Cassava genetic transformation and its application in breeding

As a major source of food, cassava (Manihot esculenta Crantz) is an important root crop in the tropics and subtropics of Africa and Latin America, and serves as raw material for the production of starches and bioethanol in tropical Asia. Cassava improvement through genetic engineering not only overcomes the high heterozygosity and serious trait separation that occurs in its traditional breeding, but also quickly achieves improved target traits. Since the first report on genetic transformation in cassava in 1996, the technology has gradually matured over almost 15 years of development and has overcome cassava genotype constraints, changing from mode cultivars to farmer-preferred ones. Significant progress has been made in terms of an increased resistance to pests and diseases, biofortification, and improved starch quality, building on the fundamental knowledge and technologies related to planting, nutrition, and the processing of this important food crop that has often been neglected. Therefore, cassava has great potential in food security and bioenergy development worldwide.     

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.     

能源木薯高淀粉抗逆分子育种研究进展与展望

木薯(Manihot esculenta Crantz)是全球重要的粮食作物,也是我国非粮生物质能源发展的主要原材料。长期以来,传统杂交育种是木薯新品种培育的主要手段。随着全球生态的变化和木薯产业发展的推进,需要加速培育抗逆能力强、高淀粉的木薯新品种,因此,利用基因工程针对特定性状开展品种创新表现出巨大的潜力。随着组学技术的发展,在木薯基础研究领域,特别是针对储藏根发育、淀粉富集、逆境响应与调控等方面的研究逐步深入。强化木薯基础理论研究和发展应用技术,对推动能源木薯的产业化发展具有重要意义。     

Cassava breeding: opportunities and challenges

Although cassava is a major food crop, its scientific breeding began only recently compared with other crops. Significant progress has been achieved, particularly in Asia where cassava is used mainly for industrial processes and no major biotic constraints affect its productivity. Cassava breeding faces several limitations that need to be addressed. The heterozygous nature of the crop and parental lines used to generate new segregating progenies makes it difficult to identify parents with good breeding values. Breeding so far has been mainly based on a mass phenotypic recurrent selection. There is very little knowledge on the inheritance of traits of agronomic relevance. Several approaches have been taken to overcome the constraints in the current methodologies for the genetic improvement of cassava. Evaluations at early stages of selection allow for estimates of general combining ability effect or breeding values of parental lines. Inbreeding by sequential self-pollination facilitates the identification of useful recessive traits, either already present in the Manihot gene pool or induced by mutagenesis.     

Cassava biology and physiology

Cassava or manioc (Manihot esculenta Crantz), a perennial shrub of the New World, currently is the sixth world food crop for more than 500 million people in tropical and sub-tropical Africa, Asia and Latin America. It is cultivated mainly by resource-limited small farmers for its starchy roots, which are used as human food either fresh when low in cyanogens or in many processed forms and products, mostly starch, flour, and for animal feed. Because of its inherent tolerance to stressful environments, where other food crops would fail, it is often considered a food-security source against famine, requiring minimal care. Under optimal environmental conditions, it compares favorably in production of energy with most other major staple food crops due to its high yield potential. Recent research at the Centro Internacional de Agricultura Tropical (CIAT) in Colombia has demonstrated the ability of cassava to assimilate carbon at very high rates under high levels of humidity, temperature and solar radiation, which correlates with productivity across all environments whether dry or humid. When grown on very poor soils under prolonged drought for more than 6 months, the crop reduce both its leaf canopy and transpiration water loss, but its attached leaves remain photosynthetically active, though at greatly reduced rates. The main physiological mechanism underlying such a remarkable tolerance to drought was rapid stomatal closure under both atmospheric and edaphic water stress, protecting the leaf against dehydration while the plant depletes available soil water slowly during long dry periods. This drought tolerance mechanism leads to high crop water use efficiency values. Although the cassava fine root system is sparse, compared to other crops, it can penetrate below 2 m soil, thus enabling the crop to exploit deep water if available. Leaves of cassava and wildManihotpossess elevated activities of the C₄ enzyme PEP carboxylase but lack the leaf Kranz anatomy typical of C₄ species, pointing to the need for further research on cultivated and wild Manihot to further improve its photosynthetic potential and yield, particularly under stressful environments. Moreover, a wide range in values of K _m (CO₂) for the C₃ photosynthetic enzyme Rubisco was found among cassava cultivars indicating the possibility of selection for higher affinity to CO₂, and consequently higher leaf photosynthesis. Several plant traits that may be of value in crop breeding and improvement have been identified, such as an extensive fine root system, long leaf life, strong root sink and high leaf photosynthesis. Selection of parental materials for tolerance to drought and infertile soils under representative field conditions have resulted in developing improved cultivars that have high yields in favorable environments while producing reasonable and stable yields under stress.     

