For centuries, foresters have watched prized timber trees grow for decades before they finally flower—a delay that frustrates breeding and conservation alike. Now, researchers have discovered that a rare tree species, Phoebe chekiangensis, uses an age-dependent decline in DNA methylation as a biological timer to switch on flowering genes. By assembling the first complete, gap-free genome of this endangered species, the team identified two MADS-box transcription factors that act as floral triggers and revealed that progressive loss of CHG methylation within gene bodies—particularly at the PcMADS19.1 locus—directly activates the reproductive program. This epigenetic mechanism explains how a woody perennial tracks its own age and decides when to transition from vegetative growth to flowering.
In most forest trees, the juvenile phase stretches from five years in pines to more than two decades in oaks, making traditional breeding programs painfully slow and limiting genetic improvement. While the molecular pathways controlling flowering are well mapped in annual plants like Arabidopsis thaliana, the regulatory logic in long-lived woody species remains largely a black box. DNA methylation is known to change with age in some trees, but whether these changes actively drive the decision to flower—or are merely passive consequences of aging—has been unclear. Based on these challenges, the authors recognized the need for an in-depth investigation into how epigenetic reprogramming might govern reproductive competence in woody perennials.
A team led by researchers at Zhejiang A&F University and Huazhong Agricultural University has now published (DOI: 10.1093/hr/uhag171) the first telomere-to-telomere genome of Phoebe chekiangensis in Horticulture Research (April 24, 2026). The assembly resolves two completely gap-free haplotypes with base-level accuracy exceeding 99.9%, providing an unprecedented reference for dissecting the vegetative-to-reproductive transition in this economically and ecologically valuable lineage.
The genome spans roughly 920 megabases per haplotype and contains nearly 29,000 high-confidence genes per haplotype, with BUSCO completeness scores above 97%. Remarkably, the telomeric repeat sequences—"AAACCCT" in one haplotype and "AAATGAG" in the other—deviate from the canonical plant motif "TTTAGGG," revealing haplotype-specific telomere biology. Age-resolved transcriptomic profiling across 1-, 4-, 5-, and 10-year-old trees identified eight distinct expression clusters, with Cluster 5 genes—enriched for MADS-box transcription factors—sharply upregulating at the transition to reproductive maturity. Two of these, PcMADS5 (an AP1-like factor) and PcMADS19.1 (a SOC1-like factor), triggered precocious flowering when overexpressed in Arabidopsis, producing as few as four rosette leaves before bolting compared to 14 in wild-type controls. Whole-genome bisulfite sequencing revealed a progressive, age-dependent decline in CHG methylation, predominantly within gene bodies, while CG methylation remained stable. At the PcMADS19.1 locus, intronic CHG methylation dropped from nearly 47% to under 37% as trees matured. Chemical demethylation with 5-azacytidine further depleted CHG methylation at this locus and selectively boosted PcMADS19.1 expression, confirming a direct causal link.
"We were surprised to find that the tree doesn't just accumulate methylation changes passively as it ages—it actively demethylates specific gene regions to unlock flowering," the authors said. "The intronic methylation of PcMADS19.1 acts like a lock, and as the tree matures, that lock is gradually released. This gives us a concrete epigenetic mechanism for how a woody perennial measures its own age and commits to reproduction. It also suggests that we might eventually be able to manipulate this timer to accelerate breeding in other valuable timber species."
The findings open several practical avenues. For conservation, the T2T genome provides a high-resolution resource for managing the genetic diversity of Phoebe chekiangensis, a species with surviving populations often numbering fewer than 70 individuals. For forestry, understanding the epigenetic switch controlling flowering could enable strategies to shorten the juvenile phase in other slow-maturing trees, potentially accelerating breeding cycles for timber quality, disease resistance, and climate adaptation. The study also establishes CHG demethylation as a previously unrecognized regulatory layer in perennial development, with implications for understanding how trees balance vegetative growth against reproductive investment—a question that becomes increasingly urgent as forest ecosystems face climate pressures.
