Plant genetics, sustainable agriculture and global food security

The United States and the world face serious societal challenges in the areas of food, environment, energy, and health. Historically, advances in plant genetics have provided new knowledge and technologies needed to address these challenges. Plant genetics remains a key component of global food security, peace, and prosperity for the foreseeable future. Millions of lives depend upon the extent to which crop genetic improvement can keep pace with the growing global population, changing climate, and shrinking environmental resources. While there is still much to be learned about the biology of plant-environment interactions, the fundamental technologies of plant genetic improvement, including crop genetic engineering, are in place, and are expected to play crucial roles in meeting the chronic demands of global food security. However, genetically improved seed is only part of the solution. Such seed must be integrated into ecologically based farming systems and evaluated in light of their environmental, economic, and social impacts-the three pillars of sustainable agriculture. In this review, I describe some lessons learned, over the last decade, of how genetically engineered crops have been integrated into agricultural practices around the world and discuss their current and future contribution to sustainable agricultural systems.     

Genetic analysis of osmotic adjustment in crop plants

Plant water deficit is a component of several different stresses, including drought, salinity and low temperatures, which severely limit plant growth and crop productivity. Genetic modification of plants to allow growth and yield under unfavourable conditions is an important component of the solution to problems of environmental stress. While disagreement and even confusion may characterize some of the discussions on what constitutes a significant and an effective osmotic adjustment (OA) is receiving increasing recognition as a major mechanism. This paper starts with review of OA functions, genetic variation and inheritance, and theories and principles involved in commonly used protocols for quantifying OA. Emphasis is placed on a summary of current molecular strategies and advanced in the improvement of plant stress resistance through manipulating OA. They include a genetic engineering approach and a QTL mapping approach. Future promising strategies for improving drought resistance lie in molecular technology that allows genes or QTLs controlling OA to be tagged and isolated, these genes to be expressed in transgenic plants, and efficiency of breeding via marker-assisted selection to be improved. Aspects of QTL utilization in plant genetics, breeding and physiology and future research directions are discussed.     

Genetic control of root exudation

The literature on genetics of root exudation and on genotypic differences in qualitative and quantitative composition of root exudates in crop and native plant species was critically assessed. Differences in exudation have been reported for genotypes that differ in tolerance to nutrient deficiencies, ion toxicities, and pathogen attack. The exudation profile of a limited number of genotypes (frequently only two genotypes with the contrasting response to the environmental stress) have been reported to date. Little is known about the variability in larger samples of the germplasm or about actual genetics behind differential qualitative and quantitative composition of root exudates. Changing the exudation profile of a given genotype may be achieved by manipulating the biosynthetic capacity and by increasing the capacity of the plasma membrane to transport the specific compound out into the rhizosphere. Overexpression of the bacterial citrate synthase gene in the cytoplasm of tobacco plants resulted in exudation of large quantities of citrate into the rhizosphere and partial alleviation of the aluminium (Al) toxicity stress. A similar strategy of transforming plants with citrate synthase gene is being tried as a way of improving plant capacity to extract phosphorus (P) from soils with notoriously low P availability.More research into the genetic basis of qualitative and quantitative differences in root exudation is warranted. Understanding the genetic control of root exudation, followed by manipulation of qualitative and quantitative composition of root exudates, will result in better adaptation of plants to environmental conditions and a greater yield of crops.     

The plant genome's methylation status and response to stress: implications for plant improvement

Plant improvement depends on generating phenotypic variation and selecting for characteristics that are heritable. Classical genetics and early molecular genetics studies on single genes showed that differences in chromatin structure, especially cytosine methylation, can contribute to heritable phenotypic variation. Recent molecular genetic and genomic studies have revealed a new importance of cytosine methylation for gene regulation and have identified RNA interference (RNAi)-related proteins that are necessary for methylation. Methylation differences among plants can be caused by cis- or trans-acting DNA polymorphisms or by epigenetic phenomena. Although regulatory proteins might be important in creating this variation, recent examples highlight the central role of transposable elements and DNA repeats in generating both genetic and epigenetic methylation polymorphisms. The plant genome's response to environmental and genetic stress generates both novel genetic and epigenetic methylation polymorphisms. Novel, stress-induced genotypes may contribute to phenotypic diversity and plant improvement.     

