Epigenetic Switch Controls How Flowers Decide When to Stop Growing

One of the most deceptively simple questions in biology is also one of the hardest to answer: how does an organ know when it has reached the right size? For plants, this question is not merely academic. Every leaf, petal, and seed-bearing structure must hit a precise target dimension to function properly, and deviations can compromise reproduction, yield, and survival. A new study published in Plant Cell Reports by Dan Xiang, Lilan Hong, and colleagues at Zhejiang University has now uncovered a surprising molecular circuit that answers this question for a small but vital floral organ, revealing that an epigenetic regulator, a gaseous hormone, and reactive oxygen species form a chain of command that tells developing flowers when to stop growing.

The organ in question is the sepal, the outermost floral whorl of the model plant Arabidopsis thaliana. Sepals are often dismissed as the plain, green wrappers of the flower, but they perform an essential protective role, shielding the delicate reproductive structures inside the bud until the moment of blooming. To do this well, a sepal must be neither too small, which would leave the bud exposed, nor excessively large, which would waste resources. Sepals have also become a favorite model system for studying organ size because their surface cells grow with striking variability, yet the organ as a whole achieves a reproducible final shape and dimensions through spatiotemporal averaging of that noisy growth. Understanding what halts sepal elongation at the right time therefore speaks to a fundamental problem in developmental biology.

The Zhejiang University team focused their investigation on a protein called histone deacetylase complex 1, or HDC1. Histone deacetylases are chromatin-associated enzymes that remove acetyl groups from histone proteins, the spools around which DNA is wound. When histones are highly acetylated, the chromatin tends to be open and genes are permissive to transcription; deacetylation generally compacts chromatin and silences genes. HDC1 was already known to be a versatile player in plant biology. Previous work had shown that its expression level titrates overall plant growth and abscisic acid sensitivity, that it facilitates hypocotyl elongation through a PIF4-dependent process, and that it helps remodel root architecture under low-phosphate conditions. What had remained unclear was whether HDC1 has any role in determining the final size of aerial reproductive organs.

Using a combination of genetic, transcriptomic, and physiological analyses in Arabidopsis, the researchers found that HDC1 acts as a critical regulator of sepal size specifically during the maturation phase of the organ. When HDC1 function is compromised, sepals fail to terminate their elongation properly, indicating that this protein is required to bring cell expansion to a controlled stop. The finding is significant because most studies of organ size have concentrated on the early, proliferative phase of growth, when cells divide rapidly and the organ primordium is established. The maturation phase, during which cells decide whether to keep expanding or to cease growth, has been far less well characterized, particularly for sepals.

The mechanistic story that emerges from the study is elegant and, in places, unexpected. During sepal maturation, HDC1 represses the expression of genes encoding protein phosphatase 2A, or PP2A, a family of serine/threonine phosphatases with well-documented roles in diverse signaling pathways, from abscisic acid responses in guard cells to microtubule organization. HDC1 achieves this repression through histone H3 deacetylation at the PP2A loci, a direct epigenetic mechanism. With PP2A expression dampened, the balance of phosphorylation within the cell shifts, and this shift has consequences for one of the most important hormonal systems in plants: ethylene biosynthesis.

Ethylene, the simple two-carbon gas that ripens fruit and promotes senescence, is synthesized from the amino acid methionine via a pathway whose rate-limiting enzymes are the ACC synthases. The activity and stability of these enzymes are famously controlled by phosphorylation. Prior work had established that mitogen-activated protein kinases can phosphorylate ACC synthases such as ACS6, stabilizing the protein and boosting ethylene production, while PP2A differentially regulates the turnover of ACC synthase isoforms. By repressing PP2A genes, HDC1 tips this regulatory balance: the ethylene biosynthetic enzymes become dephosphorylated and activated, and ethylene levels rise in the maturing sepal. In other words, an epigenetic change at the chromatin level is translated, through a phosphatase, into a change in hormone output.

The elevated ethylene then triggers the next link in the chain: accumulation of reactive oxygen species, or ROS. ROS are often portrayed as damaging byproducts of metabolism, but in the past two decades they have been recognized as bona fide signaling molecules that regulate cell expansion, vascular proliferation, root hair development, and stress adaptation throughout the plant. The interplay between ethylene and ROS is a recurring theme in plant signaling, with ethylene influencing ROS production through respiratory burst oxidase homolog enzymes such as RbohD. In the maturing sepal, the study found that increased ethylene facilitates ROS accumulation, and this oxidative burst serves a developmental purpose: it promotes sepal maturation and terminates sepal cell elongation, thereby fixing the final length of the organ.

Supporting this model, the team’s genetic evidence drew on an impressive collection of ethylene biosynthesis mutants, including the ACSDDD line and an acs octuple mutant lacking multiple ACC synthase genes, materials provided by collaborators at the Southern University of Science and Technology and elsewhere. Transcriptomic profiling of the relevant genotypes, with raw RNA sequencing data deposited in the National Genomics Data Center, allowed the researchers to connect HDC1’s chromatin activity to downstream changes in gene expression. The convergence of genetic, genomic, and physiological lines of evidence strengthens the conclusion that the HDC1–PP2A–ethylene–ROS pathway is a genuine regulatory module rather than a correlation of unrelated events.

The broader implications of the work extend beyond the sepal. Organ size control is a major determinant of crop yield, and genes that influence cell number and cell expansion, such as the maize cell number regulator CNR1, have long been viewed as targets for yield improvement. If the HDC1-mediated pathway identified here operates in other organs and crop species, it could offer breeders a handle on tuning final organ dimensions by manipulating either the epigenetic regulator itself or the hormonal module it controls. The study also adds to a growing appreciation that epigenetic machinery does not merely switch genes on and off in a static sense, but participates in dynamic developmental decisions, including the timing of growth cessation, flowering, and stress responses.

There remain open questions. How HDC1 is itself targeted to the PP2A genes specifically during sepal maturation, whether additional chromatin factors cooperate in this repression, and how the ROS signal is transduced into a cessation of cell expansion at the cellular level are all avenues for future work. It will also be interesting to see whether the same ethylene–ROS module governs size termination in other floral organs or in leaves, and whether mechanical feedback, known to restrict sepal growth and shape, intersects with this chemical signaling pathway. What is already clear, however, is that the humble sepal has once again proven its worth as a model, and that the answer to the old question of how organs know when to stop growing lies, at least in part, in a deceptively simple cascade: a histone modification silences a phosphatase, the phosphatase releases the brakes on a gaseous hormone, and the hormone ignites a burst of reactive oxygen that tells the cells their growing days are over.

Subject of Research: Epigenetic regulation of sepal organ size in Arabidopsis through the HDC1–PP2A–ethylene–ROS signaling module

Article Title: Histone deacetylase complex 1 modulates sepal length through the ethylene–ROS module

Article References: Xiang, D., Qiu, D., Zhang, R., He, X., Xu, S., Zhou, M., & Hong, L. (2026). Histone deacetylase complex 1 modulates sepal length through the ethylene–ROS module. Plant Cell Reports, 45(11), Article 330. https://doi.org/10.1007/s00299-026-04018-x

Image Credits: AI Generated

DOI: 10.1007/s00299-026-04018-x

Keywords: HDC1, histone deacetylase, sepal length, organ size, ethylene, reactive oxygen species, PP2A, Arabidopsis thaliana, epigenetics, plant development, cell elongation, Plant Cell Reports