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

Dhurrin in Sorghum: Biosynthesis, Regulation, Biological Function and Challenges for Animal Production

Sorghum (Sorghum bicolor) holds a significant position as the fifth most vital cereal crop globally. Its drought resistance and robust biomass production, coupled with commendable nutritional value, make sorghum a promising choice for animal feed. Nevertheless, the utilization of sorghum in animal production faces hurdles of dhurrin (a cyanogenic glycoside) poisoning. While dhurrin serves as a protective secondary metabolite during sorghum growth, the resulting highly toxic hydrogen cyanide poses a significant threat to animal safety. This review extensively examines the biometabolic processes of dhurrin, the pivotal genes involved in the regulation of dhurrin biosynthesis, and the factors influencing dhurrin content in sorghum. It delves into the impact of dhurrin on animal production and explores measures to mitigate its content, aiming to provide insights for advancing research on dhurrin metabolism regulation in sorghum and its rational utilization in animal production.

The Putative GATA Transcription Factor SbGATA22 as a Novel Regulator of Dhurrin Biosynthesis

Cyanogenic glucosides are specialized metabolites produced by over 3000 species of higher plants from more than 130 families. The deployment of cyanogenic glucosides is influenced by biotic and abiotic factors in addition to being developmentally regulated, consistent with their roles in plant defense and stress mitigation. Despite their ubiquity, very little is known regarding the molecular mechanisms that regulate their biosynthesis. The biosynthetic pathway of dhurrin, the cyanogenic glucoside found in the important cereal crop sorghum (Sorghum bicolor (L.) Moench), was described over 20 years ago, and yet no direct regulator of the biosynthetic genes has been identified. To isolate regulatory proteins that bind to the promoter region of the key dhurrin biosynthetic gene of sorghum, SbCYP79A1, yeast one-hybrid screens were performed. A bait fragment containing 1204 base pairs of the SbCYP79A1 5' regulatory region was cloned upstream of a reporter gene and introduced into Saccharomyces cerevisiae. Subsequently, the yeast was transformed with library cDNA representing RNA from two different sorghum developmental stages. From these screens, we identified SbGATA22, an LLM domain B-GATA transcription factor that binds to the putative GATA transcription factor binding motifs in the SbCYP79A1 promoter region. Transient assays in Nicotiana benthamiana show that SbGATA22 localizes to the nucleus. The expression of SbGATA22, in comparison with SbCYP79A1 expression and dhurrin concentration, was analyzed over 14 days of sorghum development and in response to nitrogen application, as these conditions are known to affect dhurrin levels. Collectively, these findings suggest that SbGATA22 may act as a negative regulator of SbCYP79A1 expression and provide a preliminary insight into the molecular regulation of dhurrin biosynthesis in sorghum.

Deciphering the Genetic Mechanisms of Salt Tolerance in Sorghum bicolor L.: Key Genes and SNP Associations from Comparative Transcriptomic Analyses

Sorghum bicolor L. is a vital cereal crop for global food security. Its adaptability to diverse climates make it economically, socially, and environmentally valuable. However, soil salinization caused by climate extremes poses a threat to sorghum. This study aimed to identify candidate salt-tolerant genes and single nucleotide polymorphisms (SNPs) by performing a comparative transcriptome analysis on a mutant sorghum line and its wild type. The mutant line was generated through gamma ray exposure and selection for salt tolerance. Phenotypic measurements were taken, followed by mRNA sequencing and variant calling. In this study, potential genes and non-synonymous SNPs associated with salt tolerance were inferred, including LOC8071970, LOC8067721, LOC110430887, LOC8070256, and LOC8056880. These genes demonstrated notable differences in nsSNPs in comparison to the wild type, suggesting their potential roles in salt tolerance. Additionally, LOC8060874 (cyanohydrin beta-glucosyltransferase) was suggested as a key gene involved in salt tolerance due to its possible role in dhurrin biosynthesis under salt stress. In upcoming research, additional reverse genetics studies will be necessary in order to verify the function of those candidate genes in relation to salt stress. In conclusion, this study underscores the significance of investigating salt tolerance mechanisms and the potential key genes associated with salt tolerance in sorghum. Our findings may provide insights for future breeding strategies aimed at enhancing salinity tolerance and crop productivity.

