Over-expression of hydroxynitrile lyase in transgenic cassava roots accelerates cyanogenesis and food detoxification

CRISPR/Cas9: an advanced platform for root and tuber crops improvement

Root and tuber crops (RTCs), which include cassava, potato, sweet potato, and yams, principally function as staple crops for a considerable fraction of the world population, in addition to their diverse applications in nutrition, industry, and bioenergy sectors. Even then, RTCs are an underutilized group considering their potential as industrial raw material. Complexities in conventional RTC improvement programs curb the extensive exploitation of the potentials of this group of crop species for food, energy production, value addition, and sustainable development. Now, with the advent of whole-genome sequencing, sufficient sequence data are available for cassava, sweet potato, and potato. These genomic resources provide enormous scope for the improvement of tuber crops, to make them better suited for agronomic and industrial applications. There has been remarkable progress in RTC improvement through the deployment of new strategies like gene editing over the last decade. This review brings out the major areas where CRISPR/Cas technology has improved tuber crops. Strategies for genetic transformation of RTCs with CRISPR/Cas9 constructs and regeneration of edited lines and the bottlenecks encountered in their establishment are also discussed. Certain attributes of tuber crops requiring focus in future research along with putative editing targets are also indicated. Altogether, this review provides a comprehensive account of developments achieved, future lines of research, bottlenecks, and major experimental concerns regarding the establishment of CRISPR/Cas9-based gene editing in RTCs.

Cyanogenesis, a Plant Defence Strategy against Herbivores

Plants and phytophagous arthropods have coevolved in a long battle for survival. Plants respond to phytophagous feeders by producing a battery of antiherbivore chemical defences, while herbivores try to adapt to their hosts by attenuating the toxic effect of the defence compounds. Cyanogenic glucosides are a widespread group of defence chemicals that come from cyanogenic plants. Among the non-cyanogenic ones, the Brassicaceae family has evolved an alternative cyanogenic pathway to produce cyanohydrin as a way to expand defences. When a plant tissue is disrupted by an herbivore attack, cyanogenic substrates are brought into contact with degrading enzymes that cause the release of toxic hydrogen cyanide and derived carbonyl compounds. In this review, we focus our attention on the plant metabolic pathways linked to cyanogenesis to generate cyanide. It also highlights the role of cyanogenesis as a key defence mechanism of plants to fight against herbivore arthropods, and we discuss the potential of cyanogenesis-derived molecules as alternative strategies for pest control.

Biotechnological detoxification: an unchanging source-sink balance strategy for crop improvement

The wide occurrence of natural phytotoxins renders many crops unfit for human consumption. To overcome this problem and produce detoxified crop varieties, we propose the use of biotechnological strategies that can enhance the harvest index without the need to increase crop biomass or alter whole plant architecture.

Hydroxynitrile lyase defends Arabidopsis against Tetranychus urticae

Plant-pest interactions involve multifaceted processes encompassing a complex crosstalk of pathways, molecules, and regulators aimed at overcoming defenses developed by each interacting organism. Among plant defensive compounds against phytophagous arthropods, cyanide-derived products are toxic molecules that directly target pest physiology. Here, we identified the Arabidopsis (Arabidopsis thaliana) gene encoding hydroxynitrile lyase (AtHNL, At5g10300) as one gene induced in response to spider mite (Tetranychus urticae) infestation. AtHNL catalyzes the reversible interconversion between cyanohydrins and derived carbonyl compounds with free cyanide. AtHNL loss- and gain-of-function Arabidopsis plants showed that specific activity of AtHNL using mandelonitrile as substrate was higher in the overexpressing lines than in wild-type (WT) and mutant lines. Concomitantly, mandelonitrile accumulated at higher levels in mutant lines than in WT plants and was significantly reduced in the AtHNL overexpressing lines. After mite infestation, mandelonitrile content increased in WT and overexpressing plants but not in mutant lines, while hydrogen cyanide (HCN) accumulated in the three infested Arabidopsis genotypes. Feeding bioassays demonstrated that the AtHNL gene participated in Arabidopsis defense against T. urticae. The reduced leaf damage detected in the AtHNL overexpressing lines reflected the mite's reduced ability to feed on leaves, which consequently restricted mite fecundity. In turn, mites upregulated TuCAS1 encoding β-cyanoalanine synthase to avoid the respiratory damage produced by HCN. This detoxification effect was functionally demonstrated by reduced mite fecundity observed when dsRNA-TuCAS-treated mites fed on WT plants and hnl1 mutant lines. These findings add more players in the Arabidopsis-T. urticae interplay to overcome mutual defenses.

