plant cell wall: water permeability and expansion

Tilak Ratnanather tratnana at eureka.wbme.jhu.edu
Tue Aug 9 18:08:14 EST 1994


The cochlear outer hair cell (OHC) appears to possess
properties that are characteristic of plant cells! We
have a couple of questions concerning plant cell walls
that might shed light on OHC mechanics as a result
of our osmotic experiments.

The OHC is a hydraulic skeleton i.e. the cylindrically
shaped elastic wall is reinforced and is supported by
a presurized fluid core (like inflated automobile tires);
diameter 8 microns and length 50 microns.

Recently we have found that the OHC water permeability 
coefficient, L_p, is about 10^-14 m/Pa/s (alternatively
P_f = 10^-4 cm/s) which upon review of literature is 
close to L_p values for plant cell walls.

QUESTION 1: What is the physiological evidence (by way 
of morphology or otherwise) that L_p for these plant 
cell walls are LOW?


Some background info for the next question: the OHC 
cortical lattice is composed of circumferential filaments 
(CF) which are connected in the longitudinal direction 
by crosslinks (CL). The cortical lattice lies beneath 
the plasma membrane (PM) and micropillars link the 
two together. Additionally, beneath the cortical lattice
 is a system of membranous sacs coaxial with the plasma 
membrane known as subsurface cisternae (SSC). The gap 
between the PM and the SSC is about 100 nm.

Next we have treated OHCs with aspirin or more precisely 
sodium salicylate which is known to cause the SSC to 
dilate and vesiculate (based on EM photos). Normal OHCs 
in isotonic medium did not change volume in contrast with 
aspirin treated OHCs. Examination of the strains in the 
radial and longitudinal direction suggest that aspirin 
treated OHCs have a greater tendency to deform in the 
longitudinal direction than in the radial direction. We 
speculate that the CFs were not affected by aspirin but 
that the CLs were weakened. We also know that salicylate 
causes swelling of mitochondria (which incidentally are 
sparsely poplulated along the SSC and nowhere else in the
OHC) by a H+-pump mechanism. Could it be that the H+-pump 
works in the same way as auxin causes plant cell wall 
expansion which is accompanied by microfibril orientation?

QUESTION 2: what is the precise role of auxin in plant cell
wall expansion.


Comments, suggestions or references would be greatly 
appreciated.


Thanks a lot

Tilak Ratnanather
Dept of Otolaryngology - Head & Neck Surgery
and Dept of Biomedical Engineering
The Johns Hopkins University School of Medicine
Baltimore, MD 21205-2196
-- 
Dept of Otolaryngology -- Head & Neck Surgery    || Phone 410 955 3877
The Johns Hopkins University School of Medicine  || Fax   410 955 1299
Traylor Research Bldg 522, 720 Rutland Avenue    || Fax   410 955 0549
Baltimore MD 21205-2196				 || Pager 410 351 8460



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