GmbHLH68 Integrates Short-Term Photothermal Signals to Regulate Soybean Flowering via GmFT2a.

Floral transition is crucial for crop productivity and environmental adaptability. As a photoperiod-sensitive crop, the flowering time of soybean is intricately regulated by environmental signals. Here, we show that short-term changes of photoperiod or temperature significantly affected soybean flowering time. Through an integrative analysis of transcriptomic and epigenomic data, we revealed that short-term exposure to inductive short day promoted floral transition via suppressing the expression of a bHLH family gene, GmbHLH68, along with the alteration of H3K27me3 modification on the locus, while short-term high temperature had contrary effects. We establish that GmbHLH68 directly binds to and activates GmFT2a (FLOWERING LOCUS T2a) expression, forming a critical regulatory module through which short-term photoperiod and temperature changes control flowering time in soybean. Genetic knockout of either gene abolished photothermal signal responsiveness. Interestingly, GmbHLH68 could as well bind to and promote the expression of GmRGA2L (RGA2-like) and GmFT4, two inhibitors of floral transition. This bidirectional regulation may fine-tune the impact of short-term environmental changes, enabling exquisite control of flowering time. Thus, GmbHLH68 is a central regulator of flowering in response to provisional photothermal changes. Our findings may shed light on soybean cultivar improvement for stable yield during a changing environment.