaminants in the Food Chain (CONTAM) published a scientific opinion on the acute health risks related to the presence of cyanogenic glycosides (CNGs) in raw apricot kernels in which an acute reference dose (ARfD) of 20 μg/kg body weight (bw) was established for cyanide (CN). In the present opinion, the CONTAM Panel concluded that this ARfD is applicable for acute effects of CN regardless the dietary source. To account for differences in cyanide bioavailability after ingestion of certain food items, specific factors were used. Estimated mean acute dietary exposures to cyanide from foods containing CNGs did not exceed the ARfD in any age group. At the 95th percentile, the ARfD was exceeded up to about 2.5-fold in some surveys for children and adolescent age groups. The main contributors to exposures were biscuits, juice or nectar and pastries and cakes that could potentially contain CNGs. Taking into account the conservatism in the exposure assessment and in derivation of the ARfD, it is unlikely that this estimated exceedance would result in adverse effects. The limited data from animal and human studies do not allow the derivation of a chronic health-based guidance value (HBGV) for cyanide, and thus, chronic risks could not be assessed.

Transcriptome Profiling and Genome-Wide Association Studies Reveal GSTs and Other Defense Genes Involved in Multiple Signaling Pathways Induced by Herbicide Safener in Grain Sorghum

Herbicide safeners protect cereal crops from herbicide injury by inducing genes and proteins involved in detoxification reactions, such as glutathione S-transferases (GSTs) and cytochrome P450s (P450s). Only a few studies have characterized gene or protein expression profiles for investigating plant responses to safener treatment in cereal crops, and most transcriptome analyses in response to safener treatments have been conducted in dicot model species that are not protected by safener from herbicide injury. In this study, three different approaches were utilized in grain sorghum (Sorghum bicolor (L.) Moench) to investigate mechanisms involved in safener-regulated signaling pathways. An initial transcriptome analysis was performed to examine global gene expression in etiolated shoot tissues of hybrid grain sorghum following treatment with the sorghum safener, fluxofenim. Most upregulated transcripts encoded detoxification enzymes, including P450s, GSTs, and UDP-dependent glucosyltransferases (UGTs). Interestingly, several of these upregulated transcripts are similar to genes involved with the biosynthesis and recycling/catabolism of dhurrin, an important chemical defense compound, in these seedling tissues. Secondly, 761 diverse sorghum inbred lines were evaluated in a genome-wide association study (GWAS) to determine key molecular-genetic factors governing safener-mediated signaling mechanisms and/or herbicide detoxification. GWAS revealed a significant single nucleotide polymorphism (SNP) associated with safener-induced response on chromosome 9, located within a phi-class SbGST gene and about 15-kb from a different phi-class SbGST. Lastly, the expression of these two candidate SbGSTs was quantified in etiolated shoot tissues of sorghum inbred BTx623 in response to fluxofenim treatment. SbGSTF1 and SbGSTF2 transcripts increased within 12-hr after fluxofenim treatment but the level of safener-induced expression differed between the two genes. In addition to identifying specific GSTs potentially involved in the safener-mediated detoxification pathway, this research elucidates a new direction for studying both constitutive and inducible mechanisms for chemical defense in cereal crop seedlings.

Comparative transcriptome analysis reveals significant differences in the regulation of gene expression between hydrogen cyanide- and ethylene-treated Arabidopsis thaliana

BACKGROUND

Hydrogen cyanide (HCN) is a small gaseous molecule that is predominantly produced as an equimolar co-product of ethylene (ET) biosynthesis in plants. The function of ET is of great concern and is well studied; however, the function of HCN is largely unknown. Similar to ET, HCN is a simple and diffusible molecule that has been shown to play a regulatory role in the control of some metabolic processes in plants. Nevertheless, it is still controversial whether HCN should be regarded as a signalling molecule, and the cross-talk between HCN and ET in gene expression regulation remains unclear. In this study, RNA sequencing (RNA-seq) was performed to compare the differentially expressed genes (DEGs) between HCN and ET in Arabidopsis. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were subsequently performed to investigate the function and pathway enrichment of DEGs. Parts of key genes were confirmed by quantitative real-time PCR.

