Abstract
Dof proteins are known as plant-specific transcription factors. We previously reported that Dof proteins might be involved in expression of the tobacco resistance gene N for Tobacco mosaic virus (TMV). In N-carrying tobacco cultivars such as Samsun NN, a rapid upregulation of N transcription is induced by TMV infection, which is followed by the defense response called the hypersensitive response (HR). In this study, the role of a tobacco Dof protein BBF1 in N transcription was investigated. We cloned a BBF1 ORF cDNA of Samsun NN and confirmed that a full-length recombinant BBF1 could bind in vitro to DNA with a Dof binding core motif. The transient overexpression of BBF1 alone did not induce any HR but activated the endogenous N gene expression in Samsun NN. The N promoter activation by BBF1 overexpression was also confirmed in the N-lacking Samsun nn plant by using the exogenously introduced N regulatory sequence connected to a reporter gene. Additional experiments suggested that BBF1 overexpression enhanced not only ROS production but also the transcription activity of certain defense signaling and HR marker genes even without HR induction. Regarding subcellular localization, BBF1 fused with a fluorescent protein was predominantly localized in the nucleus. Based on these data, we discuss potential roles of BBF1 and other Dof proteins as transcription factors for defense responses.
