Systematic histone H4 replacement in Arabidopsis thaliana reveals a role for H4R17 in regulating flowering time

Despite the broad array of roles for epigenetic mechanisms on regulating diverse processes in eukaryotes, no experimental system for the direct assessment of histone function is currently available in plants. In this work, we present the development of a genetic strategy in Arabidopsis thaliana in which modified H4 transgenes can completely replace the expression of endogenous histone H4. Using this strategy, we established a collection of plants expressing different H4 point mutants targeting residues that may be post-translationally modified in vivo. To demonstrate the utility of this new H4 mutant collection, we screened it to uncover substitutions in H4 that alter flowering time. We identified different mutations in the tail (H4R17A) and the globular domain (H4R36A, H4R39K, H4R39A, and H4K44A) of H4 that strongly accelerate the floral transition. Furthermore, we found a conserved regulatory relationship between H4R17 and the ISWI chromatin remodeling complex in plants. Similar to other biological systems, H4R17 regulates nucleosome spacing via ISWI. Overall, this work provides a large set of H4 mutants to the plant epigenetics community that can be used to systematically assess histone H4 function in A. thaliana and a roadmap to replicate this strategy for studying other histone proteins in plants.