Cyanogenesis in cassava and its molecular manipulation for crop improvement

While cassava is one of the most important staple crops worldwide, it has received the least investment per capita consumption of any of the major global crops. This is in part due to cassava being a crop of subsistence farmers that is grown in countries with limited resources for crop improvement. While its starchy roots are rich in calories, they are poor in protein and other essential nutrients. In addition, they contain potentially toxic levels of cyanogenic glycosides which must be reduced to safe levels before consumption. Furthermore, cyanogens compromise the shelf life of harvested roots due to cyanide-induced inhibition of mitochondrial respiration, and associated production of reactive oxygen species that accelerate root deterioration. Over the past two decades, the genetic, biochemical, and developmental factors that control cyanogen synthesis, transport, storage, and turnover have largely been elucidated. It is now apparent that cyanogens contribute substantially to whole-plant nitrogen metabolism and protein synthesis in roots. The essential role of cyanogens in root nitrogen metabolism, however, has confounded efforts to create acyanogenic varieties. This review proposes alternative molecular approaches that integrate accelerated cyanogen turnover with nitrogen reassimilation into root protein that may offer a solution to creating a safer, more nutritious cassava crop.

Two fatal cases of acetone cyanohydrin poisoning: case report and literature review

Acetone cyanohydrin (ACH), an organic cyanide, is mainly used in the production of methyl methacrylate (MMA), and it also exists in cassava roots, the main calorie source in some tropical countries. ACH can decompose spontaneously or enzymatically into acetone and highly toxic hydrogen cyanide (HCN) and be potentially toxic to its contacts. Given that limited forensic studies and case reports on fatal ACH poisoning are available, herein, we present a report of two fatal cases of ACH poisoning in which the two victims, with postmortem cyanide blood concentrations of 4.22 μg/ml and 4.07 μg/ml, suffered from acute poisoning of ACH due to a traffic accident. Furthermore, a literature review of cyanide poisoning case reports from 2000 to 2020 was carried out, and 28 subjects with cyanide poisoning were presented, including the age, sex, cause of poisoning, autopsy findings and the cyanide concentration in the blood. ACH poisoning lacks specific and reliable autopsy findings for diagnosis, and relevant toxicological studies are necessary. Due to the chemical properties of ACH that allow it to easily decompose, the toxicological analysis of acetone and cyanide in biological samples is essential for the diagnosis of ACH poisoning.

Efficient mesophyll protoplast isolation and development of a transient expression system for castor-oil plant (Ricinus communis L.)

INTRODUCTION

We investigated the main factors affecting the efficacy of protoplast isolation, including leaf-obtaining period, cutting shapes of leaf material, enzyme concentration, enzymolysis time, and centrifugal speed.

METHODS

Protoplast isolation was optimal on the condition of 20 days of leaf materials, 2-mm filament of leaves, 1.6% RS and 0.8% R-10, 80 min of enzymolysis, and 700 rpm of centrifugation, resulting in the best yield (1.19 X 106 protoplasts/g FW) and vitality (80.34%) of mesophyll protoplasts. The transient expression vector pGFPl with green fluorescent protein was transfected into the obtained protoplasts from castor by polyethylene glycol-mediated method with a transformation efficiency of 12.37%.

RESULTS

Moreover, the applicability of the system for studying the subcellular localization of Re FATA (an acyl-ACP thioesterase) was validated via the protoplast isolation and transient expression protocol in this study.

