Title

Structure and Evolution of WRKY Transcription Factors

Document Type

Book Section

Abstract

In the light of increasing numbers of sequenced plant genomes we have re-evaluated the evolution of the WRKY transcription factor family. In particular, the publication of the first charophyte genome sequence from Klebsormidium flaccidum filled a gap in the available genome sequences in the plant kingdom between unicellular green algae such as Chlamydomonas reinhardtii that typically have 1–3 WRKY genes and mosses such as Physcomitrella patens that have 30–40 genes. The K. flaccidum genome contains just two WRKY genes but the presence of a Group IIb WRKY gene was unexpected and rewrites the previously accepted evolution of WRKY transcription factors. Similarly, the presence of WRKY transcription factor genes outside of the plant lineage in some diplomonads, social amoebae, fungi incertae sedis, and amoebozoa also sheds new light on the early evolution of WRKY genes. This patchy distribution of WRKY genes outside the plant lineage suggests that lateral gene transfer has been responsible for WRKY gene transfer to nonplant species. During the evolution of flowering plants, one other type of WRKY genes evolved that contain domains typical for both intracellular type-R proteins (NBS–LRR proteins) and WRKY transcription factors. These NBS–LRR–WRKY-like proteins have been divided into eight R protein–WRKY groups (RW1–RW8). They are not found in all plant genomes but have evolved numerous times in flowering plants. These chimeric proteins also contain not only novel combinations of protein domains but also novel combinations and numbers of WRKY domains. The formation of these R protein–WRKY genes is recent with classes being restricted to specific flowering plant lineages. Once formed, R protein–WRKY genes may be selected for as they combine different components of signaling pathways that may either create new diversity in signaling or accelerate signaling by short-circuiting signaling pathways.