Cassava biology and physiology

Cassava or manioc (Manihot esculenta Crantz), a perennial shrub of the New World, currently is the sixth world food crop for more than 500 million people in tropical and sub-tropical Africa, Asia and Latin America. It is cultivated mainly by resource-limited small farmers for its starchy roots, which are used as human food either fresh when low in cyanogens or in many processed forms and products, mostly starch, flour, and for animal feed. Because of its inherent tolerance to stressful environments, where other food crops would fail, it is often considered a food-security source against famine, requiring minimal care. Under optimal environmental conditions, it compares favorably in production of energy with most other major staple food crops due to its high yield potential. Recent research at the Centro Internacional de Agricultura Tropical (CIAT) in Colombia has demonstrated the ability of cassava to assimilate carbon at very high rates under high levels of humidity, temperature and solar radiation, which correlates with productivity across all environments whether dry or humid. When grown on very poor soils under prolonged drought for more than 6 months, the crop reduce both its leaf canopy and transpiration water loss, but its attached leaves remain photosynthetically active, though at greatly reduced rates. The main physiological mechanism underlying such a remarkable tolerance to drought was rapid stomatal closure under both atmospheric and edaphic water stress, protecting the leaf against dehydration while the plant depletes available soil water slowly during long dry periods. This drought tolerance mechanism leads to high crop water use efficiency values. Although the cassava fine root system is sparse, compared to other crops, it can penetrate below 2 m soil, thus enabling the crop to exploit deep water if available. Leaves of cassava and wild Manihot possess elevated activities of the C₄ enzyme PEP carboxylase but lack the leaf Kranz anatomy typical of C₄ species, pointing to the need for further research on cultivated and wild Manihot to further improve its photosynthetic potential and yield, particularly under stressful environments. Moreover, a wide range in values of K _m (CO₂) for the C₃ photosynthetic enzyme Rubisco was found among cassava cultivars indicating the possibility of selection for higher affinity to CO₂, and consequently higher leaf photosynthesis. Several plant traits that may be of value in crop breeding and improvement have been identified, such as an extensive fine root system, long leaf life, strong root sink and high leaf photosynthesis. Selection of parental materials for tolerance to drought and infertile soils under representative field conditions have resulted in developing improved cultivars that have high yields in favorable environments while producing reasonable and stable yields under stress.     

Genetic engineering of improved nitrogen use efficiency in rice by the tissue-specific expression of alanine aminotransferase

Nitrogen is quantitatively the most essential nutrient for plants and a major factor limiting crop productivity. One of the critical steps limiting the efficient use of nitrogen is the ability of plants to acquire it from applied fertilizer. Therefore, the development of crop plants that absorb and use nitrogen more efficiently has been a long-term goal of agricultural research. In an attempt to develop nitrogen-efficient plants, rice ( Oryza sativa L.) was genetically engineered by introducing a barley AlaAT ( alanine aminotransferase ) cDNA driven by a rice tissue-specific promoter ( OsAnt1 ). This modification increased the biomass and grain yield significantly in comparison with control plants when plants were well supplied with nitrogen. Compared with controls, transgenic rice plants also demonstrated significant changes in key metabolites and total nitrogen content, indicating increased nitrogen uptake efficiency. The development of crop plants that take up and assimilate nitrogen more efficiently would not only improve the use of nitrogen fertilizers, resulting in lower production costs, but would also have significant environmental benefits. These results are discussed in terms of their relevance to the development of strategies to engineer enhanced nitrogen use efficiency in crop plants.     

