Land plant-specific H3K27 methyltransferases ATXR5 and ATXR6 control plant development and stress responses.

BACKGROUND: Histone modifications are critical for transcriptional regulation. A notable genetic innovation in land plants is the emergence of histone lysine methyltransferases ATXR5/6, which specifically catalyze the repressive histone H3 lysine 27 monomethylation (H3K27me1). Current knowledge of ATXR5/6 function is largely based on Arabidopsis studies using a weak atxr5;atxr6 hypomorphic mutant, in which ATXR6 is still partially expressed and defects are primarily observed in heterochromatin. However, the significance for land plants to evolve these enzymes remains unclear. RESULTS: In this study, we generate strong atxr5;atxr6 mutants with further reduced ATXR6 expression in Arabidopsis to explore the broader roles of ATXR5/6. Our results show that ATXR5/6 are essential for plant reproductive development and play a critical role in supporting normal plant growth by repressing the transcription of stress responsive genes. In addition, ATXR5/6 are necessary for maintaining H3K27 trimethylation (H3K27me3), likely by providing H3K27me1 as a substrate for further methylation. We also demonstrate that the function of ATXR5/6 in regulating development and responsive genes is conserved in the monocot rice. CONCLUSIONS: Our findings suggest that land plants evolved ATXR5/6 not only to maintain heterochromatin, but also to regulate development and environmental responses, providing new insights into the functional significance of ATXR5/6 in land plants.