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Loose connections
Despite advances in our knowledge of plant cell-wall biochemistry, the mechanisms involved in loosening the wall for cell expansion are not completely understood. Proteins known as expansins are certainly involved in loosening. However, other enzymes, the xyloglucan endotransglucosylases (XETs), may also participate in this process, as discussed by Van Sandt et al. (Antwerpen, Belgium, pp. 1467–1473). XETs are capable of cutting and rejoining xyloglucans that tether adjacent cellulose microfibrils. Some members of the class also exhibit xyloglucan hydrolase (XEH) activity. Further, the XETs show similar patterns of expression to those of the expansins in relation to active cell expansion, while their activity measured in situ is also correlated with growth. However, despite these indicative data, there has been to date no actual demonstration that XETs mediate cell-wall loosening for cell expansion. It is this problem that the authors have addressed using a simple in vitro system. A cDNA encoding the XET of Selaginella kraussian was cloned and over-expressed to provide a purified enzyme. The test tissue was onion epidermis, one-cell-thick, highly anisotropic tissue, expansion of which was measured in two directions with a constant-load extensiometer. Expansion in the direction parallel to the direction of the cellulose microfibrils was much less than transverse expansion. In both directions, expansion growth was greater at pH 4.5 than at pH 6.0; at pH 4.5, transverse expansion was strongly inhibited by prior heat denaturation; inhibition in the parallel direction was only slight. However, expansion in the transverse direction was largely restored (up to 66 %) by the addition of exogenous purified XET. Addition of a partly purified expansin preparation restored extension by 20 %. Enzyme addition did not stimulate expansion in parallel to the microfibrils in heat-denatured epidermis. These data thus show that XET can supply at least part of the enzyme activity needed for cell expansion and implicate XET directly in cell-wall loosening.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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