植物分子育种技术在改良木薯种质方面的应用

木薯是热带、亚热带地区的重要粮食作物和经济作物。培育出生产性状更加优良的木薯品种,是推进木薯产业更快更好发展的重要基础。分子育种技术在培育优良木薯品种方面具有传统育种技术不可比拟的优势。该文介绍了近年来在提高植物抗寒性与病虫害抗性、降低氰苷含量、提高淀粉含量及组成、改变储存物种类、防止木薯收获后变质等方面的研究进展,以便在这些研究进展的基础上,利用植物分子育种技术加快获得具有抗逆能力提高、品质改良、产量增加、耐储藏等优良特性的木薯新品种。     

Improved Cassava Starch by Antisense Inhibition of Granule-bound Starch Synthase I

Cassava is a poor man's crop which is mainly grown as a subsistence crop in many developing countries. Its commercial use was first as animal feed (also known as tapioca), but has shifted since the late sixties to a source of native starch. The availability of native starches, which on the one hand do not require substantial chemical derivatisation and on the other hand have improved properties, would make cassava also for small farmers a potentially attractive cash crop. Since breeding is difficult in this polyploid, vegetatively propagated, crop a transgenic approach would be ideal to improve certain characteristics. We have created a cassava genotype producing amylose-free starch by genetic modification. The absence of amylose increased the clarity and stability of gels made with the transgenic starch, without requiring treatment with environment-unfriendly chemicals such as epoxides (propylene oxide, ethylene oxide) and acetic anhydride, which are normally used to improve stability. The amylose-free starch showed no changes in particle size distribution, chain length distribution or phosphorous content when compared to amylose-containing starch, but the granule melting temperature was increased by almost 2°C. Furthermore, the amylose-free cassava starch shows enhanced clarity and stability properties. These improved functionalities are desired in technical applications in paper and textile manufacturing, but also in the food industry for the production of sauces, dairy products and noodles.     

木薯分子育种中若干基本科学问题的思考与研究进展

木薯作为全球重要的薯类作物,既是热带地区粮食安全的保障,也是重要的淀粉工业原材料,保障其稳产、高产、优质一直是育种家不变的研究主题．当前,木薯品种选育正处在从杂交育种转向分子育种的发展阶段,深入解析木薯特有的经济性状和生物学特点是利用生物技术进行遗传改良的基础．不同于谷物类作物,木薯光合同化物的转运和库源分配的调控机制必有其独特之处;同时,储藏根的“库容”直接影响其产量．作为热带作物,了解木薯对低温和干旱的响应可为改良其抗逆境能力提供理论依据．不同于其他薯类作物,木薯储藏根特有的“采后生理性变质”问题亟待解决,其发生和调控机制的解析对延长木薯货架期意义重大．随着分子生物学的发展,针对上述各方面的研究日益深入,不仅激发了感兴趣的公众对这些问题的认知和思考,也激励了科研人员不断努力寻找解析相关机制的方法,为最终通过分子育种手段改良木薯提供思路和技术方案,揭开木薯的层层“面纱”,推动木薯分子育种的发展．     

作物分枝的分子调控研究进展

分枝的数量及角度是决定作物株型的重要农艺性状.有效分枝数决定着作物的穗数或荚果数,进而决定着作物的产量;而分枝角度与光合效率、种植密度和抗病性密切相关,不仅影响作物的产量,也会影响作物的品质.由于分枝在作物生产中具有十分重要的作用,吸引了越来越多的研究者的注意,多个与分枝性状相关的关键基因被鉴定,分枝数目调控的分子机制...     

