Cell elongation

In plant development, cell elongation is an increase in a cell's length, often with growth biased more strongly along one axis than another. It differs from cell division, which increases cell number. Tissue extension can therefore reflect changing cell dimensions as well as production of additional cells.

For an illustrative fixed population of ten cells arranged end-to-end, increasing each length from $20$ to $25\mu \mathrm{m}$ increases the combined length from $200$ to $250\mu \mathrm{m}$ without adding any cells. Each cell has elongated by 25%. This count assumes no intervening spaces or rearrangements and is not a measured growth result.

Plant cells enlarge against a cell wall. Turgor pressure supplies mechanical loading, while controlled wall relaxation allows irreversible extension rather than only reversible elastic stretching. A landmark reconstitution experiment extracted wall-associated proteins from growing cucumber seedlings and added the extract to isolated walls. Fractionation identified two activities associated with proteins of approximately 29 and 30 kilodaltons; each induced extension without detectable hydrolytic wall breakdown.^[ref]

These proteins, associated with the expansin family, helped establish that wall loosening need not simply mean wholesale enzymatic digestion of the wall. The assay also separates a physical property from cell multiplication: isolated walls can extend in a preparation that is not dividing. The full growth of a living cell still involves water uptake, wall material and regulation; the reconstitution identifies one experimentally accessible part of that process.