Canonical histones and their variants in plants: Evolution and functions

Abstract

The DNA found inside the nuclei of eukaryotic cells is complexed with histone proteins forming the polymer called chromatin. Chromatin is organized into repeating units, nucleosomes, which are comprised of DNA wrapped around an octamer of the core histones H2A, H2B, H3, and H4. Histones are encoded by multigene families organized as clusters in animals and algae, but as dispersed copies in the genome of higher plants. The bulk of histones are expressed during the S-phase of the cell cycle in order for them to be incorporated into the chromatin of the newly replicated DNA. In addition to these canonical histones, eukaryotic genomes also encode related histone variants. Histone variants are expressed independently of the cell cycle and replace canonical histones when chromatin is disrupted by processes such as transcription, DNA repair, recombination, etc. This chapter will review the core histone families H2A, H2B, H3, and H4 in higher plants. For each family, canonical histones and their variants will be described emphasizing their evolutionary origin and the roles they play in different chromatin-mediated processes. In the plant kingdom, the core histones families have diversified allowing some isoforms to maintain their original roles, but also the emergence of new variants with novel functions. Both conserved and plant-specific histone variants participate in all aspects of plant life including development, phase transitions, flowering, responses to abiotic stresses, and germline formation among others. Many of the processes regulated by histones involve agronomically important traits highlighting their potential as targets for crop breeding and biotechnology.