The Vacuole: not just an empty hole!

The vacuole is the largest organelle of a plant cell. It accumulates proteins, ions and secondary metabolites while providing turgor for cell growth via water content. It is also a major site for the degradation of macromolecules. A full understanding of the vacuole’s roles in salt and metal ion accumulation and water uptake are hot topics in current research. It is a necessity to understand these processes for potential exploitation in production of future stress tolerant crops. How the vacuole is made and where it comes from remain unanswered questions. What we do know is a plant without vacuoles is a dead plant (Rojo et al 2001)!

The simplest view is of a single vacuole fulfilling all roles. However, to achieve such a diverse set of functions, specialised vacuoles must exist; a single vacuole cannot function as a storage compartment and dustbin at the same time! Lytic Vacuoles (LVs) are hydrolytic, acidic compartments responsible for the breakdown of a variety of macromolecules, while Protein Storage Vacuoles (PSVs) accumulate proteins in storage organs such as seeds. Up until a few years ago it was thought these two vacuole types could coexist. It now appears their coexistence in a single cell is a rare occurrence (Frigerio et al 2008). PSVs are only found in seeds, whilst LVs are prominent in vegetative tissues. Interestingly, the PSV alters it’s identity and function to become an LV during development (Marty 1999 and Zheng et al 2011).

One way researchers identify different vacuoles is by visualising proteins at the vacuolar membrane, such as aquaporins. Aquaporins are membrane channels that transport water and other small molecules; some of which do so at the vacuolar membrane, the tonoplast. These are called Tonoplast Intrinsic Proteins (TIP) and are used as vacuole ‘markers’. In the model plant Arabidopsis thaliana, 10 TIP isoforms exist (Johanson et al 2001). Different isoforms are expressed at different stages of development and in different plant organs. The most recent expression map of these isoforms to date uses translational fluorescent protein fusions to highlight isoforms at different tissues in the root (Gattolin et al 2009). By having an expression map of TIPs, researchers can begin to understand functional differences between isoforms, which could have different roles in water uptake.  The most common markers are TIP3;1 which exclusively labels PSVs in the developing embryo, and TIP1;1 which labels LVs post germination.

Confocal microscope images using these markers have most recently been compiled in video form, with a chirpy accompanying song in which the vacuole introduces itself. Created by Dr Anne Osterrieder of Oxford Brookes University in collaboration with Dr Lorenzo Frigerio and PhD student Charlotte Carroll at the University of Warwick; the video and accompanying song are part of a series of organelle teaching aids. Enjoy!

You can read more about vacuoles at The Illuminated Cell, who kindly provided the image of the tagged plant cells.

References

FRIGERIO L, HINZ G, ROBINSON D.G. (2008). Multiple vacuoles in plant cells: rule or exception? Traffic 9:1564-1570. doi:10.1111/j.1600-0854.2008.00776.x

GATTOLIN S, SORIEUL M, HUNTER P.R, KHONSARI R.H, FRIGERIO L. (2009) In vivo imaging of the tonoplast intrinsic protein family in Arabidopsis roots. BMC Plant Biology 9:133. doi:10.1186/1471-2229-9-133

JOHANSON U., KARLSSON M., JOHANSSON I., GUSTAVSSON S., SJOVALL S., FRAYSSE L., WEIG A.R., KJELLBOM P. (2001) The complete set of genes encoding major intrinsic proteins in Arabidopsis provides a framework for new nomenclature for major intrinsic proteins in plant. Plant Physiology 126:1358-1369. doi:10.1104/pp.126.4.1358

MARTY F. (1999) Plant Vacuoles. Plant Cell 11:587-600. doi:10.1105/tpc.11.4.587

ROJO E, GILLMOR S, KOVALEVA V, SOMERVILLE C.R, RAIKHEL N.V. (2001) VACUOLELESS1 Is an Essential Gene Required for Vacuole Formation and Morphogenesis in Arabidopsis. Developmental Cell 1:303-310. doi:10.1016/S1534-5807(01)00024-7

ZHENG H. and STAEHELIN A.L. (2011) Protein Storage Vacuoles Are Transformed into Lytic Vacuoles in Root Meristematic Cells of Germinating Seedlings by Multiple, Cell Type-Specific Mechanisms. Plant Physiology 155:2023–2035. doi:10.1104/pp.110.170159