A simple apparatus for measuring long-term extension of plant cell walls subjected to tensile stress

Investigations of the control of cell expansion through cell wall-tightening and -loosening require replicate measurements of long-term irreversible wall extension. A simple, inexpensive, high-throughput apparatus is described for monitoring long-term wall extension in elongating plant organs [e.g., silks (carpellary styles) from maize (Zea mays), or phloem fibre bundles from celery (Apium graveolens) petioles] subjected to a constant tensile stress. The specimen was methanol-killed, and a suitable zone of the (relaxed) specimen was marked with ink. The specimen was then hung within a glass tube filled with buffer (2 ml) plus any additives of interest, a suitable tension being applied by weights. Total creep (i.e. plastic plus elastic increase in taut length) was estimated approximately. After 16 h, the specimen was taken down and its relaxed length re-measured. Marked zones (initially 75 mm) of celery petiole fibre bundles, when stretched at pH 4.0 for 16 h with a 25-g weight, underwent an irreversible increase in length of about 20 mm, easily measured with a ruler. The capacity to undergo irreversible extension was strongly inhibited by neutral buffers, or by pre-incubation in formaldehyde or boiling water (but not in boiling methanol), suggesting a role for endogenous expansins. Irreversible extension was slightly inhibited by 1 mM Al3+, whereas total (reversible plus irreversible) creep was strongly inhibited. Thus, total creep did not always correctly predict the concomitant irreversible extension. We propose that irreversible extension is more closely related (than is total creep) to the mechanism of plant growth.