Abstract
As plants lack immune cells, each cell has to defend itself against invading pathogens. Plant cells have a large central vacuole that accumulates a variety of hydrolytic enzymes and antimicrobial compounds, raising the possibility that vacuoles play a role in plant defense. However, how plants use vacuoles to protect against invading pathogens is poorly understood. Recently, we characterized two vacuole-mediated defense strategies associated with programmed cell death (PCD). In one strategy, vacuolar processing enzyme (VPE) mediated the disruption of the vacuolar membrane, resulting in the release of vacuolar contents into the cytoplasm in response to viral infection. In the other strategy, proteasome-dependent fusion of the central vacuole with the plasma membrane caused the discharge of vacuolar antibacterial protease and cell death-promoting contents from the cell in response to bacterial infection. Intriguingly, both strategies relied on enzymes with caspase-like activities: the vacuolar membrane-collapse system required VPE, which has caspase-1-like activity, and the membrane-fusion system required a proteasome that has caspase-3-like activity. Thus, plants may have evolved a cellular immune system that involves vacuolar membrane collapse to prevent the systemic spread of viral pathogens, and membrane fusion to inhibit the proliferation of bacterial pathogens.