Cassava: The Drought, War and Famine Crop in a Changing World

Application of fluorescent and UV-Vis detection methods to profile antimicrobial activity of cassava tissues for an efficient Agrobacterium-mediated transformation

The majority of tissue culture and transformation studies in cassava (Manihot esculenta Crantz) focus on the modification of conditions in order to establish a better protocol. Although this is a standard approach for making progress in genetic transformation technology for a target plant variety, serious difficulty still remains due to the limited applicability and adaptability of the given protocol. In the present study, we aim to develop a new concept that focuses on the development of simple but adaptable genetic transformation technology in cassava. In order to establish an efficient transformation protocol, two local edible cassava varieties, R-type, with a broad leaf with reddish petiole, and S-type, with a thin leaf with shiny greenish petiole, were obtained from Okinawa, Japan. Three detection methods, i.e., fluorescence microscopy, thin-layer chromatography (TLC) with detection under an ultraviolet (UV) illumination (254 nm) and light emitting diode (LED) illuminations (365 nm and 500 nm) without any staining, and a spectrum scanning (250-700 nm) by a microplate reader system were employed to identify a series of unique features of the petioles and leaves. Antimicrobial activity of methanol extracts from the tissues was also assayed. We succeeded in the transient expression of the GUS gene using cassava leaves and also established stable introduction of the GUS gene into three organogenic cassava calli by adapting Agrobacterium-mediated transformation. With all the findings, we have identified a flexible tool to create a better match between explants and Agrobacterium strains.

African cassava whitefly, Bemisia tabaci, cassava colonization preferences and control implications

Cassava is a staple food for people across sub-Saharan Africa. Over the last 20 years, there has been an increased frequency of outbreaks and crop damage in this region caused by the cassava-adapted Bemisia tabaci putative species. Little is known about when and why B. tabaci adults move and colonize new cassava crops, especially in farming systems that contain a mixture of cultivar types and plant ages. Here, we assessed experimentally whether the age and variety of cassava affected the density of B. tabaci. We also tested whether the age and variety of the source cassava field affected the variety preference of B. tabaci when they colonized new cassava plants. We placed uninfested potted "sentinel" plants of three cassava varieties (Nam 130, Nase 14, and Njule Red) in source fields containing one of two varieties (Nam 130 or Nase 14) and one of three age classes (young, medium, or old). After two weeks, the numbers of nymphs on the sentinel plants were used as a measure of colonization. Molecular identification revealed that the B. tabaci species was sub-Saharan Africa 1 (SSA1). We found a positive correlation between the density of nymphs on sentinel plants and the density of adults in the source field. The density of nymphs on the sentinels was not significantly related to the age of the source field. Bemisia tabaci adults did not preferentially colonize the sentinel plant of the same variety as the source field. There was a significant interactive effect, however, between the source and sentinel variety that may indicate variability in colonization. We conclude that managing cassava source fields to reduce B. tabaci abundance will be more effective than manipulating nearby varieties. We also suggest that planting a "whitefly sink" variety is unlikely to reduce B. tabaci SSA1 populations unless fields are managed to reduce B. tabaci densities using other integrative approaches.

RNAi inhibition of feruloyl CoA 6'-hydroxylase reduces scopoletin biosynthesis and post-harvest physiological deterioration in cassava (Manihot esculenta Crantz) storage roots

