WRKY transcription factor

The WRKY transcription factor family (pronounced ‘worky’) is a class of DNA-binding proteins. WRKY transcription factors are primarily specific to plants and algae (Viridiplantae); although, individual WRKY proteins do appear in the human protozoan parasite Giardia lamblia and slime mold Dictyostelium discoideum. These transcription factors recognize a (T/A)TGAC(T/A) cis-regulatory element, also known as a W-box, in the promoters of target genes. WRKY transcription factors play a major role in plant defense to abiotic and biotic stresses, but also contribute to plant development and secondary metabolism. These roles are governed by an ever increasingly complex network of interactions with other DNA-binding and non-DNA-binding proteins. The WRKY transcription factor family (pronounced ‘worky’) is a class of DNA-binding proteins. WRKY transcription factors are primarily specific to plants and algae (Viridiplantae); although, individual WRKY proteins do appear in the human protozoan parasite Giardia lamblia and slime mold Dictyostelium discoideum. These transcription factors recognize a (T/A)TGAC(T/A) cis-regulatory element, also known as a W-box, in the promoters of target genes. WRKY transcription factors play a major role in plant defense to abiotic and biotic stresses, but also contribute to plant development and secondary metabolism. These roles are governed by an ever increasingly complex network of interactions with other DNA-binding and non-DNA-binding proteins. WRKY transcription factors are denoted by a 60-70 amino acid WRKY protein domain composed of a conserved WRKYGQK motif and a zinc-finger region. Based on the amino acid sequence WRKY transcription factors are classified into three major categories, group I, group II, and group III. Group I WRKY proteins are primarily denoted by the presence of two WRKY protein domains, whereas both groups II and III each possess only one domain. Group III WRKY proteins have a C2HC zinc finger instead of the C2H2 motif of group I and II factors. The structure of several plant WRKY domains has been elucidated using crystallography and nuclear magnetic resonance spectroscopy. The first WRKY transcription factor, (SPF1), was identified in 1994 for its involvement in sucrose regulation of gene expression in sweet potato (Ipomoea batatas). A year later, two WRKY transcription factors, ABF1 and ABF2, established the role of WRKY proteins as a novel family of transcriptional regulators. In 1996 three parsley (Petroselinum crispum) WRKY transcription factors (WRKY1, WRKY2, and WRKY3) were identified for their role in regulation of the PATHOGENESIS RELATED1 gene. This was the first of many studies identifying WRKY transcription factors in regulating gene expression for plant defense. By 2000 a review paper published identifying the WRKY transcription factor family in the model plant Arabidopsis thaliana (commonly called mouse-ear cress or thale cress) lead to a widespread investigation of WRKY family members. Since then WRKY transcription factors have been extensively investigated for stress tolerance in many agronomic and horticulture crops. The WRKY transcription factor family is ubiquitously present in the green plant lineage. Most algae contain one or a few WRKY proteins, whereas even the earliest land plant, moss (Physcomitrella patens), possesses at least 30 WRKY factors. Most vascular plants contain in excess of 50 WRKY transcription factors. Outside of plant WRKY proteins have been identified in Giardia lamblia and Dictyostelium discoideum. However, little is known about the function of these proteins. Of these two WRKY proteins, WRKY1 of Giardia has been demonstrated to be involved in cell wall formation, a process also likely important to algae and early land plants. WRKYs have recently been found in the white button mushroom Agaricus bisporus. It remains unclear how and why the WRKY proteins dramatically expanded and diversified in the plant lineage but not algae. Several early reports proposed that a group I WRKY transcription factor was the progenitor of the family. It was thought that a single group I WRKY domain occurred first and then duplicated to form the original ancestral WRKY transcription factor. However, more recent evidence suggests that WRKY transcription factors evolved from a single group IIc-like gene, which then diversified into group I, group IIc, and group IIa+b domains. The original WRKY protein domain has been proposed to have arisen from the GCM1 and FLYWCH zinc finger factors. GCM1 and FLYWCH are proposed ancestral proteins base on their crystal structural similarity to the WRKY domain. Both GCM1 and FLYWCH belong to families of DNA-binding factors found in metazoan. The plant specific NAC transcription factor family also shares a common structural shape and origin with WRKY transcription factors. During plant evolution the WRKY family has dramatically expanded, which is proposed to be a result of through duplication. Some species including Arabidopsis thaliana, rice (Oryza sativa), and tomato (Solanum lycopersicum) have WRKY groups which dramatically expanded and diversified in recent evolutionary history. However, differences in expression, not variation in gene sequences, have likely lead to the diverse functions of WRKY genes. Such a model is plausible as WRKY family members are part of numerous phytohormone, developmental, and defense signaling transcriptional networks. Furthermore, W-box elements for WRKY binding occur in promoters of many other WRKY transcription factors indicating not simply a hierarchical rank in gene activation, but also which genes may have arisen later during evolution after initial WRKY regulatory networks were established.