TILLING moves beyond functional genomics into crop improvement

Transgenic methods have been successfully applied to trait improvement in a number of crops. However, reverse genetics studies by transgenic means are not practical in many commercially important crops, hampering investigations into gene function and the development of novel and improved cultivars. A nontransgenic method for reverse genetics called Targeting Induced Local Lesions IN Genomes (TILLING) has been developed as a method for inducing and identifying novel genetic variation, and has been demonstrated in the model plant, Arabidopsis thaliana. Recently, TILLING has been extended to the improvement of crop plants and shows great promise as a general method for both functional genomics and modulation of key traits in diverse crops.     

Advances in plant biotechnology and their implication for forestry research

Summary Over the past few years, techniques of cell biology, genetic screening, and gene manipulation have been developed to the extent that their impact on commercial development of improved plant varieties is predicted to have a measurable impact on agriculture by the year 2000 and beyond. A review will be given of progress that has been made in each of these areas toward the manipulation of crop plants for improved field performance and product quality. There are now several opportunities in which these techniques can be employed for the improvement of forestry species. In the light of the long-time scales involved in the generation of forestry products, it is important to focus on targets that are worthwhile pursuing commercially using appropriate technical routes. Selected examples will be given of the application of plant biotechnology techniques that promise potentially significant improvement for forestry species. Presented in the Keynote address Toward the Forest of Tomorrow at the 5th Meeting of the Conifer Biotechnology Working Group, Siltingbourne, England, July 8–13, 1990.     

Current genetic research on groundnut in India

The progress in genetic improvement of groundnut in India during the last five decades has been possible largely due to understanding and utilizing the information generated on the genetics of various characters of the plant. The current genetic studies on groundnut in India are carried out both on qualitative and agronomically important quantitative characters. Research accomplishments which are noteworthy in these areas have been reported in this paper. However, the genetics of several characters of economic importance has not been analyzed so far. Efforts to incorporate resistance to abiotic and biotic factors have gained momentum and a large number of resistant sources have been identified for each of these constraints. Additional efforts are expected to be made on gaining insight into the genetics of resistance to abiotic stresses and insect pests among biotic stresses, where the available information is meagre. Future studies are also expected to concentrate on unravelling the relationship of economic yield with various other characters, including the physiological and biochemical ones. Molecular marker techniques hold promise for genetic analysis of groundnut, as in other crop plants.     

Genetic manipulation of microspores and microspore-derived embryos

Summary Recent advances in plant cell and molecular biology have furthered the genetic manipulation of many plant species and advanced the options for crop improvement. Among the many targets for genetic manipulation, microspores offer several unique advantages: they are haploid, single-celled, and highly synchronized. In many plant species microspores develop into haploid embryos, and eventually haploid and doubled haploid plants, after in vitro anther or microspore culture. This induced in vitro developmental pathway of microspores, termed microspore embryogenesis, can be used to recover individual homozygous plants from microspores and microspore-derived embryos after genetic manipulation such as mutagenesis and gene transfer. The highly efficient microspore embryogenesis system inBrassica napus has been used successfully to obtain various mutants after microspore mutagenesis, and to achieve gene transfer mediated byAgrobacterium tumefaciens. Presented in the Session-in-Depth In Vitro Gametophyte Biology at the 1991 World Congress on Cell and Tissue Culture held in Anaheim, California, June 16–20, 1991.     

