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Development and Growth Regulation

Changes in Molecular Size of Previously Deposited and Newly Synthesized Pea Cell Wall Matrix Polysaccharides ¹

Effects of Auxin and Turgor

Department of Biological Sciences, Stanford University, Stanford, California 94305

Effects of indoleacetic acid (IAA) and of turgor changes on the apparent molecular mass (M_r) distributions of cell wall matrix polysaccharides from etiolated pea (Pisum sativum L.) epicotyl segments were determined by gel filtration chromatography. IAA causes a two- to threefold decline in the peak M_r of xyloglucan, relative to minus-auxin controls, to occur within 0.5 hour. IAA causes an even larger decrease in the peak M_r concurrently biosynthesized xyloglucan, as determined by [³H]fucose labeling, but this effect begins only after 1 hour. In contrast, IAA does not appreciably affect the M_r distributions of pectic polyuronides or hemicellulosic arabinose/galactose polysaccharides within 1.5 hours. However, after epicotyl segments are cut, their peak polyuronide M_r increases and later decreases, possibly as part of a wound response. Xyloglucan also undergoes IAA-independent changes in its M_r distribution after cutting segments. In addition, the peak M_r of newly deposited xyloglucan increases from about 9 kilodaltons shortly after deposition to about 30 kilodaltons within 0.5 hour. This may represent a process of integration into the cell wall. A step increase in turgor causes the peak M_r of previously deposited xyloglucan (but not of the other major polymers) to increase about 10-fold within 0.5 hour, returning to its initial value by 1.5 hours. This upshift may comprise a feedback mechanism that decreases wall extensibility when the rate of wall extension suddenly increases. IAA-induced reduction of xyloglucan M_r might cause wall loosening that leads to cell enlargement, as has been suggested previously, but the lack of a simple relation between xyloglucan M_r and elongation rate indicates that loosening must also involve other wall factors, one of which might be the deposition of new xyloglucan of much smaller size. Although the M_r shifts in polyuronides may represent changes in noncovalent association, and for xyloglucan this cannot be completely excluded, xyloglucan seems to participate in a dynamic process that can both decrease and increase its chain length, possible mechanisms for which are suggested.

¹ Supported by grants from the National Science Foundation and the McKnight Foundation.

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