Contrasting defence mechanisms against spider mite infestation in cyanogenic and non-cyanogenic legumes

Understanding the complex interactions between plants and herbivores is essential for improving crop resistance. Aiming to expand the role of cyanogenesis in plant defence, we investigated the response of the cyanogenic Phaseolus lunatus (lima bean) and the non-cyanogenic Phaseolus vulgaris (common bean) to Tetranychus urticae (spider mite) infestation. Despite mite infesting both legumes, leaf damage infringed by this feeder was reduced in lima bean. Comparative transcriptome analyses revealed that both species exhibited substantial metabolic and transcriptional changes upon infestation, although alterations in P. lunatus were significantly more pronounced. Specific differences in amino acid homeostasis and key genes associated with the cyanogenic pathway were observed in these species, as well as the upregulation of the mandelonitrile lyase gene (PlMNL1) following T. urticae feeding. Concomitantly, the PIMNL1 activity increased. Lima bean plants also displayed an induction of β-cyanoalanine synthase (PlCYSC1), a key enzyme for cyanide detoxification, suggesting an internal regulatory mechanism to manage the toxicity of their defence responses. These findings contribute to our understanding of the legume-herbivore interactions and underscore the potential role of cyanogenesis in the elaboration of specific defensive responses, even within the same genus, which may reflect distinctive evolutionary adaptations or varying metabolic capabilities between species.

Chemical defence of a centipede (Clinopodes flavidus)

Chemical substances are of utmost importance for the biotic interactions between animals and their predators/parasites; many of these semiochemicals are emitted for defence purposes. One of the most deterrent and toxic biogenic substances we know of is hydrogen cyanide, which can be stored by certain insects, millipedes, centipedes and arachnids in the form of stable and less volatile molecules. The aim of this study was to analyse the biology and chemistry of such a defence mechanism in a geophilomorph centipede (Chilopoda). The cyanogenic secretion of Clinopodes flavidus is discharged from the ventral glands, whose glandular units are located in the space between the cuticle and the trunk muscles and do not extend deep into the segment. In addition to hydrogen cyanide, the ventral secretion contains 2-methylpentanoic acid, benzaldehyde, benzoyl cyanide, 2-methyl branched C-9 carboxylic acid (tentatively identified as 2-methyloctanoic acid), methyl 2-phenylacetate, benzoic acid and mandelonitrile as well as four major proteins with a molecular weight of 150, 66.2, 59 and 55 kDa. The correlation between the presence of ventral glands and guarding with the female's ventral side facing away from the eggs and young indicates a functional link between these two traits. We hope that the specificity of the chemical composition of the ventral secretion could serve as a criterion for chemotaxonomy and that the analysis of more species will help to clarify the phylogenetic relationships within the Geophilomorpha.

Wounding and methyl jasmonate increase cyanogenic glucoside concentrations in Sorghum bicolor via upregulation of biosynthesis

The cyanogenic glucoside, dhurrin, present in Sorghum bicolor is thought to have multiple functions, including in defence against herbivory. The hormone methyl jasmonate (MeJA) is also induced by herbivory and is key to instigating defence processes in plants. To investigate whether dhurrin is induced in response to herbivore attack and also to the associated presence of MeJA, sorghum plants were either wounded or exogenous MeJA was applied. We show that specific wounding (pin board and perforation) and the application of MeJA increases dhurrin concentration in leaves and sheath tissue 12 h after treatment. Quantitative PCR shows that the expression of two genes, SbCYP79A1 and SbUGT85B1, involved in the synthesis of dhurrin are significantly induced by exogenous MeJA and by wounding. Analysis of 2 kb of sequence upstream of the start codon of SbCYP79A1 identifies several cis-acting elements that have been linked to MeJA induction. A promoter deletion series, coupled to GFP, and transiently expressed in Nicotiana benthamiana suggests that there are potentially three sequence motifs (~-925 to -976) involved in the binding of transcription factors that result in increased expression of SbCYP79A1 and the synthesis of dhurrin in response to MeJA.

Transcript profiles of wild and domesticated sorghum under water-stressed conditions and the differential impact on dhurrin metabolism

