THE RELATIONSHIP BETWEEN XYLOGLUCAN ENDOTRANSGLYCOSYLASE AND INVITRO-CELL WALL EXTENSION IN CUCUMBER HYPOCOTYLS

It has been proposed that cell wall loosening during plant cell growth may be mediated by the endotransglycosylation of load-bearing polymer s, specifically of xyloglucans, within the cell wall. A xyloglucan end otransglycosylase (XET) with such activity has recently been identifie d in several plant species. Two cell wall proteins capable of inducing the extension of plant cell walls have also recently been identified in cucumber hypocotyls. In this report we examine three questions: (1) Does XET induce the extension of isolated cell walls? (2) Do the exte nsion-inducing proteins possess XET activity? (3) Is the activity of t he extension-inducing proteins modulated by a xyloglucan nonasaccharid e (Glc4-Xyl3-Gal2)? We found that the soluble proteins from growing cu cumber (cucumis sativum L.) hypocotyls contained high XET activity but did not induce wall extension. Highly purified wall-protein fractions from the same tissue had high extension-inducing activity but little or no XET activity. The XET activity was higher at pH 5.5 than at pH 4 .5, while extension activity showed the opposite sensitivity to pH. Re constituted wall extension was unaffected by the presence of a xyloglu can nonasaccharide (Glc,-Xyl3-Gal2), an oligosaccharide previously sho wn to accelerate growth in pea stems and hypothesized to facilitate gr owth through an effect on XET-induced cell wall loosening. We conclude that XET activity alone is neither sufficient nor necessary for exten sion of isolated walls from cucumber hypocotyls.