The Trafficking Machinery of Lytic and Protein Storage Vacuoles: How Much is Shared and How Much is Distinct?

Plant cells contain two types of vacuoles, the lytic vacuole and the protein storage vacuole. Lytic vacuoles (LVs) are present in vegetative cells, whereas protein storage vacuoles (PSVs) are found in seed cells. The physiological functions of the two vacuole types differ. Newly synthesized proteins must be transported to these vacuoles via protein trafficking through the endomembrane system for them to function. Recently, significant advances have been made in elucidating the molecular mechanisms of protein trafficking to these organelles. Despite these advances, the relationship between the trafficking mechanisms in LV and PSVs remains unclear. Some aspects of the trafficking mechanisms are common to both organelles, but certain aspects are specific to trafficking to either LV or PSVs. In this review, we summarize recent findings on the components involved in protein trafficking to both LV and PSVs and compare them to examine the extent of overlap in the trafficking mechanisms. In addition, we discuss the interconnection between the LV and PSVs in protein trafficking machinery and the implication in the identity of these organelles.

Autophagy and metacaspase determine the mode of cell death in plants

Although animals eliminate apoptotic cells using macrophages, plants use cell corpses throughout development and disassemble cells in a cell-autonomous manner by vacuolar cell death. During vacuolar cell death, lytic vacuoles gradually engulf and digest the cytoplasmic content. On the other hand, acute stress triggers an alternative cell death, necrosis, which is characterized by mitochondrial dysfunction, early rupture of the plasma membrane, and disordered cell disassembly. How both types of cell death are regulated remains obscure. In this paper, we show that vacuolar death in the embryo suspensor of Norway spruce requires autophagy. In turn, activation of autophagy lies downstream of metacaspase mcII-Pa, a key protease essential for suspensor cell death. Genetic suppression of the metacaspase–autophagy pathway induced a switch from vacuolar to necrotic death, resulting in failure of suspensor differentiation and embryonic arrest. Our results establish metacaspase-dependent autophagy as a bona fide mechanism that is responsible for cell disassembly during vacuolar cell death and for inhibition of necrosis.