Phototropin Mediates the Accurate Movement of Trifoliate Leaves in <i>Medicago truncatula</i>

Light-harvesting efficiency is crucial for plant photosynthesis, and leaves must adjust their angles to maximize sunlight capture. Leguminous plants have evolved a specialized motor organ, the pulvinus, located between the leaf blades and petioles, enabling rapid leaflet reorientation toward sunlight. Although the role of pulvinus in orienting leaflets is well understood, the exact mechanisms behind this light response remain unclear. In this study, we identified a fast-neutron radiated mutant with the back-bent leaflet (bbl) phenotype in Medicago truncatula. The mutant leaves fail to orient properly toward light and excessively open under white light (WL) or blue light (BL). The adaxial motor cells of the bbl pulvinus contain a higher potassium concentration than those of wild-type plants, causing excessive cell swelling under WL. BBL encodes a Phototropin 2 homologue from Arabidopsis thaliana, and is highly expressed in the pulvinus. BBL/MtPHOT2 is associated with the plasma membrane in the dark, and translocates to the Golgi body upon BL exposure. Our findings demonstrate that BBL/MtPHOT2 regulates leaflet movement by modulating potassium concentration in the adaxial pulvinus motor cells, ensuring accurate leaf orientation toward the light. Our study provides genetic evidence that fills the gap between pulvinus-mediated leaflet movement and light direction.