DISCUSSION

Collectively, the efficient mesophyll protoplast isolation and protoplast transient expression system facilitate to analyze the function of specific gene in castor (Ricinus communis L).

Highly efficient mesophyll protoplast isolation and PEG-mediated transient gene expression for rapid and large-scale gene characterization in cassava (Manihot esculenta Crantz)

BACKGROUND

Cassava (Manihot esculenta Crantz) is a major crop extensively cultivated in the tropics as both an important source of calories and a promising source for biofuel production. Although stable gene expression have been used for transgenic breeding and gene function study, a quick, easy and large-scale transformation platform has been in urgent need for gene functional characterization, especially after the cassava full genome was sequenced.

METHODS

Fully expanded leaves from in vitro plantlets of Manihot esculenta were used to optimize the concentrations of cellulase R-10 and macerozyme R-10 for obtaining protoplasts with the highest yield and viability. Then, the optimum conditions (PEG4000 concentration and transfection time) were determined for cassava protoplast transient gene expression. In addition, the reliability of the established protocol was confirmed for subcellular protein localization.

RESULTS

In this work we optimized the main influencing factors and developed an efficient mesophyll protoplast isolation and PEG-mediated transient gene expression in cassava. The suitable enzyme digestion system was established with the combination of 1.6% cellulase R-10 and 0.8% macerozyme R-10 for 16 h of digestion in the dark at 25 °C, resulting in the high yield (4.4 × 107 protoplasts/g FW) and vitality (92.6%) of mesophyll protoplasts. The maximum transfection efficiency (70.8%) was obtained with the incubation of the protoplasts/vector DNA mixture with 25% PEG4000 for 10 min. We validated the applicability of the system for studying the subcellular localization of MeSTP7 (an H+/monosaccharide cotransporter) with our transient expression protocol and a heterologous Arabidopsis transient gene expression system.

CONCLUSION

We optimized the main influencing factors and developed an efficient mesophyll protoplast isolation and transient gene expression in cassava, which will facilitate large-scale characterization of genes and pathways in cassava.

Cassava postharvest physiological deterioration: a complex phenomenon involving calcium signaling, reactive oxygen species and programmed cell death

Postharvest physiological deterioration (PPD) of cassava (Manihot esculenta) storage roots is a complex physiological and biochemical process which involve many regulatory networks linked with specific proteins modulation and signaling transduction pathways. However, it is poorly understood regarding biological regulation, and the interactions among protein groups and signals to determine PPD syndrome in cassava storage roots. This review sheds some light on the possible molecular mechanisms involved in reactive oxygen species (ROS), calcium signaling transduction, and programmed cell death (PCD) in cassava PPD syndrome. A model for predicting crosstalk among calcium signaling, ROS and PCD is suggested to fine-tune PPD syndrome. This would clues to cassava molecular breeding to alleviate the PPD effects on the shelf-life.

Cyanogen Metabolism in Cassava Roots: Impact on Protein Synthesis and Root Development

Cassava (Manihot esculenta Crantz), a staple crop for millions of sub-Saharan Africans, contains high levels of cyanogenic glycosides which protect it against herbivory. However, cyanogens have also been proposed to play a role in nitrogen transport from leaves to roots. Consistent with this hypothesis, analyses of the distribution and activities of enzymes involved in cyanide metabolism provides evidence for cyanide assimilation, derived from linamarin, into amino acids in cassava roots. Both β-cyanoalanine synthase (CAS) and nitrilase (NIT), two enzymes involved in cyanide assimilation to produce asparagine, were observed to have higher activities in roots compared to leaves, consistent with their proposed role in reduced nitrogen assimilation. In addition, rhodanese activity was not detected in cassava roots, indicating that this competing means for cyanide metabolism was not a factor in cyanide detoxification. In contrast, leaves had sufficient rhodanese activity to compete with cyanide assimilation into amino acids. Using transgenic low cyanogen plants, it was shown that reducing root cyanogen levels is associated with elevated root nitrate reductase activity, presumably to compensate for the loss of reduced nitrogen from cyanogens. Finally, we overexpressed Arabidopsis CAS and NIT4 genes in cassava roots to study the feasibility of enhancing root cyanide assimilation into protein. Optimal overexpression of CAS and NIT4 resulted in up to a 50% increase in root total amino acids and a 9% increase in root protein accumulation. However, plant growth and morphology was altered in plants overexpressing these enzymes, demonstrating a complex interaction between cyanide metabolism and hormonal regulation of plant growth.