RESULTS

The results showed that at least 1305 genes and 918 genes were significantly induced by HCN and ET, respectively. Interestingly, a total of 474 genes (|log₂ FC| ≥1) were co-regulated by HCN and ET. GO and KEGG analyses indicated that the co-regulated genes by HCN and ET were enriched in plant responses to stress and plant hormone signal transduction pathways, indicating that HCN may cooperate with ET and participate in plant growth and development and stress responses. However, a total of 831 genes were significantly induced by HCN but not by ET, indicating that in addition to ET, HCN is in essence a key signalling molecule in plants. Importantly, our data showed that the possible regulatory role of a relatively low concentration of HCN does not depend on ET feedback induction, although there are some common downstream components were observed.

CONCLUSION

Our findings provide a valuable resource for further exploration and understanding of the molecular regulatory mechanisms of HCN in plants and provide novel insight into HCN cross-talk with ET and other hormones in the regulation of plant growth and plant responses to environmental stresses.

The linker length of glucose-fipronil conjugates has a major effect on the rate of bioactivation by β-glucosidase

BACKGROUND

Endogenous plant β-glucosidases can be utilized to hydrolyze pro-pesticides and release the bioactive pesticide. Two related glucose-fipronil conjugates with different linkers structure, N-{3-cyano-1-[2,6-dichloro-4-(trifluoromethyl) phenyl]-4-[(trifluoromethyl) sulfinyl]-1H-pyrazol-5-yl}-1-(2-triazolethyl-β-d-glucopyranoside)-1H-1,2,3-triazole-4-methanamine (GOTF) and N-{3-cyano-1-[2,6-dichloro-4-(trifluoromethyl) phenyl]-4-[(trifluoromethyl)-sulfinyl]-1H-pyrazol-5-yl}-2-aminoethyl-β-d-glucopyranoside (GOF), were deglucolysated by β-glucosidase both in vitro and in vivo at different rates. Here, the basis for these differences was investigated by revealing the kinetics of the reaction and by modeling molecular docking between enzyme and substrate.

RESULTS

Results from kinetic study showed that the reaction rate was the main reason for the poorer rate of GOF hydrolysis with respect to GOTF. Modeling of substrate docking indicated that the spacer arm of glucose-fipronil conjugates affects the strength of non-covalent bonds within the active site and the position of fipronil within the pocket. Four glucose-fipronil conjugates and four corresponding aglycones were synthesized, and the hydrolysis data confirmed that the increased tether length between the bulky aglycone and glycone would lead to faster hydrolysis rate. The bioassay results indicated that most glucose-fipronil conjugates displayed moderate to excellent insecticidal activities in vivo against Plutella xylostella larvae.

CONCLUSION

This study provides a potential strategy to optimize the substrate structure to enhance hydrolytic specificity in order to design appropriate phloem mobile pro-pesticides. © 2018 Society of Chemical Industry.

Putative small RNAs controlling detoxification of industrial cyanide-containing wastewaters by Pseudomonas pseudoalcaligenes CECT5344