Cryopreservation of sweetpotato (Ipomoea batatas) and its pathogen eradication by cryotherapy

Sweetpotato (Ipomoea batatas) ranks as the seventh most important staple crop in the world and the fifth in developing countries after rice, wheat, maize and cassava. Sweetpotato is mainly grown in developing countries, which account for more than 95% of total production of the whole world. Genetic resources, including cultivated varieties and wild species, are a prerequisite for novel sweetpotato breeding in both conventional and genetic engineering programs. Various cryopreservation protocols have been developed for shoot tips and embryogenic tissues. The former explants are preferred for long-term conservation of sweetpotato genetic resources, while the latter are valuable for sweetpotato genetic improvement. This review provides update comprehensive information on cryopreservation of sweetpotato shoot tips and embryogenic tissues.Plant pathogens such as viruses and phytoplasma severely hamper high yield and high quality production of sweetpotato. Thus, usage of pathogen-free planting materials is pivotal for sustainable sweetpotato production. Cryotherapy of shoot tips can efficiently eradicate sweetpotato pathogens such as viruses and phytoplasma. The mechanism behind pathogen eradication by cryotherapy of shoot tips has been elucidated. Pathogen eradication by cryotherapy provides an alternative, efficient strategy for production of pathogen-free plants. In addition, cryopreserved tissues may also be considered to be safer for exchange of germplasm between countries and regions.     

Miscanthus: A fast‐growing crop for environmental remediation and biofuel production

As an herbaceous perennial, Miscanthus has attracted extensive attention in bioenergy refinery and ecological remediation due to its high yield and superior environmental adaptability. This review summarizes current research advances of Miscanthus in several aspects including biological properties, biofuels production, and phytoremediation of contaminated soil. Miscanthus has relatively high biomass yield, calorific value, and cellulose content compared with other lignocellulosic bioenergy crops, which make it one of the most promising feedstocks for the production of second‐generation biofuels. Moreover, Miscanthus can endure soil pollutions caused by various heavy metals and survive in a variety of adverse environmental conditions. Therefore, it also has potential applications in ecological remediation of contaminated soil, and reclamation of polluted soil and water resources. Nevertheless, more endeavors are still needed in the genetic improvement and elite cultivar breeding, large‐scale cultivation on marginal land, and efficient conversion to biofuels, when utilizing Miscanthus as a bioenergy crop. Furthermore, more efforts should also be undertaken to translate Miscanthus into a bioenergy crop with the phytoremediation potential.     

Cassava as an alternate host for multiplication of VAM fungi

Summary Cassava tuber skin contained VAM fungal spores, mycelial bits which embedded in between the layers and the cassava root system are non-specific to native or non-indigenous mycorrhizal fungus. Hence, an efficient strain of a VAM fungus can be multiplied in cassava and the cassava skin can be used for propagating VA-mycorrhizae in the field for crop production.     

RIKEN Cassava Initiative: Establishment of a Cassava Functional Genomics Platform

Cassava (Manihot esculenta Crantz) is an important tropical root crop that provides food security and income generation for many farmers. Cassava ranks as the third most important source of calories in the tropics and provides sustenance to 800 million people worldwide. Cassava is also an efficient source of modified starch and bioethanol. In spite of the many advantages and high potential of cassava, the research community lacks functional genomic resources. Therefore, RIKEN has been working to establish a cassava functional genomics platform, which includes full-length cDNAs, DNA microarrays, transformation capabilities, and a searchable database in collaboration with the International Center for Tropical Agriculture (CIAT) of Colombia and Mahidol University of Thailand. In this review, we summarize the present status and future perspectives of our cassava functional genomic approaches.     

The production‐ecological sustainability of cassava,sugarcane and sweet sorghum cultivation for bioethanol in Mozambique

We present an approach for providing quantitative insight into the production‐ecological sustainability of biofuel feedstock production systems. The approach is based on a simple crop‐soil model and was used for assessing feedstock from current and improved production systems of cassava for bioethanol. Assessments were performed for a study area in Mozambique, a country considered promising for biomass production. Our focus is on the potential role of smallholders in the production of feedstock for biofuels. We take cassava as the crop for this purpose and compare it with feedstock production on plantations using sugarcane, sweet sorghum and cassava as benchmarks. Production‐ecological sustainability was defined by seven indicators related to resource‐use efficiency, soil quality, net energy production and greenhouse gas (GHG) emissions. Results indicate that of the assessed systems, sugarcane performed better than cassava, although it requires substantial water for irrigation. Targeted use of nutrient inputs improved sustainability of smallholder cassava. Cassava production systems on more fertile soils were more sustainable than those on less fertile soils; the latter required more external inputs for achieving the same output, affecting most indicators negatively and reducing the feasibility for smallholders. Cassava and sweet sorghum performed similarly. Cassava production requires much more labour per hectare than production of sugarcane or sweet sorghum. Production of bioethanol feedstock on cultivated lands was more sustainable and had potential for carbon sequestration, avoiding GHG emissions from clearing natural vegetation if new land is opened.     

