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Plant and Cell Physiology Advance Access originally published online on September 16, 2009
Plant and Cell Physiology 2009 50(10):1826-1839; doi:10.1093/pcp/pcp124
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

New Dynamics in an Old Friend: Dynamic Tubular Vacuoles Radiate Through the Cortical Cytoplasm of Red Onion Epidermal Cells

Elizabeth J. Wiltshire and David A. Collings*

School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

*Corresponding author: E-mail, david.collings{at}canterbury.ac.nz; Fax, +64 (3) 364 2590.


  Abstract

The textbook image of the plant vacuole sitting passively in the centre of the cell is not always correct. We observed vacuole dynamics in the epidermal cells of red onion (Allium cepa) bulbs, using confocal microscopy to detect autofluorescence from the pigment anthocyanin. The central vacuole was penetrated by highly mobile transvacuolar strands of cytoplasm, which were also visible in concurrent transmitted light images. Tubular vacuoles also extended from the large central vacuole and radiated through the cortical cytoplasm. These tubules were thin, having a diameter of about 1.5 µm, and were connected to the central vacuole as shown by fluorescence recovery after photobleaching (FRAP) experiments. The tubules were bounded by the tonoplast, as revealed by transient expression of green fluorescent protein (GFP) targeted to the vacuolar membrane and through labeling with the dye MDY-64. Expression of endoplasmic reticulum-targeted GFP demonstrated that the vacuolar tubules were distinct from the cortical endoplasmic reticulum. Movement of the tubular vacuoles depended on actin microfilaments, as microfilament disruption blocked tubule movement and caused their collapse into minivacuoles. The close association of the tubules with GFP-tagged actin microfilaments suggests that the tubules are associated with myosin, and that tubules likely move along microfilaments. Tubular vacuoles do not require anthocyanin for their formation, as tubules were also present in white onion cells that lack anthocyanin. The function of these tubular vacuoles remains unknown, but as they greatly increase the surface area of the tonoplast, they might increase transport rates between the cytoplasm and vacuole.

Keywords: actin microfilaments - Allium cepa - anthocyanin - onion epidermis - vacuolar tubules - vacuole

Abbreviations: ER, endoplasmic reticulum; FRAP, fluorescence recovery after photobleaching; GFP, green fluorescent protein; YFP, yellow fluorescent protein

(Received July 26, 2009; Accepted September 1, 2009)
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