Ethylene and Jasmonates Signaling Network Mediating Secondary Metabolites under Abiotic Stress

Plants are sessile organisms that face environmental threats throughout their life cycle, but increasing global warming poses an even more existential threat. Despite these unfavorable circumstances, plants try to adapt by developing a variety of strategies coordinated by plant hormones, resulting in a stress-specific phenotype. In this context, ethylene and jasmonates (JAs) present a fascinating case of synergism and antagonism. Here, Ethylene Insensitive 3/Ethylene Insensitive-Like Protein1 (EIN3/EIL1) and Jasmonate-Zim Domain (JAZs)-MYC2 of the ethylene and JAs signaling pathways, respectively, appear to act as nodes connecting multiple networks to regulate stress responses, including secondary metabolites. Secondary metabolites are multifunctional organic compounds that play crucial roles in stress acclimation of plants. Plants that exhibit high plasticity in their secondary metabolism, which allows them to generate near-infinite chemical diversity through structural and chemical modifications, are likely to have a selective and adaptive advantage, especially in the face of climate change challenges. In contrast, domestication of crop plants has resulted in change or even loss in diversity of phytochemicals, making them significantly more vulnerable to environmental stresses over time. For this reason, there is a need to advance our understanding of the underlying mechanisms by which plant hormones and secondary metabolites respond to abiotic stress. This knowledge may help to improve the adaptability and resilience of plants to changing climatic conditions without compromising yield and productivity. Our aim in this review was to provide a detailed overview of abiotic stress responses mediated by ethylene and JAs and their impact on secondary metabolites.

Species-specific dynamics of specialized metabolism in germinating sorghum grain revealed by temporal and tissue-resolved transcriptomics and metabolomics

Seed germination is crucial for plant productivity, and the biochemical changes during germination affect seedling survival, plant health and yield. While the general metabolism of germination is extensively studied, the role of specialized metabolism is less investigated. We therefore analyzed the metabolism of the defense compound dhurrin during sorghum (Sorghum bicolor) grain germination and early seedling development. Dhurrin is a cyanogenic glucoside, which is catabolized into different bioactive compounds at other stages of plant development, but its fate and role during germination is unknown. We dissected sorghum grain into three different tissues and investigated dhurrin biosynthesis and catabolism at the transcriptomic, metabolomic and biochemical level. We further analyzed transcriptional signature differences of cyanogenic glucoside metabolism between sorghum and barley (Hordeum vulgare), which produces similar specialized metabolites. We found that dhurrin is de novo biosynthesized and catabolized in the growing embryonic axis as well as the scutellum and aleurone layer, two tissues otherwise mainly acknowledged for their involvement in release and transport of general metabolites from the endosperm to the embryonic axis. In contrast, genes encoding cyanogenic glucoside biosynthesis in barley are exclusively expressed in the embryonic axis. Glutathione transferase enzymes (GSTs) are involved in dhurrin catabolism and the tissue-resolved analysis of GST expression identified new pathway candidate genes and conserved GSTs as potentially important in cereal germination. Our study demonstrates a highly dynamic tissue- and species-specific specialized metabolism during cereal grain germination, highlighting the importance of tissue-resolved analyses and identification of specific roles of specialized metabolites in fundamental plant processes.

Genetic variation in hydrogen cyanide potential of perennial sorghum evaluated by colorimetry

Both annual and perennial sorghum biomass serve as important forage for ruminant animals around the world. Unfortunately, sorghum can produce hydrogen cyanide (HCN), which, if occurring in high enough concentrations, can be toxic or lethal to animals that consume it. The objectives of this study were to develop a fast and inexpensive colorimetric assay to measure the hydrogen cyanide potential (HCN-P) as well as to compare this with existing visual assays while assessing the range of variation for HCN-P among perennial and annual sorghum biomass. The HCN-P of 100 sorghum lines derived from an interspecific hybridization program was determined over 2 years (establishment and regrowth) using both visual and colorimetric assays. Visual assessment underestimated the HCN-P and was less accurate than colorimetry. Repeatability for HCN-P across all sampling dates was functionally zero in the visual assessment and low for the colorimetric assay. This was mostly explained by the significant pedigree × year interaction effects and growth stage. Growth stage substantially influenced HCN-P, which should be considered when feeding animals on fresh forage.

