Arabinoxylans (AX) and (1-->3),(1-->4)-beta-glucans are major components of wheat endosperm cell walls. Their chemical heterogeneity has been described but little is known about the sequence of their deposition in cell walls during endosperm development. The time course and pattern of deposition of the (1-->3) and (1-->3),(1-->4)-beta-glucans and AX in the endosperm cell walls of wheat (Triticum aestivum L. cv. Recital) during grain development was studied using specific antibodies. At approximately 45 degrees D (degree-days) after anthesis the developing walls contained (1-->3)-beta-glucans but not (1-->3),(1-->4)-beta-glucans. In contrast, (1-->3),(1-->4)-beta-glucans occurred widely in the walls of maternal tissues. At the end of the cellularization stage (72 degrees D), (1-->3)-beta-glucan epitopes disappeared and (1-->3),(1-->4)-beta-glucans were found equally distributed in all thin walls of wheat endosperm. The AX were detected at the beginning of differentiation (245 degrees D) in wheat endosperm, but were missing in previous stages. However, epitopes related to AX were present in nucellar epidermis and cross cells surrounding endosperm at all stages but not detected in the maternal outer tissues. As soon as the differentiation was apparent, the cell walls exhibited a strong heterogeneity in the distribution of polysaccharides within the endosperm.

It has previously been shown that down-regulation of an auxin response factor gene (DR12) results in pleiotropic phenotypes including enhanced fruit firmness in antisense transgenic tomato (AS-DR12). To uncover the nature of the ripening-associated modifications affecting fruit texture, comparative analyses were performed of pectin composition and structure in cell wall pericarp tissue of wild-type and AS-DR12 fruit at mature green (MG) and red-ripe (RR) stages. Throughout ripening, pectin showed a decrease in methyl esterification and in the content of galactan side chains in both genotypes. At mature green stage, pectin content in methyl ester groups was slightly higher in AS-DR12 fruit than in wild type, but this ratio was reversed at the red-ripe stage. The amount of water- and oxalate-soluble pectins increased at the red-ripe stage in the wild type, but decreased in AS-DR12. The distribution of methyl ester groups on the homogalaturonan backbone differed between the two genotypes. There was no evidence of more calcium cross-linked homogalacturan involved in cell-to-cell adhesion in AS-DR12 compared with wild-type fruit. Furthermore, the outer pericarp contains higher proportion of small cells in AS-DR12 fruit than in wild type and higher occurrence of (1-->5) alpha-L-arabinan epitope at the RR stage. It is concluded that the increased firmness of transgenic fruit does not result from a major impairment of ripening-related pectin metabolism, but rather involves differences in pectin fine structure associated with changes in tissue architecture.

The time course and pattern deposition of the cell wall polysaccharides in the starchy endosperm of wheat (Triticum aestivum cv. Recital) during grain development was studied using Fourier transform infrared microspectroscopy (micro-FTIR). Three stages of grain development identified as key stages for cell wall construction were retained as follows:  the end cellularization, the beginning of cell differentiation, and the beginning of maturation. Micro-FTIR revealed that β-(1→3),(1→4) glucans and arabinoglactan proteins are the main cell wall components of endosperm at the end of the cellularization stage, whereas arabinoxylans (AX) appeared only at the cell differentiation stage. Past the differentiation stage, FTIR spectra were dominated by AX features. Cell walls at the beginning of cell differentiation and at endosperm maturation could be distinguished by spectral features that were ascribed to AX substitution. AX appeared more substituted at the beginning of cell differentiation. Moreover, a difference in the degree of AX substitution was found between peripheral and central parts of the grain at the cell differentiation stage; AX in central cells was less substituted. Thus, dramatic changes in endosperm cell wall composition were detected during wheat grain development with respect to both the relative occurrence of individual constituents and the fine structure of the AX. Keywords: Arabinoxylans; endosperm; FTIR; β-(1→3),(1→4) glucans; immunolabeling; principal component analysis; Triticum aestivum