Cassava (Manihot esculenta Crantz) is a major world crop, whose storage roots provide food for over 800 million throughout the humid tropics. Despite many advantages as a crop, the development of cassava is seriously constrained by the rapid post-harvest physiological deterioration (PPD) of its roots that occurs within 24-72 h of harvest, rendering the roots unpalatable and unmarketable. PPD limits cassava's marketing possibilities in countries that are undergoing increased development and urbanisation due to growing distances between farms and consumers. The inevitable wounding of the roots caused by harvesting triggers an oxidative burst that spreads throughout the cassava root, together with the accumulation of secondary metabolites including phenolic compounds, of which the coumarin scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one) is the most abundant. Scopoletin oxidation yields a blue-black colour, which suggests its involvement in the discoloration observed during PPD. Feruloyl CoA 6'-hydroxylase is a controlling enzyme in the biosynthesis of scopoletin. The cassava genome contains a seven membered family of feruloyl CoA 6'-hydroxylase genes, four of which are expressed in the storage root and, of these, three were capable of functionally complementing Arabidopsis T-DNA insertion mutants in this gene. A RNA interference construct, designed to a highly conserved region of these genes, was used to transform cassava, where it significantly reduced feruloyl CoA 6'-hydroxylase gene expression, scopoletin accumulation and PPD symptom development. Collectively, our results provide evidence that scopoletin plays a major functional role in the development of PPD symptoms, rather than merely paralleling symptom development in the cassava storage root.

A socio-scientific analysis of the environmental and health benefits as well as potential risks of cassava production and consumption

Due to its high adaptability, cassava (Manihot esculenta Crantz) is one of the world's most cultivated and consumed plants after maize and rice. However, there are relatively few scientific studies on this important crop. The objective of this review was therefore to summarize and discuss the available information on cassava cropping in order to promote sustainable practices in terms of production and consumption. Cassava cultivation has been expanding recently at the global scale and is widely consumed in most regions of South America, Africa, and Asia. However, it is also characterized by the presence in its roots of potentially toxic hydrocyanic acid. Furthermore, cassava can also absorb pollutants as it is currently cultivated near roads or factories and generally without consideration for potential sources of soil, water, or atmospheric pollution. Careful washing, peeling, and adequate preparation before eating are therefore crucial steps for reducing human exposure to both environmental pollutants and natural hydrocyanic acid. At present, there is not enough precise data available on this staple food crop. To improve our knowledge on the nutritive benefits versus health risks associated with cassava consumption, further research is necessary to compare cassava cultivars and precisely study the influence of preparation methods.

Deep Assessment of Genomic Diversity in Cassava for Herbicide Tolerance and Starch Biosynthesis

Cassava is one of the most important food security crops in tropical countries, and a competitive resource for the starch, food, feed and ethanol industries. However, genomics research in this crop is much less developed compared to other economically important crops such as rice or maize. The International Center for Tropical Agriculture (CIAT) maintains the largest cassava germplasm collection in the world. Unfortunately, the genetic potential of this diversity for breeding programs remains underexploited due to the difficulties in phenotypic screening and lack of deep genomic information about the different accessions. A chromosome-level assembly of the cassava reference genome was released this year and only a handful of studies have been made, mainly to find quantitative trait loci (QTL) on breeding populations with limited variability. This work presents the results of pooled targeted resequencing of more than 1500 cassava accessions from the CIAT germplasm collection to obtain a dataset of more than 2000 variants within genes related to starch functional properties and herbicide tolerance. Results of twelve bioinformatic pipelines for variant detection in pooled samples were compared to ensure the quality of the variant calling process. Predictions of functional impact were performed using two separate methods to prioritize interesting variation for genotyping and cultivar selection. Targeted resequencing, either by pooled samples or by similar approaches such as Ecotilling or capture, emerges as a cost effective alternative to whole genome sequencing to identify interesting alleles of genes related to relevant traits within large germplasm collections.

Rooting for cassava: insights into photosynthesis and associated physiology as a route to improve yield potential

Contents 50 I. 50 II. 52 III. 54 IV. 55 V. 57 VI. 57 VII. 59 60 References 61 SUMMARY: As a consequence of an increase in world population, food demand is expected to grow by up to 110% in the next 30-35 yr. The population of sub-Saharan Africa is projected to increase by > 120%. In this region, cassava (Manihot esculenta) is the second most important source of calories and contributes c. 30% of the daily calorie requirements per person. Despite its importance, the average yield of cassava in Africa has not increased significantly since 1961. An evaluation of modern cultivars of cassava showed that the interception efficiency (ɛ_i ) of photosynthetically active radiation (PAR) and the efficiency of conversion of that intercepted PAR (ɛ_c ) are major opportunities for genetic improvement of the yield potential. This review examines what is known of the physiological processes underlying productivity in cassava and seeks to provide some strategies and directions toward yield improvement through genetic alterations to physiology to increase ɛ_i and ɛ_c . Possible physiological limitations, as well as environmental constraints, are discussed.