Nutritional, phytochemical composition and potential health benefits of taro (Colocasia esculenta L.) leaves: A review

Colocasia esculenta(L) or taro is a tropical crop largely produced for its tubers (corms) while leaves and stems remain underutilized and untapped by-products with promising potential applications.Colocasialeaves are low in calories, rich in proteins, dietary fiber, and micronutrients. However, its utilization as food remains limited owing to the lack of awareness vis-à-vis its nutritional profile and the presence of antinutrients such as tannins, phytates and oxalates. The antinutritional factors can be overcome by cooking and varied processing techniques thereby unveiling the nutritional benefits. The high content of bioactive compounds and antioxidative potential of colocasia leaves renders several health benefits such as anticancer, antidiabetic, anti-inflammatory activity. The paper reviews the available literature on the nutritional, antinutritional, phytochemical profile of taro leaves and the advanced analytical techniques for their identification and quantification. Further, its health benefits and food applications have been discussed.

Interactive effects of hydrogen sulphide and silicon enhance drought and heat tolerance by modulating hormones, antioxidant defence enzymes and redox status in barley (Hordeum vulgare L.)

Recent changes in climate have reduced crop productivity throughout much of the world. Drought and heat stress, particularly in arid and semi-arid regions, have seriously affected barley production. This study explored the separate and interactive effects of silicon (Si) and hydrogen sulphide (H₂ S) on plant growth and mitigation of the adverse effects of heat stress (DS) and drought stress (HS) in a barley pot experiment. The impacts of simultaneous DS + HS were more severe than individual stresses due to increased ROS production, malondialdehyde (MDA) content and higher electrolyte leakage (EL), thereby leading to reduced water, protein and photosynthetic pigment content. Exogenously applied Si and H₂ S alleviated the DS-, HS- and DS + HS-induced effects on barley by reducing ROS production, MDA and EL. A single application of H₂ S or Si + H₂ S increased plant biomass under all stress conditions, which can be ascribed to higher Si accumulation in barley shoots. A single application of Si or H₂ S significantly increased plant biomass. However, Si + H₂ S was the most effective treatment for metabolite accumulation and elevating activity of antioxidant enzymes to prevent toxicity from oxidative stress. This treatment also modulated osmolyte content, enhanced antioxidant activity and regulated the stress signalling-related endogenous hormones, abscisic acid (ABA) and indole acetic acid (IAA). Exogenous treatments regulated endogenous H₂ S and Si and resulted in higher tolerance to individual and combined drought and heat stress in barley.

Seedling growth and fall armyworm feeding preference influenced by dhurrin production in sorghum

Cyanogenic glucosides (CGs) play a key role in host-plant defense to insect feeding; however, the metabolic tradeoffs between synthesis of CGs and plant growth are not well understood. In this study, genetic mutants coupled with nondestructive phenotyping techniques were used to study the impact of the CG dhurrin on fall armyworm [Spodoptera frugiperda (J.E. Smith)] (FAW) feeding and plant growth in sorghum [Sorghum bicolor (L.) Moench]. A genetic mutation in CYP79A1 gene that disrupts dhurrin biosynthesis was used to develop sets of near-isogenic lines (NILs) with contrasting dhurrin contents in the Tx623 bmr6 genetic background. The NILs were evaluated for differences in plant growth and FAW feeding damage in replicated greenhouse and field trials. Greenhouse studies showed that dhurrin-free Tx623 bmr6 cyp79a1 plants grew more quickly than wild-type plants but were more susceptible to insect feeding based on changes in green plant area (GPA), total leaf area, and total dry weight over time. The NILs exhibited similar patterns of growth in field trials with significant differences in leaf area and dry weight of dhurrin-free plants between the infested and non-infested treatments. Taken together, these studies reveal a significant metabolic tradeoff between CG biosynthesis and plant growth in sorghum seedlings. Disruption of dhurrin biosynthesis produces plants with higher growth rates than wild-type plants but these plants have greater susceptibility to FAW feeding.