The alkaliphilic bacterium Pseudomonas pseudoalcaligenes CECT5344 uses free cyanide and several metal−cyanide complexes as the sole nitrogen source and tolerates high concentrations of metals like copper, zinc and iron, which are present in the jewelry wastewaters. To understand deeply the regulatory mechanisms involved in the transcriptional regulation of cyanide-containing wastewaters detoxification by P. pseudoalcaligenes CECT5344, RNA-Seq has been performed from cells cultured with a cyanide-containing jewelry wastewater, sodium cyanide or ammonium chloride as the sole nitrogen source. Small RNAs (sRNAs) that may have potential regulatory functions under cyanotrophic conditions were identified. In total 20 sRNAs were identified to be differentially expressed when compared the jewelry residue versus ammonium as nitrogen source, 16 of which could be amplified successfully by RT-PCR. As predicted targets of these 16 sRNAs were several components of the nit1C gene cluster encoding the nitrilase NitC essential for cyanide assimilation, the cioAB gene cluster that codes for the cyanide-insensitive cytochrome bd-type terminal oxidase, the medium length-polyhydroxyalkanoates (ml-PHAs) gene cluster, and gene clusters related with a global nitrogen limitation response like those coding for glutamine synthase and urease. Other targets were non-clustered genes (or their products) involved in metal resistance and iron acquisition, such as metal extrusion systems and the ferric uptake regulatory (Fur) protein, and a GntR-like regulatory family member probably involved in the regulation of the cyanide assimilation process in the strain CECT5344. Induction of genes targeted by sRNAs in the jewelry residue was demonstrated by qRT-PCR.

Phenylacetonitrile in locusts facilitates an antipredator defense by acting as an olfactory aposematic signal and cyanide precursor

Many aggregating animals use aposematic signals to advertise their toxicity to predators. However, the coordination between aposematic signals and toxins is poorly understood. Here, we reveal that phenylacetonitrile (PAN) acts as an olfactory aposematic signal and precursor of hypertoxic hydrogen cyanide (HCN) to protect gregarious locusts from predation. We found that PAN biosynthesis from phenylalanine is catalyzed by CYP305M2, a novel gene encoding a cytochrome P450 enzyme in gregarious locusts. The RNA interference (RNAi) knockdown of CYP305M2 increases the vulnerability of gregarious locusts to bird predation. By contrast, the elevation of PAN levels through supplementation with synthetic PAN increases the resistance of solitary locusts to predation. When locusts are attacked by birds, PAN is converted to HCN, which causes food poisoning in birds. Our results indicate that locusts develop a defense mechanism wherein an aposematic compound is converted to hypertoxic cyanide in resistance to predation by natural enemies.

Mapping QTLs and Identification of Genes Associated with Drought Resistance in Sorghum

Water limits global agricultural production. Increases in global aridity, a growing human population, and the depletion of aquifers will only increase the scarcity of water for agriculture. Water is essential for plant growth and in areas that are prone to drought, the use of drought-resistant crops is a long-term solution for growing more food for more people with less water. Sorghum is well adapted to hot and dry environments and has been used as a dietary staple for millions of people. Increasing the drought resistance in sorghum hybrids with no impact on yield is a continual objective for sorghum breeders. In this review, we describe the loci, quantitative trait loci (QTLs), or genes that have been identified for traits involved in drought avoidance (water-use efficiency, cuticular wax synthesis, trichome development and morphology, root system architecture) and drought tolerance (compatible solutes, pre- and post-flowering drought tolerance). Many of these identified genes and QTL regions have not been tested in hybrids and the effect of these genes, or their interactions, on yield must be understood in normal and drought-stressed conditions to understand the strength and weaknesses of their utility.

Metabotype variation in a field population of tansy plants influences aphid host selection

It is well known that plant volatiles influence herbivores in their selection of a host plant; however, less is known about how the nonvolatile metabolome affects herbivore host selection. Metabolic diversity between intraspecific plants can be characterized using non-targeted mass spectrometry that gives us a snapshot overview of all metabolic processes occurring within a plant at a particular time. Here, we show that non-targeted metabolomics can be used to reveal links between intraspecific chemical diversity and ecological processes in tansy (Tanacetum vulgare). First, we show that tansy plants can be categorized into five subgroups based up on their metabolic profiles, and that these "metabotypes" influenced natural aphid colonization in the field. Second, this grouping was not due to induced metabolomic changes within the plant due to aphid feeding but rather resulted from constitutive differences in chemical diversity between plants. These findings highlight the importance of intraspecific chemical diversity within one plant population and provide the first report of a non-targeted metabolomic field study in chemical ecology.