Prospects for using conventional techniques and molecular biological tools to enhance performance of `orphan' crop plants on soils low in available phosphorus

Molecular biology, combined with Mendelian and quantitative genetics in quantitative trait locus (QTL) mapping and marker-assisted selection (MAS), provides powerful new tools to facilitate efficient genetic manipulation by plant breeders of complex traits such as drought tolerance and phosphorus (P) acquisition ability. This paper examines current opportunities for genetically manipulating the ability of crop plants to more efficiently acquire (i.e. access and take up) essential soil nutrients, using as examples P and several of the crops in the genetic improvement mandate of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) – chickpea (Cicer arietinum L.), groundnut (Arachis hypogaea L.), pearl millet [Pennisetum glaucum (L.) R. Br.], pigeonpea [Cajanus cajan(L.) Millsp.], and sorghum [Sorghum bicolor (L.) Moench]. It is concluded that for at least some of these important, but often academically and economically orphaned tropical food, forage and feed-grain crops, the genetic variation and molecular tools that we will need already exist or can be expected to become available in the very near future. With appropriate, targeted research, these tools can permit empirical exploration of the potential for marker-facilitated mapping and manipulation of major genes that can contribute to enhanced ability of these crops to acquire P from sources with limited availability. With these tools, delivery of new versions of currently popular high-yielding, high quality, disease resistant crop cultivars, having genetically improved ability to acquire P currently in soils but unavailable for crop growth, could take as little as five to seven years. Sustainable use of such improved cultivars would require their utilization as components of integrated soil fertility management systems.     

Strategies for expression of foreign genes in plants. Potential use of engineered viruses

Advances in gene transfer techniques for higher plants have already permitted important achievements towards crop protection and improvement using recombinant DNA technology. Besides plant genetic engineering, the possible use of plant viruses to express foreign genes could be of considerable interest to plant biotechnology. However, insuring containment of engineered viruses for environmental use is an important safety issue that must be addressed.     

植物景观遗传学研究进展

植物景观遗传学是新兴的景观遗传学交叉学科的一个重要研究方向。目前植物景观遗传学的研究虽落后于动物,但其在生物多样性保护方面具有的巨大潜力不可忽视。从景观特征对遗传结构、环境因素对适应性遗传变异影响两个方面,系统综述了近十年来国际上植物景观遗传学的研究焦点和研究进展,比较了植物景观遗传学与动物景观遗传学研究在研究设计和研究方法上的异同,并基于将来植物景观遗传学由对空间遗传结构的描述发展为对景观遗传效应的量化分析及预测的发展框架,具体针对目前景观特征与遗传结构研究设计的系统性差、遗传结构与景观格局在时间上的误配、适应性位点与环境变量的模糊匹配、中性遗传变异与适应性遗传变异研究的分隔、景观与遗传关系分析方法的局限等五个方面提出了研究对策。     

Genetic transformation technology: Status and problems

Summary Transfer of genes from heterologous species provides the means of selectively introducing new traits into crop plants and expanding the gene pool beyond what has been available to traditional breeding systems. With the recent advances in genetic engineering of plants, it is now feasible to introduce into crop plants, genes that have previously been inaccessible to the conventional plant breeder, or which did not exist in the crop of interest. This holds a tremendous potential for the genetic enhancement of important food crops. However, the availability of efficient transformation methods to introduce foreign DNA can be a substantial barrier to the application of recombinant DNA methods in some crop plants. Despite significant advances over the past decades, development of efficient transformation methods can take many years of painstaking research. The major components for the development of transgenic plants include the development of reliable tissue culture regeneration systems, preparation of gene constructs and efficient transformation techniques for the introduction of genes into the crop plants, recovery and multiplication of transgenic plants, molecular and genetic characterization of transgenic plants for stable and efficient gene expression, transfer of genes to elite cultivars by conventional breeding methods if required, and the evaluation of transgenic plants for their effectiveness in alleviating the biotic and abiotic stresses without being an environmental biohazard. Amongst these, protocols for the introduction of genes, including the efficient regeneration of shoots in tissue cultures, and transformation methods can be major bottlenecks to the application of genetic transformation technology. Some of the key constraints in transformation procedures and possible solutions for safe development and deployment of transgenic plants for crop improvement are discussed.     