Australian native species of sorghum contain negligible amounts of dhurrin in their leaves and the cyanogenesis process is regulated differently under water-stress in comparison to domesticated sorghum species. Cyanogenesis in forage sorghum is a major concern in agriculture as the leaves of domesticated sorghum are potentially toxic to livestock, especially at times of drought which induces increased production of the cyanogenic glucoside dhurrin. The wild sorghum species endemic to Australia have a negligible content of dhurrin in the above ground tissues and thus represent a potential resource for key agricultural traits like low toxicity. In this study we investigated the differential expression of cyanogenesis related genes in the leaf tissue of the domesticated species Sorghum bicolor and the Australian native wild species Sorghum macrospermum grown in glasshouse-controlled water-stress conditions using RNA-Seq analysis to analyse gene expression. The study identified genes, including those in the cyanogenesis pathway, that were differentially regulated in response to water-stress in domesticated and wild sorghum. In the domesticated sorghum, dhurrin content was significantly higher compared to that in the wild sorghum and increased with stress and decreased with age whereas in wild sorghum the dhurrin content remained negligible. The key genes in dhurrin biosynthesis, CYP79A1, CYP71E1 and UGT85B1, were shown to be highly expressed in S. bicolor. DHR and HNL encoding the dhurrinase and α-hydroxynitrilase catalysing bio-activation of dhurrin were also highly expressed in S. bicolor. Analysis of the differences in expression of cyanogenesis related genes between domesticated and wild sorghum species may allow the use of these genetic resources to produce more acyanogenic varieties in the future.

Variation in production of cyanogenic glucosides during early plant development: A comparison of wild and domesticated sorghum

Domestication has narrowed the genetic diversity found in crop wild relatives, potentially reducing plasticity to cope with a changing climate. The tissues of domesticated sorghum (Sorghum bicolor), especially in younger plants, are cyanogenic and potentially toxic. Species of wild sorghum produce lower levels of the cyanogenic glucoside (CNglc) dhurrin than S. bicolor at maturity, but it is not known if this is also the case during germination and early growth. CNglcs play multiple roles in primary and specialised metabolism in domesticated sorghum and other crop plants. In this study, the temporal and spatial distribution of dhurrin in wild and domesticated sorghum at different growth stages was monitored in leaf, sheath and root tissues up to 35 days post germination using S. bicolor and the wild species S. brachypodum and S. macrospermum as the experimental systems. Growth parameters were also measured and allocation of plant total nitrogen (N%) to both dhurrin and nitrate (NO₃^-) was calculated. Negligible amounts of dhurrin were produced in the leaves of the two wild species compared to S. bicolor. The morphology of the two wild sorghums also differed from S. bicolor, with the greatest differences observed for the more distantly related S. brachypodum. S. bicolor had the highest leaf N% whilst the wild species had significantly higher root N%. Allocation of nitrogen to dhurrin in aboveground tissue was significantly higher in S. bicolor compared to the wild species but did not differ in the roots across the three species. The differences in plant morphology, dhurrin content and re-mobilisation, and nitrate/nitrogen allocation suggest that domestication has affected the functional roles of dhurrin in sorghum.

Nitrogen availability and allocation in sorghum and its wild relatives: Divergent roles for cyanogenic glucosides

Crop plants are assumed to have become more susceptible to pests as a result of selection for high growth rates during the process of domestication, consistent with resource allocation theories. We compared the investment by domesticated sorghum into cyanogenic glucosides, nitrogen-based specialised metabolites that break down to release hydrogen cyanide, with five wild relatives native to Australia. Plants were grown in pots in a greenhouse and supplied with low and high concentrations of nitrogen and monitored for 9 weeks. The concentrations of nitrate, total phenolics and silicon were also measured. Domesticated Sorghum bicolor had the highest leaf and root cyanogenic glucoside concentrations, and among the lowest nitrate and silicon concentrations under both treatments. Despite partitioning a much higher proportion of its stored nitrogen to cyanogenic glucosides than the wild species, S. bicolor's nitrogen productivity levels were among the highest. Most of the wild sorghums had higher concentrations of silicon and phenolics, which may provide an alternative defence system. Cyanogenic glucosides appear to be integral to S. bicolor's physiology, having roles in both growth and defence. Sorghum macrospermum displayed consistently low cyanogenic glucoside concentrations, high growth rates and high nitrogen productivity and represents a particularly attractive genetic resource for sorghum improvement.

Role of cyanogenic glycosides in the seeds of wild lima bean, Phaseolus lunatus: defense, plant nutrition or both?

Cyanogenic glycosides present in the seeds of wild lima bean plants are associated with seedling defense but do not affect seed germination and seedling growth. Wild lima bean plants contain cyanogenic glycosides (CNGs) that are known to defend the plant against leaf herbivores. However, seed feeders appear to be unaffected despite the high levels of CNGs in the seeds. We investigated a possible role of CNGs in seeds as nitrogen storage compounds that influence plant growth, as well as seedling resistance to herbivores. Using seeds from four different wild lima bean natural populations that are known to vary in CNG levels, we tested two non-mutually exclusive hypotheses: (1) seeds with higher levels of CNGs produce seedlings that are more resistant against generalist herbivores and, (2) seeds with higher levels of CNGs germinate faster and produce plants that exhibit better growth. Levels of CNGs in the seeds were negatively correlated with germination rates and not correlated with seedling growth. However, levels of CNGs increased significantly soon after germination and seeds with the highest CNG levels produced seedlings with higher CNG levels in cotyledons. Moreover, the growth rate of the generalist herbivore Spodoptera littoralis was lower in cotyledons with high-CNG levels. We conclude that CNGs in lima bean seeds do not play a role in seed germination and seedling growth, but are associated with seedling defense. Our results provide insight into the potential dual function of plant secondary metabolites as defense compounds and storage molecules for growth and development.