Selecting Native Arbuscular Mycorrhizal Fungi to Promote Cassava Growth and Increase Yield under Field Conditions

The use of arbuscular mycorrhizal fungal (AMF) inoculation in sustainable agriculture is now widespread worldwide. Although the use of inoculants consisting of native AMF is highly recommended as an alternative to commercial ones, there is no strategy to allow the selection of efficient fungal species from natural communities. The objective of this study was (i) to select efficient native AMF species (ii) evaluate their impact on nematode and water stresses, and (iii) evaluate their impact on cassava yield, an important food security crop in tropical and subtropical regions. Firstly, native AMF communities associated with cassava rhizospheres in fields were collected from different areas and 7 AMF species were selected, based upon their ubiquity and abundance. Using these criteria, two morphotypes (LBVM01 and LBVM02) out of the seven AMF species selected were persistently dominant when cassava was used as a trap plant. LBVM01 and LBVM02 were identified as Acaulospora colombiana (most abundant) and Ambispora appendicula, respectively, after phylogenetic analyses of LSU-ITS-SSU PCR amplified products. Secondly, the potential of these two native AMF species to promote growth and enhance tolerance to root-knot nematode and water stresses of cassava (Yavo variety) was evaluated using single and dual inoculation in greenhouse conditions. Of the two AMF species, it was shown that A. colombiana significantly improved the growth of the cassava and enhanced tolerance to water stress. However, both A. colombiana and A. appendicula conferred bioprotective effects to cassava plants against the nematode Meloidogyne spp., ranging from resistance (suppression or reduction of the nematode reproduction) or tolerance (low or no suppression in cassava growth). Thirdly, the potential of these selected native AMF to improve cassava growth and yield was evaluated under field conditions, compared to a commercial inoculant. In these conditions, the A. colombiana single inoculation and the dual inoculation significantly improved cassava yield compared to the commercial inoculant. This is the first report on native AMF species exhibiting multiple benefits for cassava crop productivity, namely improved plant growth and yield, water stress tolerance and nematode resistance.

Interactive effects of temperature and drought on cassava growth and toxicity: implications for food security?

Cassava is an important dietary component for over 1 billion people, and its ability to yield under drought has led to it being promoted as an important crop for food security under climate change. Despite its known photosynthetic plasticity in response to temperature, little is known about how temperature affects plant toxicity or about interactions between temperature and drought, which is important because cassava tissues contain high levels of toxic cyanogenic glucosides, a major health and food safety concern. In a controlled glasshouse experiment, plants were grown at 2 daytime temperatures (23 °C and 34 °C), and either well-watered or subject to a 1 month drought prior to harvest at 6 months. The objective was to determine the separate and interactive effects of temperature and drought on growth and toxicity. Both temperature and drought affected cassava physiology and chemistry. While temperature alone drove differences in plant height and above-ground biomass, drought and temperature × drought interactions most affected tuber yield, as well as foliar and tuber chemistry, including C : N, nitrogen and cyanide potential (CNp; total cyanide released from cyanogenic glucosides). Conditions that most stimulated growth and yield (well-watered × high temperature) effected a reduction in tuber toxicity, whereas drought inhibited growth and yield, and was associated with increased foliar and tuber toxicity. The magnitude of drought effects on tuber yield and toxicity were greater at high temperature; thus, increases in tuber CNp were not merely a consequence of reduced tuber biomass. Findings confirm that cassava is adaptable to forecast temperature increases, particularly in areas of adequate or increasing rainfall; however, in regions forecast for increased incidence of drought, the effects of drought on both food quality (tuber toxicity) and yield are a greater threat to future food security and indicate an increasing necessity for processing of cassava to reduce toxicity.

