Telomere-to-telomere genome of Phoebe chekiangensis reveals that age-dependent CHG hypomethylation promotes floral transition via MADS-box gene activation.

Abstract

Phoebe species are renowned for their highly valuable 'golden thread' timber; however, their protracted juvenile phase presents a significant obstacle to mechanistic investigations of floral induction. Phoebe chekiangensis, a rare early-flowering representative within this genus, provides a unique model system for dissecting the vegetative-to-reproductive phase transition. Nevertheless, the absence of a high-quality reference genome has severely hindered molecular insights into its developmental regulation. Here, we present the first telomere-to-telomere (T2T) genome assembly for P. chekiangensis, comprising two completely gap-free haplotypes with contig N50 values exceeding 65 Mb, base-level quality scores >36, and Long Terminal Repeat Assembly Index scores surpassing the gold standard threshold of 20. Approximately 29 000 genes were annotated per haplotype, supported by a BUSCO completeness score of >97%. Age-resolved transcriptomic landscapes identified two MADS-box transcription factors, PcMADS5 (AP1-like) and PcMADS19.1 (SOC1-like), as core activators of the floral transition. Both genes triggered precocious flowering when ectopically expressed in Arabidopsis thaliana. Whole-genome bisulfite sequencing revealed a progressive, age-dependent decline in CHG (where H is A, C, or T) DNA methylation, which was particularly pronounced at the PcMADS19.1 locus. Notably, DML1/2, which mediate active DNA demethylation, were coordinately upregulated during the onset of reproductive growth. Chemical demethylation using 5-azacytidine further diminished CHG methylation and selectively enhanced PcMADS19.1 expression, confirming a causal relationship between CHG hypomethylation and transcriptional activation. This work delivers the first chromosome-scale T2T genome within the genus Phoebe and uncovers CHG demethylation as a previously unrecognized epigenetic switch governing reproductive competence in woody perennials.