浅析我国农作物病虫草鼠害成灾特点与减灾对策

农作物病虫草鼠害是我国农业生产的重要生物灾害,是制约高产、优质、高效益农业持续发展的主导因素之一。本文回顾了农业生物灾害综合防治技术在已往农业生产中的减灾作用。针对农作物病虫草鼠发生危害此起彼伏,关键防治技术落后,预警能力差,应用基础研究薄弱等现状,分析了我国生物灾害减灾研究面临的新挑战。从加强投资力度,开展重大病虫害灾变规律、中长期预测预报及综合防治关键技术研究,重视高新技术应用研究等角度指出了持续控制生物灾害的对策与途径。     

Field testing and exploitation of genetically modified cassava with low-amylose or amylose-free starch in Indonesia

The development and testing in the field of genetically modified -so called- orphan crops like cassava in tropical countries is still in its infancy, despite the fact that cassava is not only used for food and feed but is also an important industrial crop. As traditional breeding of cassava is difficult (allodiploid, vegetatively propagated, outbreeding species) it is an ideal crop for improvement through genetic modification. We here report on the results of production and field testing of genetically modified low-amylose transformants of commercial cassava variety Adira4 in Indonesia. Twenty four transformants were produced and selected in the Netherlands based on phenotypic and molecular analyses. Nodal cuttings of these plants were sent to Indonesia where they were grown under biosafety conditions. After two screenhouse tests 15 transformants remained for a field trial. The tuberous root yield of 10 transformants was not significantly different from the control. Starch from transformants in which amylose was very low or absent showed all physical and rheological properties as expected from amylose-free cassava starch. The improved functionality of the starch was shown for an adipate acetate starch which was made into a tomato sauce. This is the first account of a field trial with transgenic cassava which shows that by using genetic modification it is possible to obtain low-amylose cassava plants with commercial potential with good root yield and starch quality.     

Cassava in South America, Brazil's contribution and the lesson to be learned from India

South America is responsible for about half of the cassava world production. In the 1970's productivity of the crop on the continent was about 15 ton/ha, and dropped continuously until reaching 12 ton/ha in 2004. India's productivity of cassava increased from 10 ton/ha in the 1970's to 28 ton/ha in 2004. Brazil contributed significantly to improving cassava crops through the Instituto Agron?mico de Campinas in the 1960's and 1970's. The Universidade de Brasília released high-protein content hybrids, apomictic clones and explored the potential of indigenous landraces.     

Cassava Production and Pest Management: Present and Potential Threats in a Changing Environment

Cassava is attacked by a complex of arthropod pests across the tropical regions of the world where the crop is grown. Root yield losses have been recorded for several pests, including mites, mealybugs, whiteflies, hornworm, lacebugs, thrips and burrower bugs. Agronomic characteristics such as vegetative propagation, a long growth cycle, drought tolerance, staggered planting dates and intercropping contribute to the considerable diversity of pests that feed on the crop. The dynamics of cassava production are evolving as trends in the food, feed and industrial starch sector are leading to an increased demand for high quality starches. The resulting shift to larger scale production units, expansion of cultivated area and modifications in crop management combined with the effects of climate change, especially warmer temperatures and altered rainfall patterns, affect the occurrence and dynamics of arthropod pests in cassava agro ecosystems. Data is presented to describe the effects of temperature and dry seasons on key pest species. Whiteflies, mites and mealybugs register a suitability increase in the same areas in South America: Northeastern Brazil, Northern Argentina, South-Central Bolivia, and Southwest Peru. In Africa increases are projected in Southeast Africa and Madagascar. In Asia, regions with greater projected suitability for these pest species are Coastal India and Southeast Asia. Future trends and important criteria that will influence the severity and management of key pests are discussed.