Thompson, Matthew V. , Wolniak, Stephen M. .
Estimating long-distance phloem behavior with sieve-element targeted, fluorescence labeled membrane proteins.
Whole-plant regulation of phloem solute exchange and membrane transport is critically dependent upon the conductivity, and thus the quantitative anatomy, of sieve tubes. If conductivity is high relative to other critical but better constrained variables, then all parts of the phloem, from leaf to root, can be expected to operate at nearly the same turgor. Conversely, if conductivity is low, then turgor will vary considerably along the length of the plant and the sieve element-companion cell complexes that comprise the phloem will be forced to regulate turgor-dependent solute exchange in a position-dependent manner.
We set out to confirm the hypothesis that the phloem operates under spatially nearly homogeneous turgor by developing a strategy for fluorescent protein tagging of sieve plates and sieve elements. N-terminal mCitrine fusions of a variety of plasma membrane-targeted proteins were expressed under the control of the AtSUC2 promoter in Arabidopsis. Successfully targeted proteins included fusions of the salt- and cold-responsive genes AtRCI2a (At3g05880) and AtRCI2b (At3g05890), and represent the first reported successful transgenic companion-cell-to-sieve-element targeting of a FP-labeled membrane-bound fusion protein.
Using these lines, we performed live-cell measurements (under confocal and wide-field fluorescence microscopy) of intact Arabidopsis sieve elements. From these measurements and others using anti-GFP Alexa Fluor 555 conjugate labeling of sectioned material, we estimated sieve element length and radius, sieve pore length and radius, and sieve pore number per sieve plate, and finally sieve tube conductivity. Our estimate of Arabidopsis stem phloem conductivity confirms our hypothesis that phloem turgor is nearly homogeneous throughout the plant.
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1 - University of Maryland, Cell Biology and Molecular Genetics, 2209 H J Patterson Hall, College Park, MD, 20742, USA
intercellular protein targeting
Presentation Type: ASPB Minisymposium
Location: International Ballroom/Hilton
Date: Sunday, July 8th, 2007
Time: 4:10 PM