The Salt-Stress Response of the Transgenic Plum Line J8-1 and Its Interaction with the Salicylic Acid Biosynthetic Pathway from Mandelonitrile

Salinity is considered as one of the most important abiotic challenges that affect crop productivity. Plant hormones, including salicylic acid (SA), are key factors in the defence signalling output triggered during plant responses against environmental stresses. We have previously reported in peach a new SA biosynthetic pathway from mandelonitrile (MD), the molecule at the hub of the cyanogenic glucoside turnover in Prunus sp. In this work, we have studied whether this new SA biosynthetic pathway is also present in plum and the possible role this pathway plays in plant plasticity under salinity, focusing on the transgenic plum line J8-1, which displays stress tolerance via an enhanced antioxidant capacity. The SA biosynthesis from MD in non-transgenic and J8-1 micropropagated plum shoots was studied by metabolomics. Then the response of J8-1 to salt stress in presence of MD or Phe (MD precursor) was assayed by measuring: chlorophyll content and fluorescence parameters, stress related hormones, levels of non-enzymatic antioxidants, the expression of two genes coding redox-related proteins, and the content of soluble nutrients. The results from in vitro assays suggest that the SA synthesis from the MD pathway demonstrated in peach is not clearly present in plum, at least under the tested conditions. Nevertheless, in J8-1 NaCl-stressed seedlings, an increase in SA was recorded as a result of the MD treatment, suggesting that MD could be involved in the SA biosynthesis under NaCl stress conditions in plum plants. We have also shown that the plum line J8-1 was tolerant to NaCl under greenhouse conditions, and this response was quite similar in MD-treated plants. Nevertheless, the MD treatment produced an increase in SA, jasmonic acid (JA) and reduced ascorbate (ASC) contents, as well as in the coefficient of non-photochemical quenching (qN) and the gene expression of Non-Expressor of Pathogenesis-Related 1 (NPR1) and thioredoxin H (TrxH) under salinity conditions. This response suggested a crosstalk between different signalling pathways (NPR1/Trx and SA/JA) leading to salinity tolerance in the transgenic plum line J8-1.

l-lysine metabolism to N-hydroxypipecolic acid: an integral immune-activating pathway in plants

l-lysine catabolic routes in plants include the saccharopine pathway to α-aminoadipate and decarboxylation of lysine to cadaverine. The current review will cover a third l-lysine metabolic pathway having a major role in plant systemic acquired resistance (SAR) to pathogen infection that was recently discovered in Arabidopsis thaliana. In this pathway, the aminotransferase AGD2-like defense response protein (ALD1) α-transaminates l-lysine and generates cyclic dehydropipecolic (DP) intermediates that are subsequently reduced to pipecolic acid (Pip) by the reductase SAR-deficient 4 (SARD4). l-pipecolic acid, which occurs ubiquitously in the plant kingdom, is further N-hydroxylated to the systemic acquired resistance (SAR)-activating metabolite N-hydroxypipecolic acid (NHP) by flavin-dependent monooxygenase1 (FMO1). N-hydroxypipecolic acid induces the expression of a set of major plant immune genes to enhance defense readiness, amplifies resistance responses, acts synergistically with the defense hormone salicylic acid, promotes the hypersensitive cell death response and primes plants for effective immune mobilization in cases of future pathogen challenge. This pathogen-inducible NHP biosynthetic pathway is activated at the transcriptional level and involves feedback amplification. Apart from FMO1, some cytochrome P450 monooxygenases involved in secondary metabolism catalyze N-hydroxylation reactions in plants. In specific taxa, pipecolic acid might also serve as a precursor in the biosynthesis of specialized natural products, leading to C-hydroxylated and otherwise modified piperidine derivatives, including indolizidine alkaloids. Finally, we show that NHP is glycosylated in Arabidopsis to form a hexose-conjugate, and then discuss open questions in Pip/NHP-related research.

