Gene activation, not silencing: how citrus turns a repressive mark on its head

DNA methylation typically silences genes, but in citrus, a surprising reversal occurs. Researchers found that CHH methylation—a type of DNA methylation usually associated with gene repression—is actually linked to higher gene expression in citrus hybrids. By analyzing a cross between mandarin (Citrus reticulata) and finger lime (Citrus australasica), the team discovered that this methylation pattern, guided by small interfering RNAs (siRNAs), accumulates at the promoters of highly active genes. This association holds across multiple citrus species and tissues, challenging long‑held assumptions about how plants regulate their genomes.

Citrus cultivation faces an unprecedented convergence of threats, from the devastating bacterial disease Huanglongbing (HLB) to drought and emerging pathogens. Most commercial varieties derive from a narrow genetic base, limiting their resilience. Wide hybridization with more distant species such as C. australasica offers a promising route to introduce new adaptive traits—but the regulatory consequences of combining genomes that have diverged for over 4 million years remain poorly understood. Understanding how these genomes interact at the molecular level is essential for predicting hybrid performance and accelerating breeding efforts. Given these challenges, there is an urgent need to investigate the epigenetic and transcriptional dynamics that shape gene expression in complex citrus hybrids.

May 4, 2026 – A team of researchers from the CNRS/University of Perpignan (LGDP), CIRAD, and the University of Montpellier has published (DOI: 10.1093/hr/uhag183) a comprehensive study in Horticulture Research revealing how DNA methylation and small RNAs shape allele‑specific gene expression in an interspecific citrus hybrid. By generating haplotype‑resolved genome assemblies and integrating transcriptomic, methylomic, and small RNA data, the researchers uncovered an unexpected positive association between a typically repressive methylation mark and transcriptional activation.

The team used a trio‑binning strategy to assemble both haplotypes of a hybrid between mandarin and finger lime at chromosome scale. Transcriptomic analysis identified nearly 3,600 genes showing biased expression toward one parental haplotype. While global methylation patterns were largely conserved, the paternal C. australasica haplotype exhibited a marked increase in CHH methylation—a hallmark of the RNA‑directed DNA methylation (RdDM) pathway. Unexpectedly, genes with CHH‑methylated promoters were significantly more highly expressed than those without, and this pattern was further associated with the presence of 24‑nucleotide siRNA clusters targeting the same promoter regions. The effect was haplotype‑specific: in the C. reticulata haplotype, the presence of siRNA clusters correlated with increased CHH methylation and higher expression among differentially expressed genes. Critically, the same association was observed in parental lines and in other citrus species including lemon, sweet orange, Cleopatra mandarin, and trifoliate orange, across both leaf and root tissues. This suggests that the link between RdDM and transcriptional activation is not a hybrid‑specific artifact but a conserved feature of the citrus genus.

The authors said: “We were genuinely surprised to find that CHH methylation, which is almost universally associated with gene silencing in model plants like Arabidopsis, actually correlates with higher gene expression in citrus. This really challenges the textbook view. What's even more striking is that this isn't just a one‑off hybrid effect—we see it across different species and tissues. It suggests that citrus has evolved a unique regulatory logic, and understanding how RdDM can activate rather than repress genes could open up new ways to think about breeding and epigenome engineering.”

These findings provide a crucial framework for interpreting gene regulation in citrus hybrids and could inform breeding strategies aimed at HLB resistance and climate adaptation. The haplotype‑resolved pipeline developed in this study offers a blueprint for analyzing complex hybrids, enabling breeders to predict which parental alleles are likely to be expressed and how epigenetic marks influence their activity. If the RdDM‑activation mechanism can be harnessed, it might be possible to boost expression of beneficial genes—such as those involved in stress tolerance or pathogen defense—without introducing foreign DNA. While the current study was conducted under non‑stressed conditions, future work will explore how these regulatory dynamics shift under biotic and abiotic challenges, and whether targeting the RdDM pathway with CRISPR‑based tools can modulate gene expression in predictable ways.

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References

DOI

10.1093/hr/uhag183

Original Source URL

https://doi.org/10.1093/hr/uhag183

Funding information

This work was supported by UPVD (BQR‑2022), Labex TULIP (ANR‑10‑LABX‑41), EUR TULIP‑GS (ANR‑18‑EURE‑0019), ANR‑23‑CE20‑0038, a PhD fellowship from Délégation Régionale Occitanie, and the Collectivité de Corse.

About Horticulture Research

Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.