WRKY proteins: signaling and regulation of expression during abiotic stress responses

Abstract

WRKY proteins are emerging players in plant signaling and have been thoroughly reported to play important roles in plants under biotic stress like pathogen attack. However, recent advances in this field do reveal the enormous significance of these proteins in eliciting responses induced by abiotic stresses. WRKY proteins act as major transcription factors, either as positive or negative regulators. Specific WRKY factors which help in the expression of a cluster of stress-responsive genes are being targeted and genetically modified to induce improved abiotic stress tolerance in plants. The knowledge regarding the signaling cascade leading to the activation of the WRKY proteins, their interaction with other proteins of the signaling pathway, and the downstream genes activated by them are altogether vital for justified targeting of the WRKY genes. WRKY proteins have also been considered to generate tolerance against multiple abiotic stresses with possible roles in mediating a cross talk between abiotic and biotic stress responses. In this review, we have reckoned the diverse signaling pattern and biological functions of WRKY proteins throughout the plant kingdom along with the growing prospects in this field of research.

Figure 1

The mitogen-activated protein kinase (MAPK) pathway induces the activity of OsWRKY30 during drought stress. Stress signals are sensed via a transmembrane receptor, which with the help of some unknown molecules and adaptor proteins activates the MPK/MAPK pathway. This leads to the phosphorylation and activation of the MPK3. MPK3 phosphorylates the target Ser residue in the SP motif of OsWRKY30 and activates the same. The activated WRKY protein then undergoes a conformational change which favourably allows it to bind to the W-box of its target gene to induce transcription. The protein product encoded by the target gene probably helps the plant system in combating the drought stress.

Figure 2

WRKY proteins regulating plant responses against multiple abiotic stresses like salinity, drought, heat, cold, nutrient starvation, light, radiation, and oxidative stresses. “At” refers to Arabidopsis thaliana, “Os” refers to Oryza sativa, “Gm” refers to Glycine max, “Vv” refers to Vitis vinifera, “Hv” refers to Hordeum vulgare, “Ta” refers to Triticum aestivum, “Bc” refers to Brassica campestris, and “Ptr” refers to Poncirus trifoliata.

Figure 3

Specific WRKY proteins like AtWRKY18, AtWRKY40, and AtWRKY60 have been depicted as mediators of cross talk between plant responses against abiotic and biotic stresses. It has been reported that these proteins get accumulated in response to SA and JA during biotic stress as well as ABA during abiotic stress responses.