|Livestock Research for Rural Development 27 (12) 2015||Guide for preparation of papers||LRRD Newsletter||
Citation of this paper
Small grain cereals such as wheat (Triticum aestivum L.) and rice together with maize constitutes three major food and animal feed crops marketed globally. Wheat is cultivated in almost all the major continents and a wide diversity of wheat germplasms and cultivars are known across the globe. Demands for new wheat varieties are consistent. Genetic transformation of cereals such as wheat is challenging and several new methods are currently being explored.
Key words: cereals, genetic transformation
Wheat (Triticum aestivum L.) was one of the first crops to be domesticated and for the past 8000 years has been a staple food used by civilizations from West Asia, Europe and North Africa (TCM 2011). Together with rice (Oryza sativa L.) and maize (Zea mays L.), it constitutes one of the three major food and animal feed crops marketed globally (Pretty et al 2010; TCM 2011). Wheat has a large genome (16.8 Mbp) consisting of three closely related genomes (A, B and D) (Akhunov et al 2003). As of 2005, 17 % of the global cultivable land was dedicated to growth of wheat (Jones 2005). In 2006 the statistical yearbook of the Food and Agricultural Organization division of the United Nations (UN) reported that the annual average global wheat production was around 600 million tonnes (FAO 2006). Currently wheat (Figure 1) is estimated to be grown on over 240 million hectares (593 million acres) of land, with an annual global production exceeding 600 million tonnes (TCM 2011) and demand for global wheat production is expected to increase by 40-50% by 2020 (UNEP 2010).
|Figure 1. Diversity of global wheat germplasms.
Photo courtesy: P Zandi, R Sengupta and S K Basu
However, social and economic constraints resulting from the population explosion, in particular in developing countries, coupled with climate change, increased farm operating costs and rising demand for food have generated pressure to release new wheat cultivars (UNEP 2010). Demand for improved agronomic traits, including biotic and abiotic stress tolerance, a shorter growing period and increased yield, is promoting use of new technologies such as genetic engineering to meet demand for new wheat germplasms, cultivars, varieties and lines (Basu et al 2010a,b,c; Pretty et al 2010). With an increasing global human population, new wheat germplasms with quality agronomic traits are essential from the perspective of long term eco-sustainability; as well as geographical, agricultural and political eco-sociological stability in dealing with issues such as global food crisis and global food security.
Unfortunately, monocots in general and in particular cereals such as wheat are extremely challenging to work with and to successfully generate stable transformants for use in cultivar development (Bhalla et al 2006). While advanced genetic delivery approaches such as microinjection, electroporation, and pollen tube mediated gene induction have been used successfully in other crop species, they have not proven very efficient in wheat (Basu et al 2011; Tzfira and Citovsky 2008). Agrobacterium-mediated gene transfer (AMGT) and biolistics (particle bombardment) are the two most common and successful approaches used for genetic transformation of wheat (Tzfira and Citovsky 2008). Although wheat is not a natural host for the transformation bacteria Agrobacterium tumefaciens, availability of virulence gene (vir) inducing agents (eg: acetosyringone and glucose) has resulted in production of super virulent Agrobacterium strains that have made Agrobacterium – Mediated Gene Transfer (AMGT) approaches successful to some extent in wheat (Tzfira and Citovsky 2008). Nevertheless, use of biolistics (particle bombardment) has been the most popular choice for wheat transformation in many laboratories. This biolistics approach is used for both transient and stable gene expression studies (Altpeter et al 2005). One of the advantages of biolistics over AMGT is that biolistics involves direct delivery of DNA into the plant cell, and is not restricted by type or nature of the cell, species or genotype (Propelka and Altepeter 2003). However, particle bombardment also produces a heterogeneous mixture of colonies of both transformed and non-transformed cells within targeted tissues; hence there has been a need to expose targeted tissues to some tissue culture regeneration method that includes antibiotic/herbicide selection for the transformed cells. Both AMGT and biolistics are traditional/conventional gene delivery approaches that have been applied successfully to a wide variety of plant species including cereals like wheat (Tzfira and Citovsky 2008).
Another approach to successful gene delivery in monocots involves use of Cell Penetrating Peptides (CPP) which have been used successfully for gene delivery in cereals like triticale (× Triticosecale Wittm. ex A. Camus), an interspecific hybrid of wheat and rye (Secale cereale L.) (Chugh et al 2009). The CPPs represent a broad class of small peptides that can effectively translocate across the cell membrane and are capable of delivering relatively large macromolecular cargos such as plasmids, nucleic acids, small interfering RNAs, liposomes, nanoparticles, peptides and proteins, oligonucleotides, drugs and pharmaceutical products into the cytoplasm of a cell (Petersen et al 2011). However, most research involving CPPs has been conducted in animal systems (Petersen et al 2011). A wide variety of CPPs (e.g., pVEC, transportan, Tat monomer and dimer, and Pep-1) are capable of successfully translocating across the cell membrane in plants, and accumulating in the nucleus; i.e., macromolecular cargoes such as DNA, protein and fluorescent dyes can be efficiently and effectively delivered into different plant cells. These studies clearly indicate that CPPs provide a means of direct cell penetration independent of any receptor based mechanism for cellular internalization; i.e., entry into plant cells may occur by a micropinocytic mode using CPPs in an energy independent fashion (Chugh et al 2009). This CPP mediated gene delivery process has been coupled with tissue culture/plant regeneration of haploid microspores which are able to spontaneously double in chromosome number, generating a doubled haploid (DH) cell line (Basu et al 2010a,b,c). The modified Isolated Microspore Culture (IMC) protocol for cereals (Eudes and Amundsen 2005) when integrated with CPP mediated transfection of microspores appears to be an efficient system for generating potential transgenic lines. New and agonomically improved wheat germplasms with better qualitative and quantitative traits are extremely important for meeting the eco-environmental and eco-sociological challenges of a hungry world; and for achieving food self sufficiency from the perspectives of the developing and under developed nations.
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# et al in this list indicates more than 10 authors
Received 15 November 2015; Accepted 29 November 2015; Published 1 December 2015
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