The role of vacuole in plant cell death

Almost all plant cells have large vacuoles that contain both hydrolytic enzymes and a variety of defense proteins. Plants use vacuoles and vacuolar contents for programmed cell death (PCD) in two different ways: for a destructive way and for a non-destructive way. Destruction is caused by vacuolar membrane collapse, followed by the release of vacuolar hydrolytic enzymes into the cytosol, resulting in rapid and direct cell death. The destructive way is effective in the digestion of viruses proliferating in the cytosol, in susceptible cell death induced by fungal toxins, and in developmental cell death to generate integuments (seed coats) and tracheary elements. On the other hand, the non-destructive way involves fusion of the vacuolar and the plasma membrane, which allows vacuolar defense proteins to be discharged into the extracellular space where the bacteria proliferate. Membrane fusion, which is normally suppressed, was triggered in a proteasome-dependent manner. Intriguingly, both ways use enzymes with caspase-like activity; the membrane-fusion system uses proteasome subunit PBA1 with caspase-3-like activity, and the vacuolar-collapse system uses vacuolar processing enzyme (VPE) with caspase-1-like activity. This review summarizes two different ways of vacuole-mediated PCD and discusses how plants use them to attack pathogens that invade unexpectedly.