Molecular approaches for improvement of medicinal and aromatic plants

Medicinal and aromatic plants (MAPs) are important sources for plant secondary metabolites, which are important for human healthcare. Improvement of the yield and quality of these natural plant products through conventional breeding is still a challenge. However, recent advances in plant genomics research has generated knowledge leading to a better understanding of the complex genetics and biochemistry involved in biosynthesis of these plant secondary metabolites. This genomics research also concerned identification and isolation of genes involved in different steps of a number of metabolic pathways. Progress has also been made in the development of functional genomics resources (EST databases and micro-arrays) in several medicinal plant species, which offer new opportunities for improvement of genotypes using perfect markers or genetic transformation. This review article presents an overview of the recent developments and future possibilities in genetics and genomics of MAP species including use of transgenic approach for their improvement.     

Practical Applications of Manipulating Plant Architecture by Regulating Gibberellin Metabolism

The international trade in floriculture is estimated to be worth about US$150 billion, with the global demand for ornamentals steadily increasing. Consumer choice is influenced by factors such as plant architecture and flower colour. Conventional breeding has been responsible for the introduction of novel traits into ornamental plants and has played an important role in the development of new cultivars. However, a restricted gene pool and failure of distant crosses have led to the exploitation of somatic cell techniques, particularly genetic transformation, to generate plants with desirable traits. Gibberellins (GAs) are endogenous plant hormones that control key aspects of growth and development. Chemical growth regulators that modify GA biosynthesis are used extensively in horticulture to control plant stature, increasing production costs, manpower, and environmental risks. An alternative strategy involves genetic manipulation of GA metabolism to induce phenotypic changes, particularly alteration of stature. Because ornamentals are not used for human consumption, genetic manipulation approaches with these plants may be more acceptable in the immediate future to the general public, in certain parts of the world, than genetically manipulated food crops.     

Polyamines and abiotic stress tolerance in plants

Environmental stresses including climate change, especially global warming, are severely affecting plant growth and productivity worldwide. It has been estimated that two-thirds of the yield potential of major crops are routinely lost due to the unfavorable environmental factors. On the other hand, the world population is estimated to reach about 10 billion by 2050, which will witness serious food shortages. Therefore, crops with enhanced vigour and high tolerance to various environmental factors should be developed to feed the increasing world population. Maintaining crop yields under adverse environmental stresses is probably the major challenge facing modern agriculture where polyamines can play important role. Polyamines (PAs)(putrescine, spermidine and spermine) are group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure and present in almost all living organisms including plants. Evidences showed that polyamines are involved in many physiological processes, such as cell growth and development and respond to stress tolerance to various environmental factors. In many cases the relationship of plant stress tolerance was noted with the production of conjugated and bound polyamines as well as stimulation of polyamine oxidation. Therefore, genetic manipulation of crop plants with genes encoding enzymes of polyamine biosynthetic pathways may provide better stress tolerance to crop plants. Furthermore, the exogenous application of PAs is also another option for increasing the stress tolerance potential in plants. Here, we have described the synthesis and role of various polyamines in abiotic stress tolerance in plants.Key words: abiotic stress tolerance, putrescine, spermidine, spermine, polyamines     

Can genetic manipulation of plant nitrogen assimilation enzymes result in increased crop yield and greater N-use efficiency? An assessment

The literature on the relations between plant nitrogen (N) assimilation enzymes and plant/crop N assimilation, growth and yield is reviewed to assess if genetic manipulation of the activities of N assimilation enzymes can result in increased yield and/or increased N use efficiency. The available data indicate that (I) levels of N assimilation enzymes do not limit primary N assimilation and hence yield; (II) root or shoot nitrate assimilation can have advantages under specific environmental conditions; (III) for cereals, cytosolic glutamine synthetase (GS1) is a key enzyme in the mobilisation of N from senescing leaves and its activity in senescing leaves is positively related to yield; and (TV) for rice (Oryza sativd), NADH-glutamate synthase (NADH-GOGAT) is important in the utilisation of N in grain filling and its activity in developing grains is positively related to yield. In our opinion, selection of plants, from either a genetically manipulated population or genetic resources, with expression of nitrate reductase/nitrite reductase primarily in the root or shoot should increase plant/crop growth and hence yield under specific environmental conditions. In addition for cereals the selection of plants with high GS1 in senescing leaves and in some cases high NADH-GOGAT in developing grains could help maximise the retrieval of plant N in seeds.