Engineered TAL effector proteins: Versatile reagents for manipulating plant genomes

Transcription activator-like (TAL) effectors are proteins produced by plant pathogens of the genus Xanthomonas. They are delivered to plant cells during infection and bind to specifi c plant gene promoters to activate transcription and promote bacterial infection. DNA binding by TAL effectors is mediated by an array of typically 14–24 repeats; each repeat is 34 amino acids in length and folds into a hairpin-like structure that contacts a single base in the target DNA. The TAL effector DNA-binding motif has proven highly modular, and custom TAL effector arrays can be made to recognize virtually any site in a plant genome, thereby providing a valuable reagent for genome manipulation. In particular, when TAL effector arrays are fused to a nuclease, they can create targeted double-strand breaks at a locus of interest. The repair of the breaks can be directed to achieve a variety of targeted genome modifi cations, with applications ranging from understanding plant gene function to creating novel traits in agronomically important crop species.