CYCLING DOF FACTOR 2 (LdCDF2) functions as a positive regulator integrating GA signaling and biosynthesis to drive bulb germination in Lilium.

Abstract

The dormancy release and initiation of germination in underground storage organs are critical for plant adaptation and agriculture. Gibberellin (GA) promotes germination, but how GA signaling achieves self-amplification at the early stage of germination to drive rapid germination remains unknown. In particular, the molecular hub that connects GA perception and biosynthesis has yet to be identified. Here, we identified CYCLING DOF FACTOR 2 (LdCDF2) from Lilium davidii var. unicolor as a novel positive regulator of germination. Using stable genetic transformation, protein‒protein interaction assays, and cell-free degradation assays, we revealed a germination module wherein LdCDF2 directly couples GA signaling with biosynthesis. Mechanistically, LdCDF2 directly activates the transcription of LdGA3ox2/3, thereby promoting GA biosynthesis. Moreover, the DELLA protein LdSLR1 interacts with LdCDF2, inhibiting the transcriptional activity of LdCDF2 and accelerating its degradation via the 26S proteasome, thereby exerting a dual inhibitory effect on germination. When GA signaling degrades LdSLR1, LdCDF2 is released, reactivating GA synthesis and enabling GA signal self-amplification. Genetic evidence revealed that stable silencing of LdCDF2 reduced the germination rate by 76.0% and endogenous GA3 level to 57.69% of that of the control, while overexpression increased the rate by 2.7-fold and the GA3 level by 6.29-fold. In summary, this study identifies LdCDF2 as a hub linking GA signaling and biosynthesis and elucidates how the DELLA-LdCDF2 dual switch mediates GA self-amplification. This discovery advances our understanding of underground organ germination mechanisms and provides new molecular targets for studying plant hormone signaling cascades and precisely controlling bulbous plant germination timing.