Study on the brassinosteroids modulated regulation of the style growth in Forsythia suspensa (Thunb.) Vahl

Forsythia suspensa (Thunb.) Vahl is a self-incompatible, heterostylous plant with style-length dimorphism. Heterostyly is a unique flower polymorphism in dioecious angiosperms. Previous research has demonstrated that in heterostylous species like Primula and Turnera, brassinosteroids (BRs) regulate style length. However, the regulatory mechanism of F. suspensa heterostyly remains poorly understood. In this study, we investigated the morphological and cytological differences between the long (L) and short (S) morphs of F. suspensa. We measured the amounts of Teasterone (TE), Typhasterol (TY), Castasterone (CS), and Brassinolide (BL) in the styles. Furthermore, we used topical treatment of the BR inhibitor propiconazole (PPZ) to investigate its impact on style elongation. To clarify the function of BRs in controlling style length, we identified key genes and evaluated their expression levels by analyzing the transcriptome data of F. suspensa. Our findings show that, in contrast to the S-morph, the L-morph shows significant elongation from the flower bud scale abscission phase to the dew corolla phase. L-morph styles have a much higher castasterone level than S-morph styles, and treatment with propiconazole, an inhibitor of BR production, prevents style elongation in the L-morph. Furthermore, transcriptome analysis revealed that the putative orthologous gene of TCH4, a BR-responsive gene, exhibits higher expression in the L-morph compared to S-morph. These findings support the role of BRs as regulators of distinct style lengths in F. suspensa. Additionally, we found that EVM0011396 gene (annotated as FsCYP749A22), encoding a putative BR-degrading enzyme, shows increased expression during S-morph style development, with significantly higher expression in S-morph styles compared to L-morph styles. In contrast, the expression level of EVM0028947 (annotated as FsCYP90D1) coding for a BR biosynthesis gene, is higher in L-morph styles compared to S-morph styles. The expression patterns of EVM0028947 and EVM0011396 suggest their involvement in BR homeostasis in F. suspensa styles. This study provides insight into the molecular mechanism underlying the heterostyly of F. suspensa and lays the foundation for further exploration of this phenomenon.