Changes in molecular size of previously deposited and newly synthesized pea cell wall matrix polysaccharides

Effects of indoleacetic acid (IAA) and of turgor changes on the apparent molecular mass (Mr) 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 Mr of xyloglucan, relative to minus-auxin controls, to occur within 0.5 hour. IAA causes an even larger decrease in the peak Mr 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 Mr distributions of pectic polyuronides or hemicellulosic arabinose/galactose polysaccharides within 1.5 hours. However, after epicotyl segments are cut, their peak polyuronide Mr increases and later decreases, possibly as part of a wound response. Xyloglucan also undergoes IAA-independent changes in its Mr distribution after cutting segments. In addition, the peak Mr 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 Mr 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 Mr might cause wall loosening that leads to cell enlargement, as has been suggested previously, but the lack of a simple relation between xyloglucan Mr 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 Mr 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.