Mechanisms and ecological consequences of plant defence induction and suppression in herbivore communities

BACKGROUND

Plants are hotbeds for parasites such as arthropod herbivores, which acquire nutrients and energy from their hosts in order to grow and reproduce. Hence plants are selected to evolve resistance, which in turn selects for herbivores that can cope with this resistance. To preserve their fitness when attacked by herbivores, plants can employ complex strategies that include reallocation of resources and the production of defensive metabolites and structures. Plant defences can be either prefabricated or be produced only upon attack. Those that are ready-made are referred to as constitutive defences. Some constitutive defences are operational at any time while others require activation. Defences produced only when herbivores are present are referred to as induced defences. These can be established via de novo biosynthesis of defensive substances or via modifications of prefabricated substances and consequently these are active only when needed. Inducibility of defence may serve to save energy and to prevent self-intoxication but also implies that there is a delay in these defences becoming operational. Induced defences can be characterized by alterations in plant morphology and molecular chemistry and are associated with a decrease in herbivore performance. These alterations are set in motion by signals generated by herbivores. Finally, a subset of induced metabolites are released into the air as volatiles and function as a beacon for foraging natural enemies searching for prey, and this is referred to as induced indirect defence.

SCOPE

The objective of this review is to evaluate (1) which strategies plants have evolved to cope with herbivores and (2) which traits herbivores have evolved that enable them to counter these defences. The primary focus is on the induction and suppression of plant defences and the review outlines how the palette of traits that determine induction/suppression of, and resistance/susceptibility of herbivores to, plant defences can give rise to exploitative competition and facilitation within ecological communities "inhabiting" a plant.

CONCLUSIONS

Herbivores have evolved diverse strategies, which are not mutually exclusive, to decrease the negative effects of plant defences in order to maximize the conversion of plant material into offspring. Numerous adaptations have been found in herbivores, enabling them to dismantle or bypass defensive barriers, to avoid tissues with relatively high levels of defensive chemicals or to metabolize these chemicals once ingested. In addition, some herbivores interfere with the onset or completion of induced plant defences, resulting in the plant's resistance being partly or fully suppressed. The ability to suppress induced plant defences appears to occur across plant parasites from different kingdoms, including herbivorous arthropods, and there is remarkable diversity in suppression mechanisms. Suppression may strongly affect the structure of the food web, because the ability to suppress the activation of defences of a communal host may facilitate competitors, whereas the ability of a herbivore to cope with activated plant defences will not. Further characterization of the mechanisms and traits that give rise to suppression of plant defences will enable us to determine their role in shaping direct and indirect interactions in food webs and the extent to which these determine the coexistence and persistence of species.

Cassava (Manihot esculenta Crantz)

Genetic transformation of plants is an indispensable technique used for fundamental research and crop improvement. Recent advances in cassava (Manihot esculenta Crantz) transformation have facilitated the effective generation of stably transformed cassava plants with favorable traits. Agrobacterium-mediated transformation of friable, embryogenic callus has evolved to become the most widely used approach and has been adopted by research laboratories in Africa. This procedure utilizes axillary meristem tissue (buds) to produce primary and secondary somatic embryos and subsequently friable, embryogenic callus. Agrobacterium harboring a binary expression cassette is used to transform this tissue, which is regenerated via cotyledons and shoot organogenesis to produce rooted in vitro plantlets. This chapter details each step of the procedure using the model cultivar 60444 and provides supplementary notes to successfully produce transgenic cassava.

