Studies on the Role of Wall Extensibility in the Control of Cell Expansion

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Abstract
When growth of stem or coleoptile sections was stimulated by auxin, cell wall [etiolated soybeans (Glycine max L. ''Hawkeye 63''), maize (Zea mays L. ''WF9XM14''), oats (Avena sativa L. ''Victory'') and peas (Pisum sativum L. ''Alaska'')] extensibility (measured by Instron analysis) was also increased. Wall loosening and elongation were uncoupled by mannitol and by mechanical restraint, demonstrating that auxin-induced wall loosening was not the result of auxin-induced cell expansion. High concentrations of Ca inhibited both the auxin-induced increase in cell expansion and wall extensibility; low concentrations increased both expansion and extensibility. Extensibility of pea sections killed by freezing or by boiling in methanol was not decreased by incubating the sections in calcium-containing solutions. Stimulation of growth by compounds other than auxins was also accompanied by increased wall extensibility. However, increased cell wall extensibility did not always support increased rates of cell expansion. At supraoptimal auxin concentration, extensibility of pea epicotyl sections increased as growth rate declined. Wall loosening may be necessary but is not sufficient for cell expansion to occur. Ca inhibition appears as a complex metabolic event rather than a simple cross-linking of anionic wall polymers.

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