Overexpression of BBF3 encoding a tobacco Dof transcription factor alters gene expression and cell death induction during N-mediated hypersensitivity to tobacco mosaic virus.

Dof (DNA binding with one finger) proteins are plant-specific transcription factors involved in gene regulation during abiotic and biotic stresses. Our previous studies suggested that tobacco Dof proteins, including BBF3, can upregulate the expression of the virus resistance N gene as well as resistance-related genes. In this study, we generated and characterized transgenic tobacco lines carrying the N gene that overexpressed BBF3 cDNA constitutively. In the absence of virus challenge, the BBF3 overexpression caused no developmental defects and no changes in the N transcript level, although the transcript levels of some defense-related genes were affected compared with levels in non-transgenic plants. TMV infection induced a hypersensitive reaction (HR) with larger necrotic lesions in transgenic lines than in non-transgenic plants, but there was no change in total virus accumulation per lesion in a standardized area. Activation of the N gene expression in non-transgenic plants during synchronously-induced HR was followed by downregulation of BBF3 gene expression. Constitutive overexpression of the BBF3 transgene resulted in higher accumulation of the N gene as well as HR marker genes (Hin1 and Hsr203j) and lower accumulation of salicylic acid- and jasmonic acid-signaling marker genes (PR1-a and PR-1b). In particular, the expression of PR-1b was remarkably suppressed in the BBF3-overexpressing transgenic lines. Dof-binding motifs were confirmed in the upstream regions of all target genes examined. Our results suggest that BBF3 has a role in modulating the expression of genes associated with virus elicitor recognition and defense responses resulting in larger areas of HR-associated cell death. [ABSTRACT FROM AUTHOR]