Induction of hairy roots in Arnica montana L. by Agrobacterium rhizogenes

The induction of hairy roots in Arnica montana L. by Agrobacterium rhizogenes mediated system was established. The frequency of genetic transformation varied from 4.8 to 12% depended on method of infection. The cefotaxime at concentration of 200 mg/l proved to suppress effectively the growth of A. rhizogenes after co-cultivation. Among the three tested nutrient media: Murashige and Skoog (MS), Gamborg’s (B5) and Schenk and Hildebrandt (SH), MS medium was superior for growth and high biomass production of transformed roots compared to other culture media. After culturing for 40 days the fresh weight of clone T4 increased 7.6 fold over the non-transformed roots. The transfer of rol A, rol B and rol C genes into Arnica genome was confirmed by PCR analysis. Established genetic transformation techniques in A. montana efficiently provided and generated a large number of transformed roots — an excellent system for studying gene function and could be used for the production of secondary metabolites synthesized in roots.

Extending cassava root shelf life via reduction of reactive oxygen species production

One of the major constraints facing the large-scale production of cassava (Manihot esculenta) roots is the rapid postharvest physiological deterioration (PPD) that occurs within 72 h following harvest. One of the earliest recognized biochemical events during the initiation of PPD is a rapid burst of reactive oxygen species (ROS) accumulation. We have investigated the source of this oxidative burst to identify possible strategies to limit its extent and to extend cassava root shelf life. We provide evidence for a causal link between cyanogenesis and the onset of the oxidative burst that triggers PPD. By measuring ROS accumulation in transgenic low-cyanogen plants with and without cyanide complementation, we show that PPD is cyanide dependent, presumably resulting from a cyanide-dependent inhibition of respiration. To reduce cyanide-dependent ROS production in cassava root mitochondria, we generated transgenic plants expressing a codon-optimized Arabidopsis (Arabidopsis thaliana) mitochondrial alternative oxidase gene (AOX1A). Unlike cytochrome c oxidase, AOX is cyanide insensitive. Transgenic plants overexpressing AOX exhibited over a 10-fold reduction in ROS accumulation compared with wild-type plants. The reduction in ROS accumulation was associated with a delayed onset of PPD by 14 to 21 d after harvest of greenhouse-grown plants. The delay in PPD in transgenic plants was also observed under field conditions, but with a root biomass yield loss in the highest AOX-expressing lines. These data reveal a mechanism for PPD in cassava based on cyanide-induced oxidative stress as well as PPD control strategies involving inhibition of ROS production or its sequestration.

The cyanogenic syndrome in rubber tree Hevea brasiliensis: tissue-damage-dependent activation of linamarase and hydroxynitrile lyase accelerates hydrogen cyanide release

BACKGROUND AND AIMS

The release of hydrogen cyanide (HCN) from injured plant tissue affects multiple ecological interactions. Plant-derived HCN can act as a defence against herbivores and also plays an important role in plant-pathogen interactions. Crucial for activity as a feeding deterrent is the amount of HCN generated per unit time, referred to as cyanogenic capacity (HCNc). Strong intraspecific variation in HCNc has been observed among cyanogenic plants. This variation, in addition to genotypic variability (e.g. in Trifolium repens), can result from modifications in the expression level of the enzymes involved in either cyanogenic precursor formation or HCN release (as seen in Sorghum bicolor and Phaseolus lunatus). Thus, a modification or modulation of HCNc in reaction to the environment can only be achieved from one to the next generation when under genetic control and within days or hours when transcriptional regulations are involved. In the present study, it is shown that in rubber tree (Hevea brasiliensis) HCNc is modulated by post-translational activity regulation of the key enzymes for cyanide release.

METHODS

Linamarase (LIN) and hydroxynitrile lyase (HNL) activity was determined by colorimetric assays utilizing dissociation of the substrates p-nitrophenyl-β-d-glucopyranoside and acetone cyanohydrin, respectively.

