JXB Advance Access published online on August 28, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm155
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RESEARCH PAPER |
Measurement of diffusion within the cell wall in living roots of Arabidopsis thaliana
1Biology Department, University of Massachusetts, 611 N. Pleasant St, Amherst, MA 01003, USA
2Physics Department, Simon's Rock College, Great Barrington, MA 01230, USA
* To whom correspondence should be addressed. E-mail: ekramer{at}simons-rock.edu
To quantify the diffusion constant of small molecules in the plant cell wall, fluorescence from carboxyfluorescein (CF) in the intact roots of Arabidopsis thaliana was recorded. Roots were immersed in a solution of the fluorescent dye and viewed through a confocal fluorescence microscope. These roots are sufficiently transparent that much of the apoplast can be imaged. The diffusion coefficient, Dcw, of CF in the cell wall was probed using two protocols: fluorescence recovery after photobleaching and fluorescence loss following perfusion with dye-free solution. Diffusion coefficients were obtained from the kinetics of the fluorescent transients and modelling apoplast geometry. Apoplastic diffusion constants varied spatially in the root. In the elongation zone and mature cortex, Dcw=(3.2±1.4)x10–11 m2 s–1, whereas in mature epidermis, Dcw=(2.5±0.7)x10–12 m2 s–1, at least an order of magnitude lower. Relative to the diffusion coefficient of CF in water, these represent reductions by approximately 1/15 and 1/195, respectively. The low value for mature epidermis is correlated with a suberin-like permeability barrier that was detected with either autofluorescence or berberine staining. This study provides a quantitative estimate of the permeability of plant cell walls to small organic acids—a class of compounds that includes auxin and other plant hormones. These measurements constrain models of solute transport, and are important for quantitative models of hormone signalling during plant growth and development.
Key words: Apoplast, Arabidopsis, carboxyfluorescein, cell wall, diffusion
Received 31 January 2006; Revised 13 May 2007 Accepted 11 June 2007