Cell wall architecture of the elongating maize coleoptile

The primary walls of grasses are composed of cellulose microfibrils, glucur onoarabinoxylans (GAXs), and mixed-linkage beta -glucans, together with sma ller amounts of xyloglucans, glucomannans, pectins, and a network of polyph enolic substances. Chemical imaging by Fourier transform infrared microspec troscopy revealed large differences in the distributions of many chemical s pecies between different tissues of the maize (Zea mays) coleoptile. This w as confirmed by chemical analyses of isolated outer epidermal tissues compa red with mesophyll-enriched preparations. Glucomannans and esterified uroni c acids were more abundant in the epidermis, whereas beta -glucans were mor e abundant in the mesophyll cells. The localization of beta -glucan was con firmed by immunocytochemistry in the electron microscope and quantitative b iochemical assays. We used field emission scanning electron microscopy, inf rared microspectroscopy, and biochemical characterization of sequentially e xtracted polymers to further characterize the cell wall architecture of the epidermis. Oxidation of the phenolic network followed by dilute NaOH extra ction widened the pores of the wall substantially and permitted observation by scanning electron microscopy of up to six distinct microfibrillar lamel lae. Sequential chemical extraction of specific polysaccharides together wi th enzymic digestion of beta -glucans allowed us to distinguish two distinc t domains in the grass primary wall. First, a beta -glucan-enriched domain, coextensive with GAXs of low degrees of arabinosyl substitution and glucom annans, is tightly associated around microfibrils. Second, a GAX that is mo re highly substituted with arabinosyl residues and additional glucomannan p rovides an interstitial domain that interconnects the beta -glucan-coated m icrofibrils. Implications for current models that attempt to explain the bi ochemical and biophysical mechanism of wall loosening during cell growth ar e discussed.