Functional characterization of HD-Zip subfamily 2 genes of pepper and tobacco related with stresses

Abstract

To adapt against various environmental conditions, plants utilize specific genetic programs for different kinds of stresses. One of such programmed engines is the transcription factors. These factors control the downstream target genes affecting the molecular, cellular, and thus physiological status. Homeodomain leucinezippers (HD-Zip) are plant-specific transcription factors and are known to be involved in plant development and adaptation to stresses. However, many HD-Zip genes in plants are remained to be characterized for their functions. Here, I characterized two HD-Zip genes, CaHB1 (Capsicum annuum homeobox 1) and NbHB1 (Nicotiana benthamiana homeobox 1). These two genes share 84\% identity and 92\% similarity in their amino acid sequences. However, through the functional characterization of these genes, it was revealed that they play different roles in pepper and N. benthamiana. CaHB1 was up-regulated upon salt or drought stress and during senescence. Overexpression of CaHB1 in tomato enhanced tolerance against salt stress and accelerated senescence. Because CaHB1 was induced by salicylic acid and ethylene, it is possible that CaHB1 was up-regulated by these hormonal signals during senescence. However, CaHB1 was down-regulated in both compatible and incompatible interactions with pathogens. The data showing accelerated pathogen-induced cell death in CaHB1-overexpressed leaves indicate that CaHB1 modulates the extent of pathogen-induced cell death. Differently from CaHB1 expression pattern, NbHB1 was up-regulated during plant-pathogen interaction. Interestingly, its overexpression induced cell death upon treatment of methyl jasmonate or darkness, and accelerated the pathogen-induced cell death. When NbHB1 was overexpressed, NbTPI expression, which is induced by jasmonic acid, was significantly suppressed. Furthermore, silencing of NbCOI1, a jasmonic acid signaling component, delayed the NbHB1-mediated cell death. These phenotypes suggest that NbHB1 may f...