New paths of cyanogenesis from enzymatic-promoted cleavage of β-cyanoglucosides are suggested by a mixed DFT/QTAIM approach

Cyanogenesis is an enzyme-promoted cleavage of β-cyanoglucosides; the release of hydrogen cyanide is believed to produce food poisoning by consumption of certain crops as Cassava (Manihot esculenta Crantz). The production of hydrogen cyanide by some disruption of the plant wall is related to the content of two β-cyanoglucosides (linamarin and lotaustralin) which are stored within the tuber. Some features about the mechanistic bases of these transformations have been published; nevertheless, there are still questions about the exact mechanism, such as the feasibility of a difference in the kinetics of cyanogenesis between both cyanoglucosides. In this work, we have performed a theoretical analysis using DFT and QTAIM theoretical frameworks to propose a feasible mechanism of the observed first step of the enzyme-catalyzed rupture of these glucosides; our results led us to explain the observed difference between linamarin and lotaustralin. Meanwhile, DFT studies suggest that there are no differences between local reactivity indexes of both glucosides; QTAIM topological analysis suggests two important intramolecular interactions which we found to fix the glucoside in such a way that suggests the linamarin as a more reactive system towards a nucleophilic attack, thus explaining the readiness to liberate hydrogen cyanide.

Metabolomics and Biochemical Approaches Link Salicylic Acid Biosynthesis to Cyanogenesis in Peach Plants

Despite the long-established importance of salicylic acid (SA) in plant stress responses and other biological processes, its biosynthetic pathways have not been fully characterized. The proposed synthesis of SA originates from chorismate by two distinct pathways: the isochorismate and phenylalanine (Phe) ammonia-lyase (PAL) pathways. Cyanogenesis is the process related to the release of hydrogen cyanide from endogenous cyanogenic glycosides (CNglcs), and it has been linked to plant plasticity improvement. To date, however, no relationship has been suggested between the two pathways. In this work, by metabolomics and biochemical approaches (including the use of [13C]-labeled compounds), we provide strong evidences showing that CNglcs turnover is involved, at least in part, in SA biosynthesis in peach plants under control and stress conditions. The main CNglcs in peach are prunasin and amygdalin, with mandelonitrile (MD), synthesized from phenylalanine, controlling their turnover. In peach plants MD is the intermediary molecule of the suggested new SA biosynthetic pathway and CNglcs turnover, regulating the biosynthesis of both amygdalin and SA. MD-treated peach plants displayed increased SA levels via benzoic acid (one of the SA precursors within the PAL pathway). MD also provided partial protection against Plum pox virus infection in peach seedlings. Thus, we propose a third pathway, an alternative to the PAL pathway, for SA synthesis in peach plants.

Utilization of a high-throughput shoot imaging system to examine the dynamic phenotypic responses of a C4 cereal crop plant to nitrogen and water deficiency over time

The use of high-throughput phenotyping systems and non-destructive imaging is widely regarded as a key technology allowing scientists and breeders to develop crops with the ability to perform well under diverse environmental conditions. However, many of these phenotyping studies have been optimized using the model plant Arabidopsis thaliana. In this study, The Plant Accelerator(®) at The University of Adelaide, Australia, was used to investigate the growth and phenotypic response of the important cereal crop, Sorghum bicolor L. Moench and related hybrids to water-limited conditions and different levels of fertilizer. Imaging in different spectral ranges was used to monitor plant composition, chlorophyll, and moisture content. Phenotypic image analysis accurately measured plant biomass. The data set obtained enabled the responses of the different sorghum varieties to the experimental treatments to be differentiated and modelled. Plant architectural instead of architecture elements were determined using imaging and found to correlate with an improved tolerance to stress, for example diurnal leaf curling and leaf area index. Analysis of colour images revealed that leaf 'greenness' correlated with foliar nitrogen and chlorophyll, while near infrared reflectance (NIR) analysis was a good predictor of water content and leaf thickness, and correlated with plant moisture content. It is shown that imaging sorghum using a high-throughput system can accurately identify and differentiate between growth and specific phenotypic traits. R scripts for robust, parsimonious models are provided to allow other users of phenomic imaging systems to extract useful data readily, and thus relieve a bottleneck in phenotypic screening of multiple genotypes of key crop plants.

