= Expansin
An <expansin> is a <protein> that promotes extension of a <plant cell wall> without requiring detectable <hydrolysis> of its structural <polysaccharides>. Wall extension and destruction of the wall are different processes: rearranging a load-bearing network can allow it to yield while leaving its components largely intact.<Two endogenous proteins that induce cell wall extension in plants by S. McQueen-Mason and others>[Abstract: reconstituted wall extension without detectable hydrolytic breakdown]{ref}
The motivating 1992 experiment extracted <proteins> from growing seedling <cell walls> and added the extract to isolated walls. Fractionation separated two active <proteins>, about $29$ and $30\,\mathrm{kDa}$, each able to restore extension. The activity depended on conditions such as <pH> and was not equally effective on every tested wall type.<Two endogenous proteins that induce cell wall extension in plants by S. McQueen-Mason and others>[Abstract: extraction, fractionation and reconstitution comparisons]{ref}
A useful control compares equally loaded isolated walls with and without the active preparation. Extension after addition connects a measurable mechanical response to material in the preparation; measuring only the abundance of a <protein> in a growing stem would provide a weaker connection. The reconstitution result does not assign every change in whole-<plant> growth to <expansin>.
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