Seedling growth and fall armyworm feeding preference influenced by dhurrin production in sorghum

Cyanogenic glucosides (CGs) play a key role in host-plant defense to insect feeding; however, the metabolic tradeoffs between synthesis of CGs and plant growth are not well understood. In this study, genetic mutants coupled with nondestructive phenotyping techniques were used to study the impact of the CG dhurrin on fall armyworm [Spodoptera frugiperda (J.E. Smith)] (FAW) feeding and plant growth in sorghum [Sorghum bicolor (L.) Moench]. A genetic mutation in CYP79A1 gene that disrupts dhurrin biosynthesis was used to develop sets of near-isogenic lines (NILs) with contrasting dhurrin contents in the Tx623 bmr6 genetic background. The NILs were evaluated for differences in plant growth and FAW feeding damage in replicated greenhouse and field trials. Greenhouse studies showed that dhurrin-free Tx623 bmr6 cyp79a1 plants grew more quickly than wild-type plants but were more susceptible to insect feeding based on changes in green plant area (GPA), total leaf area, and total dry weight over time. The NILs exhibited similar patterns of growth in field trials with significant differences in leaf area and dry weight of dhurrin-free plants between the infested and non-infested treatments. Taken together, these studies reveal a significant metabolic tradeoff between CG biosynthesis and plant growth in sorghum seedlings. Disruption of dhurrin biosynthesis produces plants with higher growth rates than wild-type plants but these plants have greater susceptibility to FAW feeding.

Dhurrin increases but does not mitigate oxidative stress in droughted Sorghum bicolor

Droughted sorghum had higher concentrations of ROS in both wildtype and dhurrin-lacking mutants. Dhurrin increased in wildtype genotypes with drought. Dhurrin does not appear to mitigate oxidative stress in sorghum. Sorghum bicolor is tolerant of high temperatures and prolonged droughts. During droughts, concentrations of dhurrin, a cyanogenic glucoside, increase posing a risk to livestock of hydrogen cyanide poisoning. Dhurrin can also be recycled without the release of hydrogen cyanide presenting the possibility that it may have functions other than defence. It has been hypothesised that dhurrin may be able to mitigate oxidative stress by scavenging reactive oxygen species (ROS) during biosynthesis and recycling. To test this, we compared the growth and chemical composition of S. bicolor in total cyanide deficient sorghum mutants (tcd1) with wild-type plants that were either well-watered or left unwatered for 2 weeks. Plants from the adult cyanide deficient class of mutant (acdc1) were also included. Foliar dhurrin increased in response to drought in all lines except tcd1 and acdc1, but not in the roots or leaf sheaths. Foliar ROS concentration increased in drought-stressed plants in all genotypes. Phenolic concentrations were also measured but no differences were detected. The total amounts of dhurrin, ROS and phenolics on a whole plant basis were lower in droughted plants due to their smaller biomass, but there were no significant genotypic differences. Up until treatments began at the 3-leaf stage, tcd1 mutants grew more slowly than the other genotypes but after that they had higher relative growth rates, even when droughted. The findings presented here do not support the hypothesis that the increase in dhurrin commonly seen in drought-stressed sorghum plays a role in reducing oxidative stress by scavenging ROS.

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

MAIN CONCLUSION

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.

Cyanogenesis in the Sorghum Genus: From Genotype to Phenotype

Domestication has resulted in a loss of genetic diversity in our major food crops, leading to susceptibility to biotic and abiotic stresses linked with climate change. Crop wild relatives (CWR) may provide a source of novel genes potentially important for re-gaining climate resilience. Sorghum bicolor is an important cereal crop with wild relatives that are endemic to Australia. Sorghum bicolor is cyanogenic, but the cyanogenic status of wild Sorghum species is not well known. In this study, leaves of wild species endemic in Australia are screened for the presence of the cyanogenic glucoside dhurrin. The direct measurement of dhurrin content and the potential for dhurrin-derived HCN release (HCNp) showed that all the tested Australian wild species were essentially phenotypically acyanogenic. The unexpected low dhurrin content may reflect the variable and generally nutrient-poor environments in which they are growing in nature. Genome sequencing of six CWR and PCR amplification of the CYP79A1 gene from additional species showed that a high conservation of key amino acids is required for correct protein function and dhurrin synthesis, pointing to the transcriptional regulation of the cyanogenic phenotype in wild sorghum as previously shown in elite sorghum.

