A Study on the Role of Arabidopsis Histone Acetyltransferase HAG1 in Developmental Transition and Plant Cell Totipotency

Abstract

Every cell in an organism carries an identical genome. However, multicellular eukaryotes are composed of different cell types that show different morphology and carry out diverse functions. Each cell types are characterized by distinct gene expression patterns that are generated during development and stably maintained. The main mechanism by which the information stored in the genome can be selectively controlled is based on epigenetics. Epigenetic regulation is referred to the stable control of gene expression caused by mechanisms that are not dependent on underlying DNA sequence. Epigenetic mechanisms include DNA methylation, chromatin remodeling by ATP-dependent remodelers or histone modifications, nuclear positioning, histone variant incorporation, and regulation by noncoding RNAs. Histone acetylation is an epigenetic modification associated with euchromatin and is catalyzed by histone acetyltransferases (HATs). HATs transfer the acetyl group from acetyl-CoA to lysine residues within the N-terminal histone tails. The negative charge of the acetyl group (CH3COO-) neutralizes the positive charges of histones, reducing their affinity to negatively charged DNA. This renders DNA more accessible to the transcriptional machinery or generates binding sites for acetyl lysine-binding proteins to spread the modification, leading to transcriptional activation. In this study, I address two distinct roles HAG1/AtGCN5, a member of the Arabidopsis Gcn5-related N-acetyltransferase (GNAT) family of HATs. In the first part, I show that the HAG1 and PRZ1/ADA2b-containing SAGA-like histone acetyltransferase (HAT) complex controls the juvenile-to-adult phase transition. In Arabidopsis, vegetative phase transitions are regulated by SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) factors that are post-transcriptionally regulated by miR156 in an age-dependent manner. I demonstrate that HAG1 and PRZ1 directly controls the transcription of SPLs by catalyzing H3Ac of their chromatin, and thus determines the time for juvenile-to-adult phase transition. Furthermore, this epigenetic mechanism is also crucial for miR156-independent induction of SPLs and acceleration of phase transition by light and photoperiod or during post-embryonic growth. In the second part, I address the role of HAG1 in the acquisition of pluripotency in callus during de novo organogenesis. For decades, callus was considered as an unorganized mass of undifferentiated cells. However, recent studies have provided evidences that callus is formed by the proliferation of a subset of existing stem cell-like pericycle cells adjacent to the xylem poles. These studies have also shown that callus cells have gene-expression profiles similar to those of root primordia. I demonstrate that HAG1 protein is induced in developing calli and plays a pivotal role in the activation of root meristem genes. Upon callus induction, histone acetylation at the root meristem genes, namely WOX5/WOX14, SCARECROW (SCR), PLETHORA 1 (PLT1), and PLT2, is increased in a HAG1-dependent manner. This allows a high transcriptional output of the root meristem genes, and then the transcription factors encoded by the root meristem genes act as potency factors conferring competence for regeneration that is necessary for de novo shoot formation.