Activation of a Novel Transcription Factor through Phosphorylation by WIPK, a Wound-Induced MitogenActivated Protein Kinase in Tobacco Plants

Wound-induced protein kinase (WIPK) is a tobacco (Nicotiana tabacum) mitogen-activated protein kinase known to play an essential role in defense against wounding and pathogens, although its downstream targets have yet to be clarified. This study identified a gene encoding a protein of 648 amino acids, which directly interacts with WIPK, designated as N. tabacum WIPK-interacting factor (NtWIF). The N-terminal region with approximately 250 amino acids showed a high similarity to the plant-specific DNA binding domain, B3, but no other similarity with known proteins. The C terminus of approximately 200 amino acids appeared to be essential for the interaction with WIPK, and a Luciferase-reporter gene assay using Bright Yellow 2 cells indicated the full-length protein to possess trans-activation activity, located to the middle region of approximately 200 amino acids. In vitro phosphorylation assays indicated that WIPK efficiently phosphorylates the full-length protein and the N terminus but not the C terminus. When full-length NtWIF was coexpressed with WIPK in Bright Yellow 2 cells, the Luciferase transcriptional activity increased up to 5-fold that of NtWIF alone, whereas no effect was observed with a kinase-deficient WIPK mutant. Transcripts of NtWIF began to simultaneously accumulate with those of WIPK 30 min after wounding and 1 h after the onset of hypersensitive response upon tobacco mosaic virus infection. These results suggest that NtWIF is a transcription factor that is directly phosphorylated by WIPK, thereby being activated for transcription of target gene(s) involved in wound and pathogen responses.