Resilience of cassava (Manihot esculenta Crantz) to salinity: implications for food security in low-lying regions

Rising sea levels are threatening agricultural production in coastal regions due to inundation and contamination of groundwater. The development of more salt-tolerant crops is essential. Cassava is an important staple, particularly among poor subsistence farmers. Its tolerance to drought and elevated temperatures make it highly suitable for meeting global food demands in the face of climate change, but its ability to tolerate salt is unknown. Cassava stores nitrogen in the form of cyanogenic glucosides and can cause cyanide poisoning unless correctly processed. Previous research demonstrated that cyanide levels are higher in droughted plants, possibly as a mechanism for increasing resilience to oxidative stress. We determined the tolerance of cassava to salt at two different stages of development, and tested the hypothesis that cyanide toxicity would be higher in salt-stressed plants. Cassava was grown at a range of concentrations of sodium chloride (NaCl) at two growth stages: tuber initiation and tuber expansion. Established plants were able to tolerate 100mM NaCl but in younger plants 40mM was sufficient to retard plant growth severely. Nutrient analysis showed that plants were only able to exclude sodium at low concentrations. The foliar cyanogenic glucoside concentration in young plants increased under moderate salinity stress but was lower in plants grown at high salt. Importantly, there was no significant change in the cyanogenic glucoside concentration in the tubers. We propose that the mechanisms for salinity tolerance are age dependent, and that this can be traced to the relative cost of leaves in young and old plants.

Current knowledge and future research perspectives on cassava (Manihot esculenta Crantz) chemical defenses: An agroecological view

Cassava (Manihot esculenta Crantz) is one of the most important staple crops worldwide. It constitutes the major source of carbohydrates for millions of low-income people living in rural areas, as well as a cash crop for smallholders in tropical and sub-tropical regions. The Food and Agriculture Organization of the United Nations predicts that cassava plantations will increase and production systems will intensify in the future, highlighting the need for developing strategies that improve the sustainability of production. Plant chemical defenses hold the potential for developing pest management strategies, as these plant traits can influence the behavior and performance of both pests and beneficial arthropods. Cassava plants are well-defended and produce a number of compounds involved in direct defense, such as cyanogenic glycosides, flavonoid glycosides, and hydroxycoumarins. In addition, volatile organic compounds induced upon herbivory and the secretion of extrafloral nectar act as indirect defense against herbivores by recruiting natural enemies. Here, cassava chemical defenses against pest arthropods are reviewed, with the aim of identifying gaps in our knowledge and areas of research that deserve further investigation for developing sound pest control strategies to improve sustainable production of this crop, and how these defenses can be used to benefit other crops. Cyanogenic content in cassava is also highly toxic to humans, and can cause irreversible health problems even at sub-lethal doses when consumed over prolonged periods. Therefore, the promotion of chemical defense in this crop should not aggravate these problems, and must be accompanied with the education on processing methods that reduce human exposure to cyanide.

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.

Development and application of a novel screening method and experimental use of the mutant bacterial strain Clostridium beijerinckii NCIMB 8052 for production of butanol via fermentation of fresh cassava

Classic chemical mutagenesis has a demonstrated potential to create a strain ofClostridiumwith improved fermentation performance for obtaining high butanol yield.