Evaluation of the molecular and physiological response to dehydration of two accessions of the model plant Setaria viridis

Water deficits are responsible for countless agricultural losses. Among the affected crops, C4 plants are of special interest due to their high water and nitrogen use efficiency. Two accessions of Setaria viridis (Ast-1 and A10.1) with contrasting responses to water deficit were used in the current work to better understand the mechanisms behind drought tolerance in C4 species. Our results showed that although the A10.1 accession exhibited a reduced size and lower Rfd values in comparison to Ast-1, it had overall higher Fv/Fm and lower NPQ values in well-watered conditions. The water deficit induction was performed with PEG-8000 at the grain-filling stage using dehydration cycles. Analysis of physiological measurements showed the A10.1 accession as being more tolerant to multiple water deficit exposures. In addition, PCA identified a clear difference in the pattern of drought response of the accessions. Four drought marker genes previously described in the literature were chosen to evaluate the response at the molecular level: SvP5CS2, SvDHN1, SvNAC6, and SvWRKY1. Besides confirming that Ast-1 is a more sensitive accession, the expression analysis revealed that SvNAC1 might better monitor drought stress, while SvWRKY1 was able to differentiate the two accessions. Distinct evolutionary histories of each accession may be behind their differences in response to water deficits.

Regulation of dhurrin pathway gene expression during Sorghum bicolor development

Developmental and organ-specific expression of genes in dhurrin biosynthesis, bio-activation, and recycling offers dynamic metabolic responses optimizing growth and defence responses in Sorghum. Plant defence models evaluate the costs and benefits of resource investments at different stages in the life cycle. Poor understanding of the molecular regulation of defence deployment and remobilization hampers accuracy of the predictions. Cyanogenic glucosides, such as dhurrin are phytoanticipins that release hydrogen cyanide upon bio-activation. In this study, RNA-seq was used to investigate the expression of genes involved in the biosynthesis, bio-activation and recycling of dhurrin in Sorghum bicolor. Genes involved in dhurrin biosynthesis were highly expressed in all young developing vegetative tissues (leaves, leaf sheath, roots, stems), tiller buds and imbibing seeds and showed gene specific peaks of expression in leaves during diel cycles. Genes involved in dhurrin bio-activation were expressed early in organ development with organ-specific expression patterns. Genes involved in recycling were expressed at similar levels in the different organ during development, although post-floral initiation when nutrients are remobilized for grain filling, expression of GSTL1 decreased > tenfold in leaves and NITB2 increased > tenfold in stems. Results are consistent with the establishment of a pre-emptive defence in young tissues and regulated recycling related to organ senescence and increased demand for nitrogen during grain filling. This detailed characterization of the transcriptional regulation of dhurrin biosynthesis, bioactivation and remobilization genes during organ and plant development will aid elucidation of gene regulatory networks and signalling pathways that modulate gene expression and dhurrin levels. In-depth knowledge of dhurrin metabolism could improve the yield, nitrogen use efficiency and stress resilience of Sorghum.

Effects of Salinity on the Growth and Nutrition of Taro (Colocasia esculenta): Implications for Food Security

Taro (Colocasia esculenta (L.) Schott) is a staple food crop in the Asia-Pacific region in areas where rising sea levels are threatening agricultural production. However, little is known about its response to salinity. In this study, we investigated the effects of salinity on the growth, morphology, physiology, and chemical traits of taro to predict the impacts of rising sea levels on taro production and nutritional value in the Pacific. We grew taro (approximately 4 months old) with a range of NaCl treatments (0–200 mM) for 12 weeks. Full nutrient, micronutrient, and secondary metabolite analyses were conducted, including measures of calcium oxalate (CaOx), an irritant that reduces palatability. Significant reductions in growth and biomass were observed at and above 100 mM NaCl. Concentrations of macro- and micronutrients, including sodium, were higher on a per mass basis in corms of plants experiencing salt stress. Foliar sodium concentrations remained stable, indicating that taro may utilize a salt exclusion mechanism. There was a large amount of individual variation in the concentrations of oxalate and phenolics, but overall, the concentrations were similar in the plants grown with different levels of salt. The total contents of CaOx and phenolics decreased in plants experiencing salt stress. Taro’s ability to survive and produce corms when watered with a 200 mM NaCl solution places it among the salt-tolerant non-halophytes. The nutritional quality of the crop is only marginally affected by salt stress. Taro is, therefore, likely to remain a useful staple in the Pacific region in the future.

