Scientists have discovered a molecular bridge that links two important signaling pathways involved in pollen development in plants. Led by Associate Professor Emi Ito and Professor Takashi Ueda at the National Institute for Basic Biology (NIBB), along with researchers from multiple universities and institutions, the study identifies a new signaling pathway involving small GTPase proteins. These proteins act as molecular switches that control various cellular processes. The research team discovered a protein called REAP1/SWAP70, which connects RAB5—a small GTPase involved in moving materials within cells—with ROP7, a member of the plant-specific ROP family of GTPases that influence cell shape and growth. The newly identified RAB5-REAP1-ROP7 signaling pathway plays a crucial role in normal pollen development. The study shows that REAP1 interacts with both ARA6, a plant-specific version of RAB5, and the more widely conserved RAB5 proteins found in many types of cells. REAP1 is found in endosomes, which are cellular structures that act as sorting centers for transporting materials within the cell. Its presence in these compartments depends on interactions with RAB5 and specific types of lipids in the cell membrane. Further experiments revealed that REAP1 also binds to ROP7. When RAB5 was overproduced in cells, ROP7 was recruited to endosomes, but this did not happen in cells without REAP1. These findings suggest that REAP1 functions as a molecular bridge, connecting RAB5 and ROP7. When this signaling network was disrupted, pollen development was affected. Specifically, when both RAB5 and ROP7 functions were reduced, problems arose during the transition from a single-celled to a two-celled pollen grain, leading to a higher number of pollen grains that did not mature properly. The study provides new understanding of how different small GTPase systems work together in plants. It suggests that plants have developed their own interconnected signaling networks to manage complex processes needed for reproduction. This research was published in the journal Nature Plants, offering valuable insights into plant biology and potential applications in agricultural science.