Assessing the Effect of Composting Cassava Peel Based Substrates on the Yield, Nutritional Quality, and Physical Characteristics of Pleurotus ostreatus (Jacq. ex Fr.) Kummer

Cassava peel based substrate formulations as an alternative substrate were used to grow mushrooms. The effect of two compost heights, three composting periods on the mycelia growth, physical characteristics, yield, and nutritional qualities of Pleurotus ostreatus (Jacq. ex Fr.) Kummer was studied. Mean mycelia growth of 16.2 cm after a period of seven (7) weeks was the best for 1.5 m compost height. Cap diameter and stipe length differed significantly (P < 0.05) with the compost heights (0.8 m and 1.5 m). The yield on compost height of 1.5 m, composted for 5 days, differed significantly (P < 0.05) from that of 0.8 m and gave increasing yields as follows: cassava peels and manure, cassava peels only, cassava peels and corn cobs (1 : 1 ratio), and cassava peels and corn cobs (1 : 1 ratio) with chicken manure. Composting periods (3 and 7 days) gave varying yields depending on the compost height. Based on the findings an interaction of 1.5 m compost height and 5 days composting period on cassava peels and corncobs (1 : 1 ratio) with chicken manure produced the best results. The nutritional quality of the mushrooms also differed significantly (P < 0.05), indicating that cassava peels could be used as a possible substrate in cultivation of mushroom.

In silico genome comparison and distribution analysis of simple sequences repeats in cassava

We conducted a SSRs density analysis in different cassava genomic regions. The information obtained was useful to establish comparisons between cassava's SSRs genomic distribution and those of poplar, flax, and Jatropha. In general, cassava has a low SSR density (~50 SSRs/Mbp) and has a high proportion of pentanucleotides, (24,2 SSRs/Mbp). It was found that coding sequences have 15,5 SSRs/Mbp, introns have 82,3 SSRs/Mbp, 5' UTRs have 196,1 SSRs/Mbp, and 3' UTRs have 50,5 SSRs/Mbp. Through motif analysis of cassava's genome SSRs, the most abundant motif was AT/AT while in intron sequences and UTRs regions it was AG/CT. In addition, in coding sequences the motif AAG/CTT was also found to occur most frequently; in fact, it is the third most used codon in cassava. Sequences containing SSRs were classified according to their functional annotation of Gene Ontology categories. The identified SSRs here may be a valuable addition for genetic mapping and future studies in phylogenetic analyses and genomic evolution.

Food safety: importance of composition for assessing genetically modified cassava (Manihot esculenta Crantz)

The importance of food composition in safety assessments of genetically modified (GM) food is described for cassava ( Manihot esculenta Crantz) that naturally contains significantly high levels of cyanogenic glycoside (CG) toxicants in roots and leaves. The assessment of the safety of GM cassava would logically require comparison with a non-GM crop with a proven "history of safe use". This study investigates this statement for cassava. A non-GM comparator that qualifies would be a processed product with CG level below the approved maximum level in food and that also satisfies a "worst case" of total dietary consumption. Although acute and chronic toxicity benchmark CG values for humans have been determined, intake data are scarce. Therefore, the non-GM cassava comparator is defined on the "best available knowledge". We consider nutritional values for cassava and conclude that CG residues in food should be a priority topic for research.

Drought adversely affects tuber development and nutritional quality of the staple crop cassava (Manihot esculenta Crantz)

Cassava (Manihot esculenta Crantz) is the staple food source for over 850million people worldwide. Cassava contains cyanogenic glucosides and can be toxic to humans, causing paralysing diseases such as konzo, and even death if not properly processed. Konzo epidemics are often associated with times of drought. This may be due to a greater reliance on cassava as it is drought tolerant, but it may also be due to an increase in cyanogenic glucosides. Episodic droughts are forecast to become more common in many cassava-growing regions. We therefore sought to quantify the effect of water-stress on both yield and cyanogenic glucoside concentration (CNc) in the developing tubers of cassava. Five-month-old plants were grown in a glasshouse and either well watered or droughted for 28 days. A subset of droughted plants was re-watered half way through the experiment. Droughted plants had 45% fewer leaves and lower tuber yield, by 83%, compared with well-watered plants. CNc was 2.9-fold higher in the young leaves of droughted plants, whereas CNc in tubers from droughted plants was 4-fold greater than in tubers from well-watered plants. Re-watered plants had a similar biomass to control plants, and lower CNc than droughted plants. These findings highlight the important link between food quality and episodic drought.

