MFP1 defines the subchloroplast location of starch granule initiation

In leaf chloroplasts, starch granules are recurrently synthesized in the stromal space between the thylakoid membranes. Here, we provide unambiguous evidence that the thylakoid-associated protein MAR-BINDING FILAMENT-LIKE PROTEIN 1 (MFP1) determines this location of starch granule biogenesis within the chloroplast. Together with its interacting protein partners, MFP1 can initiate starch granules at a specific subchloroplast location given that it is anchored to a membrane compartment. This study establishes the first step of starch granule initiation and significantly advances our understanding of starch biosynthetic machinery.