Recent collaborative research has revealed the specific regulatory mechanism involved in phase separation during plant flowering. This study was led by Professor HOU Xingliang from the South China Botanical Garden of the Chinese Academy of Sciences, and Professor MIAO Yansong from Nanyang Technological University in Singapore. The findings were published in The EMBO Journal.

Phase separation is the process by which biomacromolecular structures form within cells without the presence of membranes. This mechanism is essential for organismal development and signal transduction. The plant photoperiod regulatory factor CONSTANS (CO) protein interacts with nuclear transcription factors NF-YB2 and NF-YC9 to co-activate the expression of the key flowering gene FLOWERING LOCUS T (FT), promoting the transition to flowering. However, the role of phase separation in this mechanism remains unclear, and its specific molecular details have yet to be fully elucidated.

Researchers discovered that the proteins CO, NF-YB2, and NF-YC9 can form phase-separated complexes in plants. To further explore the mechanisms and regulatory roles of this formation, they used protoplasts, in vitro experimental systems, and transgenic plants to examine how these transcription factors influence FT gene expression. The results showed that in response to light signals, CO proteins create functional percolation clusters from a previously diffuse distribution, a process dependent on the B-box motif. The multivalent co-assembly with NF-YC9 and NF-YB2 is crucial for maintaining proper CO assembly and transcriptional activity. This assembly prevents the formation of slow-diffusing condensates that could inhibit FT activation.

To clarify how NF-Y factors maintain the fluidity of CO/NF-YB2/NF-YC9 condensates for the transcriptional activation of flowering genes, the researchers found that the intrinsically disordered region (IDR) of NF-YC9 contains a polyglutamine (polyQ) motif. This motif adjusts the functional properties of the CO/NF-YB/NF-YC condensates. Additionally, specific recognition of the FT promoter, along with polyelectrolyte partitioning, enhances the fluid functional properties of these condensates. This enables the transcriptional activation of FT.

This study highlights the essential role of phase separation in the transition to flowering, explaining how plants regulate flowering by modulating the state of transcription factor condensates (Figure 1). It provides new insights into the photoperiodic flowering pathway in plants.

The research was funded by the Guangzhou Science and Technology Program and the Guangdong Provincial Key Research and Development Program. 

Figure 1. A model of fine-tuned CO condensation in regulating FT expression and flowering.(Image by HOU et al)