Resumen de Role of histone H3 variant, HTR6, during stress response
Chromatin organization and function are essential for development and growth of all organisms as well as for their response to environmental challenges. Histones are key to channel intra- and extracellular signals and modulate chromatin dynamics, ultimately influencing genome functions. This is primarily achieved by post-translational modifications of histones and the exchange of canonical by variant histones. Here, we have sought to define the role of Arabidopsis thaliana HTR6, a histone H3 variant of unknown function.
We found that HTR6 expression is induced in plants exposed to abscisic acid (ABA), a hormone that is highly involved in several types of abiotic stresses, such as to high salt. A spatiotemporal expression analysis in ABA-treated roots unveiled that HTR6 is expressed in the transition/elongation zone, where cells switch from the mitotic cell cycle to the endocycle and increase their nuclear ploidy. HTR6 expression is largely excluded from the root apical meristem and also, from the differentiated zone. Moreover, HTR6 expression is confined to external cell layers, such as epidermis and cortex. At the cellular level, we found that HTR6 is mainly expressed during the S-phase in endocycling cells. Interestingly, its incorporation into chromatin relies on the HIRA and DEK3 chaperones and is independent of CAF-1. Consistent with this, HTR6 is an euchromatic histone deposited in a DNA replication-independent manner.
This is a feature shared with the histone H3.3 variant. However, HTR6 possesses unique amino acid residues, with F41 being a striking one because it is shared with the canonical H3.1. We found that F41 residue is important to increase HTR6 turnover, as revealed by the increased amount of HTR6 carrying a F41Y mutation. This is similar to the role of the F41 of H3.1 in euchromatic regions. Other changes in the C-terminal region of HTR6 to the amino acids present in H3.3, such as N63I, V124I or A132G, are sufficient to increase HTR6 accumulation. In particular, the A132 residue of HTR6 seems important for the proteasome-mediated degradation of HTR6 beyond the transition zone.
HTR6 plays a crucial role in restricting growth in response to ABA and salt, as demonstrated by the larger root length, root apical meristem and higher cortical cell number of the htr6-1 loss of function mutants. Since the ploidy level of htr6-1 root nuclei remains unaffected, HTR6 might play a role in controlling the boundary between the root apical meristem and the transition/ elongation zone in response to stress. Furthermore, our transcriptomic analysis showed that the genes that failed to be activated in the htr6-1 mutant early after ABA treatment (10 minutes), are involved in cell wall biogenesis genes and lipid metabolism, and encode members of the TCP family of transcription factors. Furthermore, several LEA family genes failed to be downregulated in the htr6-1 mutant.
Together, our studies have uncovered a key role of HTR6, a previously uncharacterized histone H3 variant, in the early response of Arabidopsis to the abiotic stress.
