Overexpression of autophagy genes activates lipophagy in Arabidopsis

Cells possess an intrinsic recycling system called macroautophagy/autophagy, which delivers obsolete or damaged cytoplasmic components into the vacuole for degradation and reuse. Autophagy-mediated breakdown of organellar membrane lipids supplies fatty acids for the synthesis of triacylglycerols (TAGs), which are then packaged into subcellular organelles called lipid droplets (LDs). Conversely, autophagy contributes to TAG turnover by delivering LDs into vacuoles for breakdown by resident acid lipases. Additionally, LDs can undergo degradation through cytosolic lipolysis mediated by SDP1 (Sugar-Dependent1) lipase in Arabidopsis. Autophagy-mediated LD, a process referred to as lipophagy, has been described in plants. Nevertheless, the precise mechanisms of lipophagy and the specific types of vacuoles involved remain unclear. Here, we show that overexpression of autophagy genes ATG5 or ATG8 promotes autophagic activity and significantly reduces LD accumulation. We demonstrate that the decreased LD abundance is not due to increased LD degradation mediated by SDP1 but is dependent on autophagy. The abundance of vacuoles specifically labeled by δ-TIP3 (delta-tonoplast intrinsic protein 3) was markedly decreased in ATG5-overexpressing lines. In addition, disruption of autophagic genes prevented the formation of both δ-TIP3-positive vacuoles and the mysterious vacuolar lumen structures previously termed bulbs, indicating their autophagic origin. Furthermore, confocal imaging analysis revealed close associations between LDs and δ-TIP3-labeled vacuoles, as well as the presence of LDs within vacuoles delimited by δ-TIP3. Together, our findings indicate that overexpression of autophagy genes triggers lipophagy and identify a distinct type of vacuole involved in this process.