Exogenous silicon and hydrogen sulfide alleviates the simultaneously occurring drought stress and leaf rust infection in wheat

Silicon (Si) and hydrogen sulfide (H₂S) are known to enhance plant defense against multiple stresses. Current study was conducted to investigate the application of Si and H₂S alone as well as in combination, improved physiological resilience of wheat plants to drought stress (DS) and pathogen-Puccinia triticina (Pt) infection. We aimed to increase the wheat plant growth and to enhance the DS tolerance and Pt resistance with the concurrent applications of H₂S and Si. In the first experiment, we selected the best growth enhancing concentration of H₂S (0.3 mM) and Si (6 mM) to further investigate their tolerance and resistance potential in the pot experiment under DS and pathogen infection conditions. The obtained results reveal that DS has further increased the susceptibility of wheat plants to leaf rust pathogen infection while, the sole application of Si and the simultaneous exogenous treatments of H₂S + Si enhanced the plant growth, decreased disease incidence, and significantly improved tolerance and defense mechanisms of wheat under individual and interactive stress conditions. The exogenous treatment of H₂S + Si improved the growth criteria, photosynthetic pigments, osmoprotectants, and defense related enzyme activities. The same treatment also reinforced the endogenous H₂S, Si, ABA and SA contents while decreased the disease incidence and oxidative stress indicators under individual and combined stress conditions. Overall, results from this study presents the influence of combined drought and P. triticina stress in wheat and reveal the beneficial impacts of concurrent exogenous treatment of H₂S + Si to mitigate the drought and pathogen (P. triticina) induced adverse effects.

Genome-wide association study of cyanogenic glycosides, proline, sugars, and pigments in Eucalyptus cladocalyx after 18 consecutive dry summers

Natural variation of cyanogenic glycosides, soluble sugars, proline, and nondestructive optical sensing of pigments (chlorophyll, flavonols, and anthocyanins) was examined in ex situ natural populations of Eucalyptus cladocalyx F. Muell. grown under dry environmental conditions in the southern Atacama Desert, Chile. After 18 consecutive dry seasons, considerable plant-to-plant phenotypic variation for all the traits was observed in the field. For example, leaf hydrogen cyanide (HCN) concentrations varied from 0 (two acyanogenic individuals) to 1.54 mg cyanide g^-1 DW. Subsequent genome-wide association study revealed associations with several genes with a known function in plants. HCN content was associated robustly with genes encoding Cytochrome P450 proteins, and with genes involved in the detoxification mechanism of HCN in cells (β-cyanoalanine synthase and cyanoalanine nitrilase). Another important finding was that sugars, proline, and pigment content were linked to genes involved in transport, biosynthesis, and/or catabolism. Estimates of genomic heritability (based on haplotypes) ranged between 0.46 and 0.84 (HCN and proline content, respectively). Proline and soluble sugars had the highest predictive ability of genomic prediction models (PA = 0.65 and PA = 0.71, respectively). PA values for HCN content and flavonols were relatively moderate, with estimates ranging from 0.44 to 0.50. These findings provide new understanding on the genetic architecture of cyanogenic capacity, and other key complex traits in cyanogenic E. cladocalyx.

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.

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.