KEY RESULTS

In rubber tree leaves, LIN and HNL show up to ten-fold increased activity in response to tissue damage. This enzyme activation occurs within seconds and results in accelerated HCN formation. It is restricted to the damaged leaf area and depends on the severity of tissue damage.

CONCLUSIONS

LIN and HNL activation (in contrast to genetic and transcriptional regulations) allows an immediate, local and damage type-dependent modulation of the cyanogenic response. Accordingly, this post-translational activation plays a decisive role in the defence of H. brasiliensis against herbivores as well as pathogens and may allow more flexible reactions in response to these different antagonists.

Overexpression of hydroxynitrile lyase in cassava roots elevates protein and free amino acids while reducing residual cyanogen levels

Cassava is the major source of calories for more than 250 million Sub-Saharan Africans, however, it has the lowest protein-to-energy ratio of any major staple food crop in the world. A cassava-based diet provides less than 30% of the minimum daily requirement for protein. Moreover, both leaves and roots contain potentially toxic levels of cyanogenic glucosides. The major cyanogen in cassava is linamarin which is stored in the vacuole. Upon tissue disruption linamarin is deglycosylated by the apolplastic enzyme, linamarase, producing acetone cyanohydrin. Acetone cyanohydrin can spontaneously decompose at pHs >5.0 or temperatures >35°C, or is enzymatically broken down by hydroxynitrile lyase (HNL) to produce acetone and free cyanide which is then volatilized. Unlike leaves, cassava roots have little HNL activity. The lack of HNL activity in roots is associated with the accumulation of potentially toxic levels of acetone cyanohydrin in poorly processed roots. We hypothesized that the over-expression of HNL in cassava roots under the control of a root-specific, patatin promoter would not only accelerate cyanogenesis during food processing, resulting in a safer food product, but lead to increased root protein levels since HNL is sequestered in the cell wall. Transgenic lines expressing a patatin-driven HNL gene construct exhibited a 2–20 fold increase in relative HNL mRNA levels in roots when compared with wild type resulting in a threefold increase in total root protein in 7 month old plants. After food processing, HNL overexpressing lines had substantially reduced acetone cyanohydrin and cyanide levels in roots relative to wild-type roots. Furthermore, steady state linamarin levels in intact tissues were reduced by 80% in transgenic cassava roots. These results suggest that enhanced linamarin metabolism contributed to the elevated root protein levels.

The BioCassava plus program: biofortification of cassava for sub-Saharan Africa

More than 250 million Africans rely on the starchy root crop cassava (Manihot esculenta) as their staple source of calories. A typical cassava-based diet, however, provides less than 30% of the minimum daily requirement for protein and only 10%-20% of that for iron, zinc, and vitamin A. The BioCassava Plus (BC+) program has employed modern biotechnologies intended to improve the health of Africans through the development and delivery of genetically engineered cassava with increased nutrient (zinc, iron, protein, and vitamin A) levels. Additional traits addressed by BioCassava Plus include increased shelf life, reductions in toxic cyanogenic glycosides to safe levels, and resistance to viral disease. The program also provides incentives for the adoption of biofortified cassava. Proof of concept was achieved for each of the target traits. Results from field trials in Puerto Rico, the first confined field trials in Nigeria to use genetically engineered organisms, and ex ante impact analyses support the efficacy of using transgenic strategies for the biofortification of cassava.

Cassava: an appraisal of its phytochemistry and its biotechnological prospects

The present state of knowledge of the phytochemistry of small molecules isolated from the roots and leaves of cassava, Manihot esculenta Crantz (Euphorbiaceae), is reviewed. Cassava roots are an important source of dietary and industrial carbohydrates, mainly eaten as a source of starch, forming the staple food to over 500 million; additionally, the roots have value as a raw material for industrial starch production and for animal feed giving the crop high economic value, but it suffers markedly from post-harvest physiological deterioration (PPD). The hydroxycoumarins scopoletin and its glucoside scopolin as well as trace quantities of esculetin and its glucoside esculin are identified from cassava roots during PPD. The biotechnological prospects for cassava are also reviewed including a critical appraisal of transgenic approaches for crop improvement, together with its use for bioethanol production, due to cassava's efficient ability to fix carbon dioxide into carbohydrate.

