Plant vacuoles

The vacuoles of plant cells are multifunctional organelles that are central to cellular strategies of plant development. They share some of their basic properties with the vacuoles of algae and yeast and the lysosomes of animal cells. They are lytic compartments, function as reservoirs for ions and metabolites, including pigments, and are crucial to processes of detoxification and general cell homeostasis. They are involved in cellular responses to environmental and biotic factors that provoke stress. In the vegetative organs of the plant, they act in combination with the cell wall to generate turgor, the driving force for hydraulic stiffness and growth. In seeds and specialized storage tissues, they serve as sites for storing reserve proteins and soluble carbohydrates. In this way, vacuoles serve physical and metabolic functions that are essential to plant life. Plant cell vacuoles were discovered with the early microscope and, as indicated in the etymology of the word, originally defined as a cell space empty of cytoplasmic matter. Technical progress has variously altered the operating definition of the plant vacuole over time. Today, definitions continue to be colored by the tools and concepts brought to bear in any given study. Indeed, the combination of microscopy, biochemistry, genetics, and molecular biology is fundamental to research into the plant vacuole. In this review, vacuoles are provisionally defined as the intracellular compartments that arise as a terminal product of the secretory pathway in plant cells. They are ontogenetically and functionally linked with other components of the vacuolar apparatus (i.e., vacuoles and those membranous bodies that are either committed to becoming vacuolar or have immediately completed a vacuolar function). Experimental evidence suggests that material within the vacuolar system in plants derives confluently from both an intracellular biosynthetic pathway and a coordinated endocytotic pathway. The biogenetic pathways include (1) sorting of proteins destined for the vacuole away from those to be delivered to the cell surface after transit through the early stages of the secretory pathway; (2) endocytosis of materials from the plasma membrane; (3) autophagy pathways for vacuole formation; and (4) direct cytoplasm-to-vacuole delivery. Ultimately, sorting and targeting mechanisms ensure that specific proteins are faithfully assigned to conduct the vacuolar functions. The reader is referred to other contributions to this issue (i.e., Battey et al., 1999; Sanderfoot and Raikhel, 1999) and to previous reviews (Herman, 1994; Okita and Rogers, 1996; Bassham and Raikhel, 1997; Marty, 1997; Robinson and Hinz, 1997; Neuhaus and Rogers, 1998; Herman and Larkins, 1999) for detailed information on specific aspects of vacuole biology.