Correlation-based network analysis combined with machine learning techniques highlight the role of the GABA shunt in Brachypodium sylvaticum freezing tolerance

Perennial grasses will account for approximately 16 billion gallons of renewable fuels by the year 2022, contributing significantly to carbon and nitrogen sequestration. However, perennial grasses productivity can be limited by severe freezing conditions in some geographical areas, although these risks could decrease with the advance of climate warming, the possibility of unpredictable early cold events cannot be discarded. We conducted a study on the model perennial grass Brachypodium sylvaticum to investigate the molecular mechanisms that contribute to cold and freezing adaption. The study was performed on two different B. sylvaticum accessions, Ain1 and Osl1, typical to warm and cold climates, respectively. Both accessions were grown under controlled conditions with subsequent cold acclimation followed by freezing stress. For each treatment a set of morphological parameters, transcription, metabolite, and lipid profiles were measured. State-of-the-art algorithms were employed to analyze cross-component relationships. Phenotypic analysis revealed higher adaption of Osl1 to freezing stress. Our analysis highlighted the differential regulation of the TCA cycle and the GABA shunt between Ain1 and Osl1. Osl1 adapted to freezing stress by repressing the GABA shunt activity, avoiding the detrimental reduction in fatty acid biosynthesis and the concomitant detrimental effects on membrane integrity.

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.

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.

Counting the costs: nitrogen partitioning in Sorghum mutants

Long-standing growth/defence theories state that the production of defence compounds come at a direct cost to primary metabolism when resources are limited. However, such trade-offs are inherently difficult to quantify. We compared the growth and nitrogen partitioning in wild type Sorghum bicolor (L.) Moench, which contains the cyanogenic glucoside dhurrin, with unique mutants that vary in dhurrin production. The totally cyanide deficient 1 (tcd1) mutants do not synthesise dhurrin at all whereas mutants from the adult cyanide deficient class 1 (acdc1) have decreasing concentrations as plants age. Sorghum lines were grown at three different concentrations of nitrogen. Growth, chemical analysis, physiological measurements and expression of key genes in biosynthesis and turnover were determined for leaves, stems and roots at four developmental stages. Nitrogen supply, ontogeny, tissue type and genotype were all important determinants of tissue nitrate and dhurrin concentration and turnover. The higher growth of acdc1 plants strongly supports a growth/defence trade-off. By contrast, tcd1 plants had slower growth early in development, suggesting that dhurrin synthesis and turnover may be beneficial for early seedling growth rather than being a cost. The relatively small trade-off between nitrate and dhurrin suggests these may be independently regulated.

Label-free Raman hyperspectral imaging analysis localizes the cyanogenic glucoside dhurrin to the cytoplasm in sorghum cells

Localisation of metabolites in sorghum coleoptiles using Raman hyperspectral imaging analysis was compared in wild type plants and mutants that lack cyanogenic glucosides. This novel method allows high spatial resolution in situ localization by detecting functional groups associated with cyanogenic glucosides using vibrational spectroscopy. Raman hyperspectral imaging revealed that dhurrin was found mainly surrounding epidermal, cortical and vascular tissue, with the greatest amount in cortical tissue. Numerous "hotspots" demonstrated dhurrin to be located within both cell walls and cytoplasm adpressed towards the plasmamembrane and not in the vacuole as previously reported. The high concentration of dhurrin in the outer cortical and epidermal cell layers is consistent with its role in defence against herbivory. This demonstrates the ability of Raman hyperspectral imaging to locate cyanogenic glucosides in intact tissues, avoiding possible perturbations and imprecision that may accompany methods that rely on bulk tissue extraction methods, such as protoplast isolation.

Comparative Transcriptome and Lipidome Analyses Reveal Molecular Chilling Responses in Chilling-Tolerant Sorghums

Chilling temperatures (0 to 15°C) are a major constraint for temperate cultivation of tropical-origin crops, including the cereal crop sorghum ( [L.] Moench). Northern Chinese sorghums have adapted to early-season chilling, but molecular mechanisms of chilling tolerance are unknown. We used RNA sequencing of seedlings to compare the chilling-responsive transcriptomes of a chilling-tolerant Chinese accession with a chilling-sensitive US reference line, and mass spectrometry to compare chilling-responsive lipidomes of four chilling-tolerant Chinese accessions with two US reference lines. Comparative transcriptomics revealed chilling-induced up-regulation of cold-response regulator C-repeat binding factor (CBF) transcription factor and genes involved in reactive oxygen detoxification, jasmonic acid (JA) biosynthesis, and lipid remodeling phospholipase Dα1 (α) gene in the chilling-tolerant Chinese line. Lipidomics revealed conserved chilling-induced increases in lipid unsaturation, as well as lipid remodeling of photosynthetic membranes that is specific to chilling-tolerant Chinese accessions. Our results point to CBF-mediated transcriptional regulation, galactolipid and phospholipid remodeling, and JA as potential molecular mechanisms underlying chilling adaptation in Chinese sorghums. These molecular systems underlying chilling response could be targeted in molecular breeding for chilling tolerance.