Analysis of cyanogens with resorcinol and picrate

The total cyanogenic potential of various substrates (flax seed, stones of peach, plum, nectarine and apricot as well as apple seeds, and various model compounds) was investigated by using the acid hydrolysis method, picrate method, and a novel method based on the reaction of cyanide liberated from plants with resorcinol and picrate. The hydrocyanic acid liberated from cyanogens was trapped by using a 1% sodium bicarbonate. Then, 1 ml of extract was mixed with 1 ml of working reagent containing 160 microg of resorcinol, 320 microg of picric acid, and 30 mg of sodium carbonate, and heated on a boiling water bath for 10 min. The absorbance was measured at 488 nm in 1cm glass cuvettes at room temperature. The color system obeys Beer's law in the range of 0-5 microg ml(-1) total HCN. Using model compounds and real samples including replicate analyses on prunasin, the resorcinol method proved to be more accurate, reproducible, and especially more sensitive than the known spectrophotometric methods such as the acid hydrolysis method and the picrate method.

Genetic modification of cassava for enhanced starch production

To date, transgenic approaches to biofortify subsistence crops have been rather limited. This is particularly true for the starchy root crop cassava (Manihot esculenta Crantz). Cassava has one of the highest rates of CO(2) fixation and sucrose synthesis for any C3 plant, but rarely reaches its yield potentials in the field. It was our hypothesis that starch production in cassava tuberous roots could be increased substantially by increasing the sink strength for carbohydrate. To test this hypothesis, we generated transgenic plants with enhanced tuberous root ADP-glucose pyrophosphorylase (AGPase) activity. This was achieved by expressing a modified form of the bacterial glgC gene under the control of a Class I patatin promoter. AGPase catalyses the rate-limiting step in starch biosynthesis, and therefore the expression of a more active bacterial form of the enzyme was expected to lead to increased starch production. To facilitate maximal AGPase activity, we modified the Escherichia coli glgC gene (encoding AGPase) by site-directed mutagenesis (G336D) to reduce allosteric feedback regulation by fructose-1,6-bisphosphate. Transgenic plants (three) expressing the glgC gene had up to 70% higher AGPase activity than control plants when assayed under conditions optimal for plant and not bacterial AGPase activity. Plants having the highest AGPase activities had up to a 2.6-fold increase in total tuberous root biomass when grown under glasshouse conditions. In addition, plants with the highest tuberous root AGPase activity had significant increases in above-ground biomass, consistent with a possible reduction in feedback inhibition on photosynthetic carbon fixation. These results demonstrate that targeted modification of enzymes regulating source-sink relationships in crop plants having high carbohydrate source strengths is an effective strategy for increasing carbohydrate yields in sink tissues.

Cassava plants with a depleted cyanogenic glucoside content in leaves and tubers. Distribution of cyanogenic glucosides, their site of synthesis and transport, and blockage of the biosynthesis by RNA interference technology

