Genetics of Crop Improvement

SYNOPSIS. Man has been engaged in the genetic manipulation ofcrop plants for millennia. At its most elemental level, plantgenetic manipulation has three requirements: (1) a source ofgenetic variability that can be utilized for plant improvement;(2) methods for propagating desirable plant genotypes; and (3)strategies for the transfer and selection of useful genes orgenecombinations. The modern science of genetics has providedmany new approaches to each of these three aspects of plantimprovement. Modern plant genetics has also led to a varietyof environmental, political and economic problems. These problemsinclude the loss of valuable plant genetic resources, the widespreadadoption of monocultures that may be unstable in the face ofpathogen epidemics, and the current political debates concerningthe regulation and environmental implications of plant geneticengineering. It is impossible tojudge these and related issueswithout a basic understanding of the ways in which geneticsis applied in improving crop plants. The goal of this articleis to provide an account of the applications of plant geneticsin crop improvement.     

Apomixis for crop improvement

Summary Apomixis is a genetically controlled reproductive process by which embryos and seeds develop in the ovule without female meiosis and egg cell fertilization. Apomixis produces seed progeny that are exact replicas of the mother plant. The major advantage of apomixis over sexual reproduction is the possibility to select individuals with desirable gene combinations and to propagate them as clones. In contrast to clonal propagation through somatic embryogenesis or in vitro shoot multiplication, apomixis avoids the need for costly processes, such as the production of artificial seeds and tissue culture. It simplifies the processes of commercial hybrid and cultivar production and enables a large-scale seed production economically in both seed- and vegetatively propagated crops. In vegetatively reproduced plants (e.g., potato), the main applications of apomixis are the avoidance of phytosanitary threats and the spanning of unfavorable seasons. Because of its potential for crop improvement and global agricultural production, apomixis is now receiving increasing attention from both scientific and industrial sectors. Harnessing apomixis is a major goal in applied plant genetic engineering. In this regard, efforts are focused on genetic and breeding strategies in various plant species, combined with molecular methods to analyze apomictic and sexual modes of reproduction and to identify key regulatory genes and mechanisms underlying these processes. Also, investigations on the components of apomixis, i.e., apomeiosis, parthenogenesis, and endosperm development without fertilization, genetic screens for apomictic mutants and transgenic approaches to modify sexual reproduction by using various regulatory genes are receiving a major effort. These can open new avenues for the transfer of the apomixis trait to important crop species and will have far-reaching potentials in crop improvement regarding agricultural production and the quality of the products.     

Halophytes as a source of genes for abiotic stress tolerance

Agricultural productivity is majorly impacted due to various abiotic stresses, particularly salinity and drought. Halophytes serve as an excellent resource for identifying and developing new crop systems, as these grow very luxuriously in very high saline soils. Understanding salinity stress tolerance mechanisms in such plants is an important step towards generating crop varieties that can cope with environmental stresses. Use of modern tools of ‘omics’ analyses and small RNA sequencing has helped to gain insights into the complex plant stress responses. Salinity tolerance being a multigenic trait requires a combination of strategies and techniques to successfully develop improved crops varieties. Many transgenic crops are being developed through genetic transformation. Besides marker-assisted breeding/QTL approaches are also being used to improve abiotic stress tolerance. In this review, we focus on the recent developments in the utilization of halophytes as a source of genes for genetic improvement in abiotic stress tolerance of crops.     

From plant immunity to crop disease resistance

Plant diseases caused by diverse pathogens lead to a serious reduction in crop yield and threaten food security worldwide. Genetic improvement of plant immunity is considered as the most effective and sustainable approach to control crop diseases. In the last decade, our understanding of plant immunity at both molecular and genomic levels has improved greatly. Combined with advances in biotechnologies, particularly clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-based genome editing, we can now rapidly identify new resistance genes and engineer disease-resistance crop plants like never before. In this review, we summarize the current knowledge of plant immunity and outline existing and new strategies for disease resistance improvement in crop plants. We also discuss existing challenges in this field and suggest directions for future studies.     

