Development and Application of Genetic Engineering for Wheat Improvement

Wheat is one of the most important staple food crops of the world, and continuous genetic improvement is vital to meet the demands of the rapidly growing world population. Conventional breeding has led to the development of current high yielding wheat varieties, and recent achievements in genetic engineering are expected to augment conventional breeding to further increase production. Advances in genome sequencing and molecular breeding have increased the rate of gene discovery, leading to a need for highly efficient and robust transformation systems. Targeted genome editing will require efficient delivery of sequence-specific nucleases, such as zinc fingers (ZFNs), transcription activator-like effector nucleases (TALENs), and RNA-guided engineered nucleases such as CRISPR-Cas9. Since the first report of fertile transgenic wheat in 1992, optimization of plant tissue culture techniques, DNA delivery methods, gene expression cassettes, and marker genes have led to reliable transformation protocols for a range of wheat model cultivars. However, like other cereal crops, wheat transformation has also been hampered by genotype effects. The limited range of transformable tissues in wheat is considered another challenge. Several excellent review papers have described the progress made towards developing robust genetic transformation systems for wheat, so we have focused our attention on a detailed analysis of selectable markers and promoters that have been used. The choice of selectable marker and promoter can dramatically influence the outcome of a transformation project. Both Agrobacterium tumefaciens and microprojectile-mediated transformation systems have been employed successfully for genetic transformation of wheat using genes of agronomic importance. Since improvement in agronomic traits of wheat will affect a sizeable population, we have provided an analysis of the progress made towards developing genetically superior wheat containing gene(s) of agronomic importance. Recent efforts on targeted genome editing in wheat are also discussed.