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 [(3)H]
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.