PEG-induced osmotic stress in Mentha x piperita L.: Structural features and metabolic responses

The present study investigated whether osmotic stress induced by the exposure of peppermint (Mentha x piperita L.) to moderate and severe stress for short periods of time changes the plant's physiological parameters, leaf anatomy and ultrastructure and essential oil. Plants were exposed to two levels of polyethyleneglycol (50 g L(-1) and 100 g L(-1) of PEG) in a hydroponic experiment. The plants exposed to 50 g L(-1) maintained metabolic functions similar to those of the control group (0 g L(-1)) without changes in gas exchange or structural characteristics. The increase in antioxidant enzyme activity reduced the presence of free radicals and protected membranes, including chloroplasts and mitochondria. In contrast, the osmotic stress caused by 100 g L(-1) of PEG inhibited leaf gas exchange, reduced the essential oil content and changed the oil composition, including a decrease in menthone and an increase in menthofuran. These plants also showed an increase in peroxidase activity, but this increase was not sufficient to decrease the lipid peroxidation level responsible for damaging the membranes of organelles. Morphological changes were correlated with the evaluated physiological features: plants exposed to 100 g L(-1) of PEG showed areas with collapsed cells, increases in mesophyll thickness and the area of the intercellular space, cuticle shrinkage, morphological changes in plastids, and lysis of mitochondria. In summary, our results revealed that PEG-induced osmotic stress in M. x piperita depends on the intensity level of the osmotic stress applied; severe osmotic stress changed the structural characteristics, caused damage at the cellular level, and reduced the essential oil content and quality.

Occurrence of Benzoic Acid Esters as Putative Catabolites of Prunasin in Senescent Leaves of Prunus laurocerasus

Prunus laurocerasus is an evergreen shrub containing large quantities of the cyanogenic glycoside prunasin (1) in its leaves, which decomposes to prunasin amide (2) or glucose-1-benzoate (4) when the leaves become chlorotic as a result of senescence or pseudosenescence. This study was aimed at the systematic identification of senescence-associated metabolites to contribute further insight into the catabolism of 1. LC-ESIMS profiles of senescent and green leaves were analyzed by principal component analysis. In senescent leaves, the concentrations of 36 compounds were increased significantly including several benzoic acid derivatives, of which prunasin amide-6'-benzoate (5) and prunasin acid-6'-benzoate (6) were isolated and identified. The observed metabolic changes were also induced by treatment of P. laurocerasus shrubs with exogenous ethylene. The data presented support an oxidative catabolism of 1 without release of hydrogen cyanide and the remobilization of its nitrogen in the course of senescence. The results are discussed in the context of functional diversification and drug discovery, where senescent plant material represents a widely unexplored source for the discovery of natural products.

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.

Cyanogenic glycosides: synthesis, physiology, and phenotypic plasticity

Cyanogenic glycosides (CNglcs) are bioactive plant products derived from amino acids. Structurally, these specialized plant compounds are characterized as α-hydroxynitriles (cyanohydrins) that are stabilized by glucosylation. In recent years, improved tools within analytical chemistry have greatly increased the number of known CNglcs by enabling the discovery of less abundant CNglcs formed by additional hydroxylation, glycosylation, and acylation reactions. Cyanogenesis--the release of toxic hydrogen cyanide from endogenous CNglcs--is an effective defense against generalist herbivores but less effective against fungal pathogens. In the course of evolution, CNglcs have acquired additional roles to improve plant plasticity, i.e., establishment, robustness, and viability in response to environmental challenges. CNglc concentration is usually higher in young plants, when nitrogen is in ready supply, or when growth is constrained by nonoptimal growth conditions. Efforts are under way to engineer CNglcs into some crops as a pest control measure, whereas in other crops efforts are directed toward their removal to improve food safety. Given that many food crops are cyanogenic, it is important to understand the molecular mechanisms regulating cyanogenesis so that the impact of future environmental challenges can be anticipated.