HAC-NPR1-TGA 복합체에 의한 애기장대 면역의 후성유전학적 조절

Abstract

Unlike animals, plants lack specialized immune cells. Instead, plants have developed multiple layers of sophisticated immune responses through massive expression of immune-related genes including the pathogenesis related (PR) genes to respond to bacterial or viral pathogenic attacks. Upon pathogenic attack, plants turn on the innate immune system as the first line of defense. The immune response is initiated at the site of infection by accumulating salicylic acid (SA). Then, as a major signaling molecule in plant immunity, SA is accumulated at distal tissues to protect the entire plant against successive attacks by various pathogen. This whole plant resistance response is referred as systemic acquired resistance (SAR). SA signal results in the activation of the master immune regulator, NPR1, which is recruited by TGACG SEQUENCE-SPECIFIC BINDING PROTEIN (TGA) transcription factors to numerous downstream PR genes. However, despite the critical role of NPR1 in SA-triggered immunity, the biochemical mechanism of NPR1 as a transcriptional co-activator remain largely unknown. Epigenetics is the study of heritable changes in gene expression which are not based on changes in DNA sequence. Histone acetylation is an epigenetic modification that occurs at the lysine residue of N-terminal histone tail. Histone acetyltransferases (HATs) transfer the acetyl group (COCH3) from acetyl coenzyme A (acetyl-CoA) to the NH3+ amino group of histones. HATs are also known as transcription co-activators, leading to transcriptional activation. In Arabidopsis, recent studies reported that CBP/p300 HAC family proteins possess histone acetyltransferase activities. The two types of zinc finger domains ZnF-TAZ and ZnF-ZZ in CBP/p300 families are known to be important for mediating protein–protein interactions. Epigenetic regulation through several HACs is known to play crucial roles in flowering, various developmental processes, and ethylene signaling pathway. In this study, I showed that the CBP/p300-family histone acetyltransferases, HAC1 and HAC5 (HAC1/5) are required for SA-triggered immunity and PR induction in Arabidopsis. During SA-triggered immune response, HAC1 form a complex with NPR1 and TGAs to activate PR genes by histone acetylation. Thus, this study reveals the function of HAC1 as a co-activator of NPR1 and the precise biochemical mechanism of NPR1-mediated transcriptional activation. Furthermore, this study also proposes epigenetic reprogramming acts as an essential part of plant immune system which allows plants to efficiently switch their regular developmental program to a defense program upon pathogenic attack.