Diversity of dicotyledenous-infecting geminiviruses and their associated DNA molecules in southern Africa, including the South-west Indian ocean islands

The family Geminiviridae comprises a group of plant-infecting circular ssDNA viruses that severely constrain agricultural production throughout the temperate regions of the world, and are a particularly serious threat to food security in sub-Saharan Africa. While geminiviruses exhibit considerable diversity in terms of their nucleotide sequences, genome structures, host ranges and insect vectors, the best characterised and economically most important of these viruses are those in the genus Begomovirus. Whereas begomoviruses are generally considered to be either monopartite (one ssDNA component) or bipartite (two circular ssDNA components called DNA-A and DNA-B), many apparently monopartite begomoviruses are associated with additional subviral ssDNA satellite components, called alpha- (DNA-αs) or betasatellites (DNA-βs). Additionally, subgenomic molecules, also known as defective interfering (DIs) DNAs that are usually derived from the parent helper virus through deletions of parts of its genome, are also associated with bipartite and monopartite begomoviruses. The past three decades have witnessed the emergence and diversification of various new begomoviral species and associated DI DNAs, in southern Africa, East Africa, and proximal Indian Ocean islands, which today threaten important vegetable and commercial crops such as, tobacco, cassava, tomato, sweet potato, and beans. This review aims to describe what is known about these viruses and their impacts on sustainable production in this sensitive region of the world.

Drying and processing protocols affect the quantification of cyanogenic glucosides in forage sorghum

BACKGROUND

Cyanogenic glucosides are common bioactive products that break down to release toxic hydrogen cyanide (HCN) when combined with specific β-glucosidases. In forage sorghum, high concentrations of the cyanogenic glucoside dhurrin lead to reduced productivity and sometimes death of grazing animals, especially in times of drought, when the dhurrin content of stunted crops is often higher. The aim of this study was to develop harvesting protocols suitable for sampling in remote areas.

RESULTS

Dhurrin concentration in air- and oven-dried leaves was the same as in fresh leaves, with no subsequent losses during storage. Dhurrin concentration was halved when leaves were freeze-dried, although activity of the endogenous dhurrinase was preserved. Direct measurement of dhurrin concentration in methanolic extracts using liquid chromatography/mass spectrometry (LC/MS) gave similar results to methods that captured evolved cyanide. A single freezing event was as effective as fine grinding in facilitating complete conversion of dhurrin to cyanide.

CONCLUSION

Direct measurement of dhurrin using LC/MS is accurate but expensive and not appropriate for fieldwork. Air drying provides an accurate, low-cost method for preparing tissue for dhurrin analysis, so long as the specific β-glucosidase is added. It is recommended that comparative studies like the one presented here be extended to other cyanogenic species.

Variations in the chemical composition of cassava ( Manihot esculenta Crantz) leaves and roots as affected by genotypic and environmental variation

The purpose of this study was to assess the quality of cassava cultivars, in terms of cyanogenic potential and composition of macro- and micronutrients, sampled from different locations in rural Mozambique. Total cyanide concentrations in fresh cassava tissues were measured using portable cyanide testing kits, and elemental nutrients were later analyzed from dried plant tissue. Variation in cyanogenic potential and nutrient composition occurred both among cultivars and across locations. The majority of cultivars contained >100 ppm total cyanide, fresh weight, and are therefore considered to be dangerously poisonous unless adequately processed before consumption. Leaf cyanogenic and nutrient content varied with plant water status, estimated using carbon isotope discrimination (δ(13)C). The colonization of roots of all cultivars by arbuscular mycorrhizal fungi was also quantified and found to be high, indicating that mycorrhizas could play a key role in plant nutrient acquisition in these low-input farming systems.