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Expansin activity may be detected by a cell-wall extension assay. For this
technique, cell walls are clamped in an extensometer, expansin protein is added,
and the resulting lengthening of the cell walls is measured. The figure below
illustrates the principle of the method, which is based on articles by Cosgrove,
1989 and McQueen-Mason, Durachko and Cosgrove,
1992.
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Top: Heat-inactivated walls are prepared
from the growing region of young seedlings and clamped at constant load in
an extensometer, which measures changes in length of the wall specimen.
To prepare expansin proteins, cell walls are collected from growing cells and extracted for wall
proteins (red arrows), which are added to the solution surrounding the wall sample.
Bottom: Time course for extension of wall samples. The heat
treatment kills endogenous acid-induced extension. Upon addition of expansin
proteins (at arrow), the wall begins to extend.
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