Genetic improvement of wheat via alien gene transfer, an assessment
Introduction
Wheat is a member of the Triticeae group of cereals and indisputably one of the major food crops of the world and a foundation of human nutrition worldwide. In addition to its basic caloric value, wheat, with its high protein content, is the single most important source of plant protein in the human diet [1]. Traditionally, genetic improvement in wheat is generated by using extensive crossing program and then systematically selection of useful new combinations [2], [3]. Although, with the help of the genetic variation present in wheat, plant breeders have tried for decades to improve the yield of wheat by using conventional methods of breeding, but their efforts were reaching plateau especially with respect to yield. Growth rates of yields have slowed during the period between 1987 and 2001 [4]. Moreover, the world's population will reach 8 billion by 2025 and it has been estimated that food and feed production must continue to rise annually by 1.2% to satisfy the demand of the world's population [5]. Past success, therefore, does not guarantee a food abundant world in coming decades. Hence the genetic improvement of wheat has received considerable attention worldwide over the years with the purpose of increasing the grain yield, to minimize crop loss due to unfavorable environmental conditions and development of resistance against various pests and pathogens [6], [7].
The last decade has witnessed the extensive use of recombinant DNA technology for introduction of exogenous DNA into major cereal crops including wheat [8]. The technology is based on the delivery of defined foreign genes into plant cells, obtaining integration of the genes into plant genome and subsequently plant regeneration from transformed cells or tissues. Although, in vitro culture techniques exist for several decades but they are now playing a key role in the applicability of genetic engineering techniques for the improvement of plant species. The efficiency of transformation is greatly influenced by genotype, explant source and also medium composition. Therefore, most of the approaches to transform wheat have attempted to develop a genotype independent and cost effective procedure for the introduction of alien genes. The first transgenic wheat plants were produced by Vasil et al. [9], followed by Vasil et al. [10], Weeks et al. [11], Nehra et al. [12], Becker et al. [13] and Altpeter et al. [14] employing microprojectile bombardment as a method of DNA delivery. Subsequently, the development of methodology for the delivery of genes into intact plant tissues by bombardment of DNA-coated gold or tungsten particles has revolutionized the field of wheat transformation. In recent years, sincere efforts are being made to transform wheat genetically with different alien genes of agronomically importance [15], [16], [17], [18], [19], [20], [21]. However, in majority of reports, genetic transformation with a single target gene has been used for the production of transgenic wheat expressing tolerance to herbicide, resistance to fungal and viral diseases [7], [8]. As most agronomic traits are polygenic in nature, wheat genetic engineering will require the integration of multiple transgenes into the plant genome, while ensuring their stable inheritance and expression in succeeding generations. Therefore, recent developments in wheat genetic engineering are aimed to the integration of multiple transgenes into the plant genome and coordinated expression of these transgenes in transformed plants. In next decade, therefore, it is assumed that wheat transformation is going to play a very crucial role in complementing the conventional wheat breeding for generating novel cultivars with desirable characters [22].
Irrespective of methods and genotypes used in attempts to transform wheat, the best results have been obtained by direct bombardment of scutellum of immature embryos. Therefore, the present review is devoted to the achievements made in obtaining transgenic plants by employing microprojectile bombardment as a device for direct DNA delivery into the immature embryos of wheat. In recent years, Agrobacterium-mediated transformation has emerged highly efficient alternative to direct gene delivery in a number of economically important crop plants including wheat [23], [24], [25]. Therefore, we have also focused our attention to the attempts made in obtaining transgenic wheat using Agrobacterium as a vehicle of DNA delivery.
Section snippets
Tissue culture and regeneration: a prerequisite for genetic transformation of wheat
Genetic transformation of cereals including wheat largely depends on the ability of transformed tissues to proliferate on selection medium and subsequently regeneration of plants from transformed cells. Indeed, it is the totipotency of plant cells that underlies in the success of most plant transformation systems. In cereals, immature embryos are considered the most responsive explant in culture because of their ability to produce readily embryogenic callus and subsequently large number of
Genetic transformation of wheat employing microprojectile bombardment
Successful genetic manipulation requires the ability to deliver biologically active and functional DNA into plant cells followed by recovery of transgenic plants expressing a foreign gene. Success in genetic transformation of cereals was difficult to achieve and often limited to transient gene expression because of the lack of suitable regenerative systems and incapability of Agrobacterium to infect cereal tissues. This is the main reason that the method of introducing DNA into cells by
Genetic transformation of wheat mediated by Agrobacterium tumefaciens
Biotechnological strategies for crop improvement demand efficient procedures for routine introduction of low copy number of defined foreign genes into plant genome. The use of Agrobacterium vectors for genetic transformation confers advantages, over direct DNA delivery techniques, which include a high frequency of stable genomic integration and single/low copy number of the intact transgene. Infact, these characteristics of Agrobacterium-mediated transformation have encouraged researcher to
Reporter and selectable marker genes for wheat transformation
Irrespective of methods used for the delivery of foreign DNA into target tissue of wheat, the choice of markers, reporter, promoters and introns greatly influences the final outcome. In addition, better selection of transformed cells minimize the risk of escapes and allows better chance for the recovery of transformed cells and subsequently transgenic plants. The reporter genes produce a visible effect and therefore, allows detection of expression of foreign DNA within the transformed tissue at
Promoters for driving transgene in wheat
Efficient expression of a gene in target tissue critically dependent on the selection of an appropriate promoter. Several promoters have been used to control the expression of selectable marker and reporter genes in transgenic wheat. These include heterologous promoters such as those of shrunken gene of maize [132], copia LTR from Drosophila [133], the cauliflower mosaic virus (CaMV) 35S RNA transcript (35S) promoter [133], [134] and mannopine synthase 2 gene [135]. However, because level of
Agronomically important genes introduced into wheat
Recent advances in transformation technology have resulted in the routine production of transgenic wheat plants for the introduction of not only marker genes but also agronomically important genes for quality improvement, male sterility, transposon tagging, resistance to drought stress, resistance against fungal pathogen and insect resistance (Table 3). However, unlike rice and maize, only a few agronomically useful genes have been introduced in wheat. The first report of an agronomically trait
Conclusion and future prospective
Although production of fertile transgenic plants of wheat is no longer a dream now, but genotype-dependent response and low transformation frequency are still major hurdle in utilizing the powerful recombinant DNA technology for genetic improvement of this important crop with agronomically important genes. In the majority of the published reports, the transformation frequency lies between 0.15 and 2.34% and therefore, the question, how to increase the efficiency of wheat transformation is still
Acknowledgements
A.K. Sahrawat wishes to thank the Alexander von Humbolt Foundation of Germany for the award of long-term Alexander von Humboldt Research Fellowship. The authors thank Dr Manfred Gahrtz for helpful discussions.
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