Effects of PEG-induced osmotic stress on growth and dhurrin levels of forage sorghum

Sorghum (Sorghum bicolor L. Moench) is a valuable forage crop in regions with low soil moisture. Sorghum may accumulate high concentrations of the cyanogenic glucoside dhurrin when drought stressed resulting in possible cyanide (HCN) intoxication of grazing animals. In addition, high concentrations of nitrate, also potentially toxic to ruminants, may accumulate during or shortly after periods of drought. Little is known about the degree and duration of drought-stress required to induce dhurrin accumulation, or how changes in dhurrin concentration are influenced by plant size or nitrate metabolism. Given that finely regulating soil moisture under controlled conditions is notoriously difficult, we exposed sorghum plants to varying degrees of osmotic stress by growing them for different lengths of time in hydroponic solutions containing polyethylene glycol (PEG). Plants grown in medium containing 20% PEG (-0.5 MPa) for an extended period had significantly higher concentrations of dhurrin in their shoots but lower dhurrin concentrations in their roots. The total amount of dhurrin in the shoots of plants from the various treatments was not significantly different on a per mass basis, although a greater proportion of shoot N was allocated to dhurrin. Following transfer from medium containing 20% PEG to medium lacking PEG, shoot dhurrin concentrations decreased but nitrate concentrations increased to levels potentially toxic to grazing ruminants. This response is likely due to the resumption of plant growth and root activity, increasing the rate of nitrate uptake. Data presented in this article support a role for cyanogenic glucosides in mitigating oxidative stress.

The rare cyanogen proteacin, and dhurrin, from foliage of Polyscias australiana, a tropical Araliaceae

The tyrosine-derived cyanogenic di-glucoside proteacin and related mono-glucoside dhurrin were identified as the cyanogens in foliage of the tropical tree species Polyscias australiana, present in the approximate ratio 9:1. To date cyanogenic glycosides have not been characterised from the Araliaceae or the Apiales. Concentrations of cyanogenic glycosides varied significantly between plant parts and with leaf age, with the highest concentrations in young emerging leaves (mean 2217.1 μg CN g(-1) dry wt), petioles (rachis; 1487.1 μg CN g(-1) dry wt) and floral buds (265.8 μg CN g(-1) dry wt). Between 2% and 10% of nitrogen in emerging leaves and petioles was present as cyanogenic glycosides. With the exception of floral buds, all tissues apparently lack a specific cyanogenic β-glucosidase to catalyse the first step in the breakdown of these cyanogenic glycosides. Only with the addition of emulsin, an exogenous non-specific β-glucosidase from almonds, were high concentrations of cyanogenic glycosides detected, as much as 20-fold greater than the low to negligible cyanogenic glycoside concentrations determined in the absence of exogenous enzyme. High concentrations of cyanogens in young tissues may confer protection, but may also be a nitrogen source during leaf expansion.

Light affects in vitro organogenesis of Linum usitatissimum L. and its cyanogenic potential

The relationships between organogenesis of oil flax (Linum usitatissimum L., cv. 'Szafir') in vitro, cyanogenic potential (HCN-p) of these tissues and light were investigated. Shoot multiplication obtained on Murashige and Skoog medium containing 0.05 mg L^-1 2,4-dichloro-phenoxyacetic acid and 1 mg L^-1 6-benzyladenine (BA), was about twice higher in light-grown cultures than those in darkness. Light-grown explants showed also higher rate of roots regeneration (in medium containing 1 mg L^-1 α-naphtaleneacetic acid and 0.05 mg L^-1 BA) than dark-grown ones. The cyanogenic potential (expressed both as linamarin and lotaustralin content and linamarase activity) of flax cultured in vitro was tissue-specific and generally was higher under light conditions than in darkness. The highest concentration of linamarin and lotaustralin was detected in light-regenerated shoots, and its amount was twice as high as in roots, and about threefold higher than in callus tissue. The activities of linamarase and β-cyanoalanine synthase in light-regenerated organs were also higher than those in darkness. Thus, higher frequency of regeneration of light-grown cultures than dark-grown ones seems to be correlated with higher HCN-p of these tissues. We suggest that free HCN, released from cyanoglucosides potentially at higher level under light conditions, may be involved in some organogenetic processes which improve regeneration efficiency.

Selective grazing of acyanogenic white clover: Variation in behaviour among populations of the slug Deroceras reticulatum

Collections of the slug Deroceras reticulatum were made from grassland sites containing contrasting frequencies of the cyanogenic morph of white clover, Trifolium repens. In choice chamber experiments, slugs obtained from sites with a low frequency of cyanogenic clover showed a significantly greater degree of selective eating of acyanogenic morphs than slugs taken from a site containing a high frequency of cyanogenic clover. Differences in selectivity between populations were caused both by differences in the rate of initiation of feeding on cyanogenic morphs, and by differences in the extent of damage once feeding had been initiated. The implications of these results for the cyanogenic polymorphism of T. repens are discussed.