代谢基因工程提高作物产量的分子靶标

应用基因工程技术对植物细胞内的代谢途径进行遗传修饰,已成功地使细胞代谢发生改变或合成新的化合物。光合作用,淀粉合成,氮素同化和水分利用等是形成作物产量的基础代谢。对这些代谢途径中的关键步骤和靶分子进行基因修饰以提高作物产量的研究已取得长足的进展,并正在发展成为提高作物产量的新途径。本文着重论述应用代谢基因工程提高作物产量的技术策略,研究现状,存在的问题,所面临的挑战和应用前景。     

SOMACLONAL AND GAMETOCLONAL VARIATION

For several years it has been recognized that introduction of plant cells into culture results in genetic changes. These genetic alterations have been recovered in the plants regenerated from cell cultures. More recently it has been recognized that this method of introducing genetic changes into crop plants could be used to develop new breeding lines. The technology of introducing genetic variation by using cell culture has been termed somaclonal and gametoclonal variation. This paper reviews the history of this technology and offers genetic documentation of somaclonal variation in tomato. As this variation represents a new tool for the plant breeder, breeding strategies for the use of this variation are presented and discussed. Somaclonal and gametoclonal variation are new tools for the geneticist and plant breeder that permit reduction in the time period for new variety development and that permit access to new classes of genetic variation.     

The role and basics of computer simulation in support of critical decisions in plant breeding

A number of crucial decisions face the plant breeder before any field activities directed to crop genetic improvement are actually initiated, primarily related to choice of parents and breeding strategy options. Because of the impact, the complexity of these decisions, and the cost of implementing multiple options, computer simulation can be an important resource for the modern breeder. To maximize utility, the simulation tool must be based on effective models of the genome, the breeding process, and other ‘processes’ involved in genetic recombination, identification, and production of new cultivars. Additionally, the statistical methodology employed has ramifications for predicting performance and breeding outcome. The objective of this work is to highlight the role of computer simulation in the planning phases of crop genetic improvement, the basics of model building, statistical considerations, and key issues to be addressed. Examples of publicly available simulation software for plant breeding scenarios are described (features, functionalities, and assumptions) and new directions for improved/expanded approaches and tools are discussed.     

Harvesting the potential of induced biological diversity

For most of the past century, chemical and physical mutagens have been used in plant genetic research to introduce novel genetic variation. In crop improvement, more than 2000 plant varieties that contain induced mutations have been released for cultivation having faced none of the regulatory restrictions imposed on genetically modified material. In plant science, mutational approaches have found extensive use in forward genetics and for enhancer and suppressor screens - particularly in model organisms where positional cloning is easily achieved. However, new approaches that combine mutagenesis with novel and sensitive methods to detect induced DNA sequence variation are establishing a new niche for mutagenesis in the expanding area of (crop) plant functional genomics and providing a bridge that links discovery in models to application in crops.     

Engineering plant virus resistance: from RNA silencing to genome editing strategies

Viral diseases severely affect crop yield and quality, thereby threatening global food security. Genetic improvement of plant virus resistance is essential for sustainable agriculture. In the last decades, several modern technologies were applied in plant antiviral engineering. Here we summarized breakthroughs of the two major antiviral strategies, RNA silencing and genome editing. RNA silencing strategy has been used in antiviral breeding for more than thirty years, and many crops engineered to stably express small RNAs targeting various viruses have been approved for commercial release. Genome editing technology has emerged in the past decade, especially CRISPR/Cas, which provides new methods for genetic improvement of plant virus resistance and accelerates resistance breeding. Finally, we discuss the potential of these technologies for breeding crops, and the challenges and solutions they may face in the future.     

A new mechanism in plant engineering: The potential roles of microRNAs in molecular breeding for crop improvement

MicroRNAs (miRNAs) are small, endogenous, noncoding RNAs that negatively modulate the expression of genes by inhibiting translation or by promoting the degradation of target mRNAs. miRNAs are now known to have greatly expanded roles in a variety of plant developmental processes, in signal transduction, and in the response to environmental stress and pathogen invasion. Because of their ability to inactivate either specific genes or entire gene families, artificial miRNAs function as dominant suppressors of gene activity when brought into a plant. Consequently, miRNA-based manipulations have emerged as promising new approaches for the improvement of crops. This includes the development of breeding strategies and the genetic modification of agronomic traits. Herein, we highlight new findings regarding the roles of miRNAs in plant traits, and describe the current miRNA-based plant engineering approaches. Finally, we consider the feasibility of modulating current approaches to address future challenges such as breeding programs to increase crop yield.     

