Protein dynamics and proteolysis in plant vacuoles.

Plant cells cannot live without their vacuoles. The tissues and organs of a plant contain a wide variety of differentiated and specialized vacuoles -- even a single plant cell can possess two or more types of vacuoles. Vacuolar proteins are encoded by nuclear genes and synthesized in the cytoplasm. Their transport into the vacuolar compartment is under cytoplasmic control. Transcription seems to be a major control level for differential protein supply to the vacuoles. It is at this level that vacuole differentiation and functions are mainly integrated into cellular processes. Recycling amino acids generated by protein degradation is a major function of the vacuole. This is most evident when storage proteins are mobilized in storage tissues of generative or vegetative organs in order to nourish the embryo of germinating seeds or sprouting buds. When specific proteins are transferred to the vacuole for immediate degradation this compartment contributes to the adaptation of protein complexes in response to changes in developmental or environmental conditions. Vacuolar proteases are involved in protein degradation during reversible senescence and programmed cell death, which is also called irreversible senescence. Vacuoles contribute to defence against pathogens and herbivores by limited and unlimited proteolysis. Our present knowledge on functions and processes of vacuolar protein dynamics in plants is reviewed. Research perspectives are deduced.