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Etxeberria - Plant Science Journal
In and out of the plant storage vacuole
Ed Etxeberria, Javier Pozueta-Romero and Pedro Gonzalez
Abstract. No longer considered metabolically inactive and almost exclusively a storage/lytic compartment, the plant vacuole is now recognized as a multifunctional organelle, a notion supported by current data implicating the vacuole in a wide variety of metabolic functions (1, 2, 3). Despite its relatively large volume and seemingly uncomplicated structure, the vacuole is a highly dynamic compartment capable of transforming both in terms of shape (1) and, in specific cell types, acquiring separate individual functions (4, 5, 6). In special cases, vacuoles contain storage and lytic functional zones within the same vacuolar space (3). Technological advancements during the past two decades have allowed us to identify the involvement of the vacuole in what once appeared unthinkable and improbable distant functions. We have long acknowledged the vacuole as being intimately involved in the maintenance of cell turgor (7), a function achieved by the regulatory exchange of osmotically active solutes across the tonoplast (8). As a lytic compartment, the vacuole is known to participate in the degradation of proteins and recycling of amino acids (7), and in the degrading of entire organelles through autophagosomes, especially during senescence (9). Plant cells also use the vacuole as part of their defense strategies (10), for detoxification (11), nastic movements (12, 13), and as interim location in the process of ion secretion by salt glands (14). During nectar production, the vacuole intervenes as the source of sugars (15) and also participates directly in the secretion of viscin (an elastic, mucilaginous and sticky tissue that adhere falling seeds to branches) in mistletoe seeds dispersal mechanism (16). In Basidiomycetes, vacuoles serve as conduit for longitudinal transport of solutes (17), whereas in the green algae Mesostigma viride, contractile vacuoles regulate water content (18). Long accepted as the site of fructan synthesis (19), more recently the vacuole has been implicated in the production of other metabolites (e.g. nicotine, 20) and in the final processing stages in the synthesis of cytosolic proteins (21).
