The double flower variant of yellowhorn is due to a LINE1 transposon-mediated insertion.

As essential organs of reproduction in angiosperms, flowers and the genetic mechanisms of their development have been well characterized in many plant species but not in the woody tree yellowhorn (Xanthoceras sorbifolium). Here, we focused on the double flower phenotype in yellowhorn, which has high ornamental value. We found a candidate C-class gene, AGAMOUS1 (XsAG1), through BSA sequencing and genetics analysis with a LONG INTERSPERSED NUCLEAR ELEMENTS 1 (LINE1) transposable element fragment (Xsag1-LINE1-1) inserted into its second intron that caused a loss-of-C-function and therefore the double-flower phenotype. In situ hybridization of XsAG1 and analysis of the expression levels of other ABC genes were used to identify differences between single and double flower development processes. These findings enrich our understanding of double flower formation in yellowhorn and provide evidence that transposon insertions into genes can reshape plant traits in forest trees.