Biomolecular condensates of ATG18 reshape ER to promote autophagy in plants

Autophagosomes originate from and maintain association with the endoplasmic reticulum (ER) during their formation, yet how these processes are molecularly coordinated in plants remains poorly understood. Here, we demonstrate that Arabidopsis ATG18a, a key organizer of early autophagosome formation, undergoes phase separation to form biomolecular condensates on the ER membrane, which progress from highly mobile droplets to stable ring-like structures, while the ER is reshaped. We discovered that ATG18a condensates work together with RHD3, an ER membrane-shaping protein, with RABC1 serving as a molecular linker between them. Importantly, RABC1 facilitates both RHD3 assembly necessary for formation of ringlike ER structures and its interaction with ATG18a condensates. These findings reveal a novel mechanism whereby biomolecular condensates, work together with membrane shaping proteins to reshape specialized membrane domains through wetting interactions, providing fundamental insights into autophagosome formation in plant stress responses. Here all original and full western blots are included.