Crop improvement through tissue culture

Plant tissue culture comprises a set of in vitro techniques, methods and strategies that are part of the group of technologies called plant biotechnology. Tissue culture has been exploited to create genetic variability from which crop plants can be improved, to improve the state of health of the planted material and to increase the number of desirable germplasms available to the plant breeder. Tissue-culture protocols are available for most crop species, although continued optimization is still required for many crops, especially cereals and woody plants. Tissueculture techniques, in combination with molecular techniques, have been successfully used to incorporate specific traits through gene transfer. In vitro techniques for the culture of protoplasts, anthers, microspores, ovules and embryos have been used to create new genetic variation in the breeding lines, often via haploid production. Cell culture has also produced somaclonal and gametoclonal variants with crop-improvement potential. The culture of single cells and meristems can be effectively used to eradicate pathogens from planting material and thereby dramatically improve the yield of established cultivars. Large-scale micropropagation laboratories are providing millions of plants for the commercial ornamental market and the agricultural, clonally-propagated crop market. With selected laboratory material typically taking one or two decades to reach the commercial market through plant breeding, this technology can be expected to have an ever increasing impact on crop improvement as we approach the new millenium.D.C.W. Brown is with Agriculture and Agri-Food Canada, Central Experimental Farm, Plant Research Centre, Ottawa, Ontario, K1A 0C6, Canada. T.A. Thorpe is with the Plant Physiology Research Group, Department of Biological Sciences, University of Calgary, Calgary, Alberta, T2N 1N4, Canada     

Production of transgenic rice with agronomically useful genes: an assessment

Rice is the most important food crop in tropical and subtropical regions of the world. Yield enhancement to increase rice production is one of the essential strategies to meet the demand for food of the growing population. Both abiotic and biotic features limit adversely the productivity of rice growing areas. Conventional breeding has been an effective means for developing high yielding varieties, however; it is associated with its own limitations. It is envisaged that recent trends in biotechnology can contribute to the agronomic improvement of rice in terms of yield and nutritional quality as a supplement to traditional breeding methods. Genetic transformation of rice has demonstrated numerous important opportunities resulting in the genetic improvement of existing elite rice varieties and production of new plant types. Significant advances have been made in the genetic engineering of rice since the first transgenic rice plant production in the late 1980s. Several gene transfer protocols have been employed successfully for the introduction of foreign genes to rice. In more than 60 rice cultivars belonging to indica, japonica, javanica, and elite African cultivars, the protocol has been standardized for transgenic rice production. Selection and use of appropriate promoters, selectable markers, and reporter genes has been helpful for development of efficient protocols for transgenic rice in a number of rice cultivars. The present review is an attempt to assess the current state of development in transgenic rice for the transfer of agronomically useful genes, emphasizing the application and future prospects of transgenic rice production for the genetic improvement of this food crop.     

In vitro mutagenesis of Chrysanthemum morifolium cultivars using ethylmethanesulphonate (EMS) and mutation assessment by ISSR and IRAP markers

Plant Cell, Tissue and Organ Culture (PCTOC) - Mutation induction is a feasible and established breeding method for crop improvement and genetic diversity creation to introduce new plant cultivars....     

MicroRNA-mediated gene regulation: potential applications for plant genetic engineering

Food security is one of the most important issues challenging the world today. Any strategies to solve this problem must include increasing crop yields and quality. MicroRNA-based genetic modification technology (miRNA-based GM tech) can be one of the most promising solutions that contribute to agricultural productivity directly by developing superior crop cultivars with enhanced biotic and abiotic stress tolerance and increased biomass yields. Indirectly, the technology may increase usage of marginal soils and decrease pesticide use, among other benefits. This review highlights the most recent progress of transgenic studies utilizing various miRNAs and their targets for plant trait modifications, and analyzes the potential of miRNA-mediated gene regulation for use in crop improvement. Strategies for manipulating miRNAs and their targets in transgenic plants including constitutive, stress-induced, or tissue-specific expression of miRNAs or their targets, RNA interference, expressing miRNA-resistant target genes, artificial target mimic and artificial miRNAs were discussed. We also discussed potential risks of utilizing miRNA-based GM tech. In general, miRNAs and their targets not only provide an invaluable source of novel transgenes, but also inspire the development of several new GM strategies, allowing advances in breeding novel crop cultivars with agronomically useful characteristics.     