Prunasin production using engineered Escherichia coli expressing UGT85A47 from Japanese apricot and UDP-glucose biosynthetic enzyme genes

Japanese apricot, Prunus mume Sieb. et Zucc., biosynthesizes the l-phenylalanine-derived cyanogenic glucosides prunasin and amygdalin. Prunasin has biological properties such as anti-inflammation, but plant extraction and chemical synthesis are impractical. In this study, we identified and characterized UGT85A47 from Japanese apricot. Further, UGT85A47 was utilized for prunasin microbial production. Full-length cDNA encoding UGT85A47 was isolated from Japanese apricot after 5'- and 3'-RACE. Recombinant UGT85A47 stoichiometrically catalyzed UDP-glucose consumption and synthesis of prunasin and UDP from mandelonitrile. Escherichia coli C41(DE3) cells expressing UGT85A47 produced prunasin (0.64 g/L) from racemic mandelonitrile and glucose. In addition, co-expression of genes encoding UDP-glucose biosynthetic enzymes (phosphoglucomutase and UTP-glucose 1-phosphate uridiltransferase) and polyphosphate kinase clearly improved prunasin production up to 2.3 g/L. These results showed that our whole-cell biocatalytic system is significantly more efficient than the existing prunasin production systems, such as chemical synthesis.

Molecular identification and characterization of rhodaneses from the insect herbivore Pieris rapae

The association of cabbage white butterflies (Pieris spec., Lepidoptera: Pieridae) with their glucosinolate-containing host plants represents a well-investigated example of the sequential evolution of plant defenses and insect herbivore counteradaptations. The defensive potential of glucosinolates, a group of amino acid-derived thioglucosides present in plants of the Brassicales order, arises mainly from their rapid breakdown upon tissue disruption resulting in formation of toxic isothiocyanates. Larvae of P. rapae are able to feed exclusively on glucosinolate-containing plants due to expression of a nitrile-specifier protein in their gut which redirects glucosinolate breakdown to the formation of nitriles. The release of equimolar amounts of cyanide upon further metabolism of the benzylglucosinolate-derived nitrile suggests that the larvae are also equipped with efficient means of cyanide detoxification such as β-cyanoalanine synthases or rhodaneses. While insect β-cyanoalanine synthases have recently been identified at the molecular level, no sequence information was available of characterized insect rhodaneses. Here, we identify and characterize two single-domain rhodaneses from P. rapae, PrTST1 and PrTST2. The enzymes differ in their kinetic properties, predicted subcellular localization and expression in P. rapae indicating different physiological roles. Phylogenetic analysis together with putative lepidopteran rhodanese sequences indicates an expansion of the rhodanese family in Pieridae.

Freeze-induced cyanide toxicity does not maintain the cyanogenesis polymorphism in white clover (Trifolium repens)

PREMISE OF THE STUDY

The maintenance of adaptive polymorphisms within species requires fitness trade-offs reflecting selection for each morph. Cyanogenesis, the ability to produce hydrogen cyanide (HCN) after tissue damage, occurs in >3000 plant species and exists as a discrete polymorphism in white clover. This polymorphism is spatially distributed in recurrent clines, with higher frequencies of cyanogenic plants in warmer climates. The HCN autotoxicity hypothesis proposes that cyanogenic plants are selected against where frosts are common, as freezing liberates HCN and could impair cellular respiration.

METHODS

We tested the HCN autotoxicity hypothesis using a freezing chamber to examine survival, tissue damage, and physiological recovery as assessed via chlorophyll fluorescence following mild and severe freezing treatments. We utilized 65 genotypes from a single polymorphic population to eliminate effects of population structure.

KEY RESULTS

Cyanogenic plants did not differ from acyanogenic plants in survival, tissue damage, or recovery following freezing. However, plants producing either of the two required cyanogenic precursors had lower survival and tissue damage after freezing than plants lacking both precursors.

