The endoplasmic reticulum-localized acyltransferase LPAT2 interacts with ATI1/2 and mediates lipid droplet proliferation.

Lipid droplet (LD) biogenesis from the endoplasmic reticulum (ER) is accompanied by the conversion of phospholipids to triacylglycerol and the expansion of the phospholipid monolayer surface. The determinants of this process are largely unknown in plants. We demonstrate that the Arabidopsis thaliana lysophosphatidic acid acyltransferase, LPAT2, and its product phosphatidic acid (PA), are critical lipid-based cues for LD proliferation. Suppression of LPAT2 decreases PA accumulation at the monolayer surface of LDs, consequently compromising dark- or stress-induced LD biogenesis, whereas overexpression of LPAT2 exhibits opposite effects. LPAT2 physically interacts with ATG8-interacting protein 1/2 (ATI1/2), which serves as autophagy cargo receptors. Mutations of ATI1/2 drastically reduce LPAT2 translocation from the ER to the forming LDs. Our data suggest that the LPAT2-ATI1/2 complex is an important machinery that facilitates LD biogenesis to mediate plant responses to developmental and environmental stimuli.