TAL effectors: highly adaptable phytobacterial virulence factors and readily engineered DNA-targeting proteins

• Recent literature concerning transcription activator-like (TAL) effectors is reviewed.
• TAL effectors are plant bacterial virulence factors that activate host genes.
• DNA sequence recognition by TAL effectors is structurally encoded and customizable.
• Understanding the roles of TAL effectors opens avenues for engineered plant resistance.
• TAL effector-based DNA targeting is advancing research, biotechnology, and medicine.

Transcription activator-like (TAL) effectors are transcription factors injected into plant cells by pathogenic bacteria of the genus Xanthomonas. They function as virulence factors by activating host genes important for disease, or as avirulence factors by turning on genes that provide resistance. DNA-binding specificity is encoded by polymorphic repeats in each protein that correspond one-to-one with different nucleotides. This code has facilitated target identification and opened new avenues for engineering disease resistance. It has also enabled TAL effector customization for targeted gene control, genome editing, and other applications. This article reviews the structural basis for TAL effector-DNA specificity, the impact of the TAL effector-DNA code on plant pathology and engineered resistance, and recent accomplishments and future challenges in TAL effector-based DNA targeting.