Conformational changes in three-dimensional chromatin structure in Paulownia fortunei after phytoplasma infection.

Higher-order chromatin structures play important roles in regulating multiple biological processes, growth and development, biotic and abiotic stress response. However, little is known about three-dimensional chromatin structures in Paulownia, or about whole-genome chromatin conformational changes that occur in response to Paulownia witches' broom (PaWB) disease. We used high-throughput chromosome conformation capture (Hi-C) to obtain genome-wide profiles of chromatin conformation in healthy and phytoplasma-infected Paulownia fortunei genome. The heatmap results indicated that the strongest interactions between chromosomes were in the telomeres. We confirmed the main structural characteristics, such as A/B compartments, topologically associated domains, and chromatin loops were prominent in Paulownia genome and clearly altered in phytoplasma-infected plants. By combining chromatin Immunoprecipitation sequencing, Hi-C signals, and RNA sequencing data, we inferred that the chromatin structure changed and the modification levels of three histones (H3K4me3/K9ac/K36me3) increased in phytoplasma-infected P. fortunei, which was associated with changes of transcriptional activity. We concluded that epigenetic modifications, transcriptional activity might function in combination to shape chromatin packing in healthy and phytoplasm-infected Paulownia. Finally, 11 genes (such as RPN6, Sec61 subunit alpha), commonly located at specific TAD boundaries, A/B compartment switching, specific loops, and associated with histone marks, were identified and considered as closely related to PaWB stress. Our results provide new insights into the nexus between gene regulation and chromatin conformational alterations in non-model plants upon phytopathogen infection and plant disease resistance. Key words: Paulownia witches' broom; chromatin conformation; histone modification; transcription regulation.