Transgenic cassava (Manihot esculenta Crantz, cv MCol22) plants with a 92% reduction in cyanogenic glucoside content in tubers and acyanogenic (<1% of wild type) leaves were obtained by RNA interference to block expression of CYP79D1 and CYP79D2, the two paralogous genes encoding the first committed enzymes in linamarin and lotaustralin synthesis. About 180 independent lines with acyanogenic (<1% of wild type) leaves were obtained. Only a few of these were depleted with respect to cyanogenic glucoside content in tubers. In agreement with this observation, girdling experiments demonstrated that cyanogenic glucosides are synthesized in the shoot apex and transported to the root, resulting in a negative concentration gradient basipetal in the plant with the concentration of cyanogenic glucosides being highest in the shoot apex and the petiole of the first unfolded leaf. Supply of nitrogen increased the cyanogenic glucoside concentration in the shoot apex. In situ polymerase chain reaction studies demonstrated that CYP79D1 and CYP79D2 were preferentially expressed in leaf mesophyll cells positioned adjacent to the epidermis. In young petioles, preferential expression was observed in the epidermis, in the two first cortex cell layers, and in the endodermis together with pericycle cells and specific parenchymatic cells around the laticifers. These data demonstrate that it is possible to drastically reduce the linamarin and lotaustralin content in cassava tubers by blockage of cyanogenic glucoside synthesis in leaves and petioles. The reduced flux to the roots of reduced nitrogen in the form of cyanogenic glucosides did not prevent tuber formation.

Engineering cyanogen synthesis and turnover in cassava (Manihot esculenta)

Cassava is the major root crop for a quarter billion subsistence farmers in sub-Saharan Africa. It is valued for its ability to grow in adverse environments and the food security it provides. Cassava contains potentially toxic levels of cyanogenic glycosides (linamarin) which protect the plant from herbivory and theft. The cyanogens, including linamarin and its deglycosylated product, acetone cyanohydrin, can be efficiently removed from the root by various processing procedures. Short-cuts in processing, which may occur during famines, can result in only partial removal of cyanogens. Residual cyanogens in cassava foods may cause neurological disorders or paralysis, particularly in nutritionally compromised individuals. To address this problem and to further understand the function of cyanogenic glycosides in cassava, we have generated transgenic cassava in which cyanogenic glycoside synthesis has been selectively inhibited in leaves and roots by antisense expression of CYP79D1/D2 gene fragments. The CYP79D1/D2 genes encode two highly similar cytochrome P450s that catalyze the first-dedicated step in cyanogenic glycoside synthesis. Transgenic plants in which the expression of these genes was selectively inhibited in leaves had substantially reduced (60- 94% reduction) linamarin leaf levels. Surprisingly, these plants also had a greater than a 99% reduction in root linamarin content. In contrast, transgenic plants in which the CYP79D1/D2 transcripts were reduced to non-detectable levels in roots had normal root linamarin levels. These results demonstrate that linamarin synthesized in leaves is transported to the roots and accounts for nearly all of the root linamarin content. Importantly, transgenic plants having reduced leaf and root linamarin content were unable to grow in the absence of reduced nitrogen (NH3) . Cassava roots have previously been demonstrated to have an active cyanide assimilation pathway leading to the synthesis of amino acids. We propose that cyanide derived from linamarin is a major source of reduced nitrogen for cassava root protein synthesis. Disruption of linamarin transport from leaves in CYP79D1/D2 anti-sense plants prevents the growth of cassava roots in the absence of an alternate source of reduced nitrogen. An alternative strategy for reducing cyanogen toxicity in cassava foods is to accelerate cyanogenesis and cyanide volatilization during food processing. To achieve this objective, we have expressed the leaf-specific enzyme hydroxynitrile lyase (HNL) in roots. HNL catalyzes the breakdown of acetone cyanohydrin to cyanide. Expression of HNL in roots accelerated cyanogenesis by more than three-fold substantially reducing the accumulation of acetone cyanohydrin during processing relative to wild-type roots.

Development and application of transgenic technologies in cassava

The capacity to integrate transgenes into the tropical root crop cassava (Manihot esculenta Crantz) is now established and being utilized to generate plants expressing traits of agronomic interest. The tissue culture and gene transfer systems currently employed to produce these transgenic cassava have improved significantly over the past 5 years and are assessed and compared in this review. Programs are underway to develop cassava with enhanced resistance to viral diseases and insects pests, improved nutritional content, modified and increased starch metabolism and reduced cyanogenic content of processed roots. Each of these is described individually for the underlying biology the molecular strategies being employed and progress achieved towards the desired product. Important advances have occurred, with transgenic plants from several laboratories being prepared for field trails.