CONCLUSIONS

These results suggest that freezing-induced HCN toxicity is unlikely to be responsible for the maintenance of the cyanogenesis polymorphism in white clover. However, energetic trade-offs associated with costs of producing the cyanogenic precursors may confer a fitness benefit to acyanogenic plants under stressful climatic conditions. The lack of evidence for HCN toxicity suggests that cyanogenic clover uses physiological mechanisms mediated by β-cyanoalanine synthase and alternative oxidase to maintain cellular function in the presence of HCN.

The Vacuolar Transportome of Plant Specialized Metabolites

The plant vacuole is a cellular compartment that is essential to plant development and growth. Often plant vacuoles accumulate specialized metabolites, also called secondary metabolites, which constitute functionally and chemically diverse compounds that exert in planta many essential functions and improve the plant's fitness. These metabolites provide, for example, chemical defense against herbivorous and pathogens or chemical attractants (color and fragrance) to attract pollinators. The chemical composition of the vacuole is dynamic, and is altered during development and as a response to environmental changes. To some extent these alterations rely on vacuolar transporters, which import and export compounds into and out of the vacuole, respectively. During the past decade, significant progress was made in the identification and functional characterization of the transporters implicated in many aspects of plant specialized metabolism. Still, deciphering the molecular players underlying such processes remains a challenge for the future. In this review, we present a comprehensive summary of the most recent achievements in this field.

Micro- and macroevolutionary adaptation through repeated loss of a complete metabolic pathway

There is growing evidence for the convergent evolution of physically linked gene clusters encoding chemical defense pathways. Metabolic clusters are proposed to evolve because they ensure co-inheritance of all required genes where the defense is favored, and prevent inheritance of toxic partial pathways where it is not. This hypothesis rests on the assumption that clusters evolve in species where selection favors intraspecific polymorphism for the defense; however, they have not been examined in polymorphic species. We examined metabolic cluster evolution in relation to an adaptive polymorphism for cyanogenic glucoside (CNglc) production in clover. Using 163 accessions, we performed CNglc assays, BAC sequencing, Southern hybridizations and molecular evolutionary analyses. We find that the CNglc pathway forms a 138-kb cluster in white clover, and that the adaptive polymorphism occurs through presence/absence of the complete cluster. Component genes are orthologous to those in the distantly related legume Lotus japonicus. These findings provide empirical support for the co-inheritance hypothesis, and they indicate that adaptive CNglc variation in white clover evolves through recurrent deletions of the entire pathway. They further indicate that the shared ancestor of many important legume crops was likely cyanogenic and that this defense was lost repeatedly over the last 50 Myr.

Glutathione transferases catalyze recycling of auto-toxic cyanogenic glucosides in sorghum

Cyanogenic glucosides are nitrogen-containing specialized metabolites that provide chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide. It has been suggested that cyanogenic glucosides are also a store of nitrogen that can be remobilized for general metabolism via a previously unknown pathway. Here we reveal a recycling pathway for the cyanogenic glucoside dhurrin in sorghum (Sorghum bicolor) that avoids hydrogen cyanide formation. As demonstrated in vitro, the pathway proceeds via spontaneous formation of a dhurrin-derived glutathione conjugate, which undergoes reductive cleavage by glutathione transferases of the plant-specific lambda class (GSTLs) to produce p-hydroxyphenyl acetonitrile. This is further metabolized to p-hydroxyphenylacetic acid and free ammonia by nitrilases, and then glucosylated to form p-glucosyloxyphenylacetic acid. Two of the four GSTLs in sorghum exhibited high stereospecific catalytic activity towards the glutathione conjugate, and form a subclade in a phylogenetic tree of GSTLs in higher plants. The expression of the corresponding two GSTLs co-localized with expression of the genes encoding the p-hydroxyphenyl acetonitrile-metabolizing nitrilases at the cellular level. The elucidation of this pathway places GSTs as key players in a remarkable scheme for metabolic plasticity allowing plants to reverse the resource flow between general and specialized metabolism in actively growing tissue.