Transgenic Approaches

Transgenic approaches remove all taxonomic limits to plant improvement. The manipulation of genes through biotechnology has provided the opportunity of genetic engineering plant responses to abiotic stresses such as heat, drought, and salinity (Qin et al. 2011). The regulated expression of stress-induced transcription factors, for example, has been identified as an attractive tool for improving stress tolerance, since transcription factors can regulate the expression of a large number of relevant downstream genes associated to abiotic stress responses in genetically modified plants (Nakashima et al. 2009). Transcription factors such as DREB1/ CBF, DREB2, AREB/ABF, and NAC, are used to improve stress tolerance to abiotic stresses in various grasses including wheat and rice.

Recent efforts to test the functionality of various transcription factors involved in complex physiological responses and to evaluate the effect of these under either constitutive or inducible promoters have been made in wheat. Transgenic wheat lines were developed at CIMMYT using new constructs generated at the Japan International Research Center for Agricultural Sciences (JIRCAS) and Rikagaku

Kenkyūjo (RIKEN). The resulting lines were then characterized under glasshouse conditions and open field trials conducted at CIMMYT's subtropical experimental station in Tlaltizapán, Morelos, Mexico (18° 41′ N, 99° 10′ W, 940 m asl). In these open-field trials, physiological traits and grain yield performance of 116 homozy-

gous low copy number GM wheat lines (14 gene-promoter combinations) were evaluated under different water regimes. The lines evaluated in the field showed no pleiotropic effect, nor unpredictable unwanted effects when compared to control lines (Saint Pierre et al. 2012). Based on a 3-year analysis, the most promising lines showed an average increase of yield under drought of 10 % when compared to nontransformed line (var. Fielder).

Even though promising results have been observed, several challenges have yet to be overcome before anticipating a high impact of genetically modified lines on wheat production. Efforts need to address the identification of appropriate gene and gene-promoter combinations, the insertion of the proper transgene in appropriate backgrounds, and field selection strategies. Regulation processes, commercialization, and marketing of genetically modified products are parallel challenges to face. From the technical side, a critical step in the case of genetically modified wheat is that more efficient transformation protocols as well as precise expression systems would need to be defined. The wheat variety Fielder is commonly used in transformation due to its good embryogenesis capacity and regeneration efficiency; however it has lower yield potential than modern elite wheat lines. Then, after insertion

Fig. 41.3 Yield increases as percentage over non-transformed Fielder line, observed for the most promising lines from transformations using different gene-promoter combinations. Field trials were grown during 3-year, in open field trial in Mexico, 2010–2013

of the transgenes in wheat backgrounds (Fielder) it is necessary to transfer them to modern elite lines, which is currently achieved by backcrossing. Predictions based on extrapolating yield increases from the transformed line Fielder to new high performing germplasm (elite lines) should be made with caution. The challenge is, therefore, to break the yield barriers in modern elite wheat lines to finally achieve a significant improvement in wheat grain yield under stress. Collective and cooperative interventions from molecular biologists, physiologists, and breeders are required (Fig. 41.3).

 
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