Recent advances in genetic engineering for improvement of fruit crops

Fruits are one of the major sources of vitamins, essential nutrients, antioxidants and fibers in human diet. During the last two–three decades, genetic engineering methods based on the use of transgenes have been successfully adopted to improve fruit plants and focused mainly on enhanced tolerance to biotic and abiotic stresses, increased fruit yield, improved post harvest shelf life of fruit, reduced generation time and production of fruit with higher nutritional value. However, the development of transgenic fruit plants and their commercialization are hindered by many regulatory and social hurdles. Nowadays, new genetic engineering approaches i.e. cisgenesis or intragenesis receive increasing interest for genetic modification of plants. The absence of selectable marker gene in the final product and the introduced gene(s) derived from the same plant or plants sexually compatible with the target crop should increase consumer’s acceptance. In this article, we attempt to summarize the recent progress achieved on the genetic engineering in fruit plants and their applications in crop improvement. Challenges and opportunities for the deployment of genetic engineering in crop improvement programs of fruit plants are also discussed.     

Genetic improvement of Purslane (Portulaca oleracea L.) and its future prospects

Common purslane (Portulaca oleracea), also known as pigweed, fatweed, pusle, and little hogweed, is an annual succulent herb in the family Portulacaceae that is found in most corners of the globe. From the ancient ages purslane has been treated as a major weed of vegetables as well as other crops. However, worldwide researchers and nutritionists have studied this plant as a potential vegetable crop for humans as well as animals. Purslane is a nutritious vegetable with high antioxidant properties and recently has been recognized as the richest source of α-linolenic acid, essential omega-3 and 6 fatty acids, ascorbic acid, glutathione, α-tocopherol and β-carotene. The lack of vegetable sources of ω-3 fatty acids has resulted in a growing level of attention to introduce purslane as a new cultivated vegetable. In the rapid-revolutionizing worldwide atmosphere, the ability to produce improved planting material appropriate to diverse and varying rising conditions is a supreme precedence. Though various published reports on morphological, physiological, nutritional and medicinal aspects of purslane are available, research on the genetic improvement of this promising vegetable crop are scant. Now it is necessary to conduct research for the genetic improvement of this plant. Genetic improvement of purslane is also a real scientific challenge. Scientific modernization of conventional breeding with the advent of advance biotechnological and molecular approaches such as tissue culture, protoplast fusion, genetic transformation, somatic hybridization, marker-assisted selection, qualitative trait locus mapping, genomics, informatics and various statistical representation have opened up new opportunities of revising the relationship between genetic diversity, agronomic performance and response to breeding for varietal improvement. This review is an attempt to amalgamate the assorted scientific information on purslane propagation, cultivation, varietal improvement, nutrient analyses, medicinal uses and to describe prospective research especially for genetic improvement of this crop.     

The myth of plant transformation

Technology development is innovative to many aspects of basic and applied plant transgenic science. Plant genetic engineering has opened new avenues to modify crops, and provided new solutions to solve specific needs. Development of procedures in cell biology to regenerate plants from single cells or organized tissue, and the discovery of novel techniques to transfer genes to plant cells provided the prerequisite for the practical use of genetic engineering in crop modification and improvement. Plant transformation technology has become an adaptable platform for cultivar improvement as well as for studying gene function in plants. This success represents the climax of years of efforts in tissue culture improvement, in transformation techniques and in genetic engineering. Plant transformation vectors and methodologies have been improved to increase the efficiency of transformation and to achieve stable expression of transgenes in plants. This review provides a comprehensive discussion of important issues related to plant transformation as well as advances made in transformation techniques during three decades.     

Plant biotechnology for crop improvement

The typical crop improvement cycle takes 10–15 years to complete and includes germplasm manipulations, genotype selection and stabilization, variety testing, variety increase, proprietary protection and crop production stages. Plant tissue culture and genetic engineering procedures that form the basis of plant biotechnology can contribute to most of these crop improvement stages. This review provides an overview of the opportunities presented by the integration of plant biotechnology into plant improvement efforts and raises some of the societal issues that need to be considered in their application.