Epigenetic “switch” underlying hemizygote-dependent super-early flowering in soybean transformants

Researchers in China uncover the genetic and epigenetic basis of a unique super-early flowering phenomenon in soybean transformants, where the very phenotype is active in hemizygotes and silenced in homozygotes.

Seeking to develop soybean varieties adapted to diverse climates and growing seasons, researchers are exploring precise controlling of the flowering time. Now, scientists report that a super-early flowering phenotype driven by the key gene GmFT2a is expressed in hemizygous soybeans and becomes epigenetically silenced in homozygotes. This hemizygote-dependent dominance is caused by a two distinct rounds of DNA methylation triggered by different small RNAs (siRNAs). This finding opens new possibilities for designing flexible and reversible systems for manipulating target traits in future breeding program.

Soybean (Glycine max (L.) Merrill) is a globally important crop. The flowering timing of soybean plays a critical role in determining its adaptation to maturity groups. In particular, developing early-flowering (maturing) varieties is important for high-latitude regions. To that end, understanding how to precisely regulate flowering time, as well as maturing, has long been studied by researchers.

In a new study published in The Crop Journal, researchers from Chinese Academy of Agricultural Sciences discovered that in a specific transgenic soybean line overexpressing GmFT2a, a super-early and early-flowering phenotype was observed in the hemizygous state but was epigenetically silenced in the homozygous state. This highlights a case of hemizygote-dependent dominance and offers new insights into precise flowering time control through epigenetic regulation.

“We attributed this switching-off (silencing) behavior to an siRNA-mediated epigenetic mechanism in homozygous plants, where siRNAs triggered DNA methylation that silenced the gene expression,” shares senior and corresponding author Tianfu Han. “Notably, two distinct rounds of DNA methylation establishment occur, each mediated by a different mechanism.”

The homozygotes derived from the hemizygous mother plants was associated with the initiation of CHHcontext DNA methylation at 35S promoters mediated by 21- and 22- nucleotide (nt) siRNAs. Subsequently, 24 nt siRNAs contribute to additional CHG- and CG-context DNA methylation at 35S promoters during the homozygosity of genes in plants already homozygous in maternal lineage. The silenced trait could be reactivated through hybridization, restoring the hemizygous state and recovering the early-flowering phenotype.

“These finding inspire us to design gene expression systems that respond to genetic dosage through the manipulation of epigenetic elements,” adds Han. “These could in turn serve as new tools for the precise regulation of flowering and maturation, particularly in crops that require strict adaptation.

This study lays a foundation for developing “switchable” genetic platform, where important traits can be intentionally turned on or off, or triggered by environmental cues.

Titles of original papers: Molecular dissection of hemizygote-dependent dominance of super-early flowering in soybean

Journal: The Crop Journal

DOI: 10.1016/j.cj.2025.02.007