TIME DOMAIN 'H-NMR OF ARABINOXYLANS AND 1 P-GLUCANS FILMS, MODELS OF A LAMELLAR ORGANISATION IN ENDOSPERM CELL WALLS OF CEREAL GRAINS

摘要

Plant cell walls are partly responsible for the variability of plant crops in their different agro-industrial food and non-food uses. Control of this variability is essential for the improvement of plant productions quality. Endosperm cell walls have important impact on the uses of cereal grains (milling, baking, human nutrition.. .). Arabinoxylans (AX) and mixed (1-3)(1-4)-3-D-glucans (BG) are the main components of the cereal grains endosperm cell walls. They constitute respectively 70 and 30% of the walls, which represents 2-4% of the dry matter in endosperm. AX are characterized by a linear backbone of xylose to which arabinose substituents are attached while BG are linear homopolymers of D-glucose linked by P-(l-4) and p-(l-3) linkages. The arabinofuranose units are essentially found as mono- or di-substitution in position 0-3 or 0-3 and 0-2 of xylose residues. AX exhibit large natural variations on their structure, which mainly differ by the arabinose to xylose ratio (A/X) with values ranging from 0.31 to 1.06. Arabinosyl residues can also be esterified on the O-5 position mainly with ferulic acid. The local composition heterogeneity of endosperm cell walls has been investigated on samples recovered from fractioning processes, by direct detection of fluorescence, by coupling imaging and spectroscopic techniques and by immunolabelling techniques . It was concluded that BG are present in higher proportion in cell walls of aleurone and subaleurone layers than in the starchy endosperm, which contains in its central zone, a higher content of AX. Aleurone cell walls are characterised by a low A/X ratio in the range 0.3-0.4, while it attains 0.5-0.6 in the starch endosperm walls. This chemical heterogeneity has been extensively described but little is known about the time-course and the pattern of the AX and BG deposition in cell walls during endosperm development. Various observations using different microscopy techniques (MCBL, AFM) have indicated the lamellar organisation of wheat endosperm cells walls that possibly reflects the assemblies of AX and BG. In order to study the impact of their fine structure and interactions on cell walls properties, in particular hydration properties, we have prepared AX and BG films as models of their lamellar distribution within endosperm cell walls. The second moment M2 and spin-spin relaxation times T2 were measured respectively with single pulse free induction decay (FID) and Carr-Purcell-Meiboom-Gill (CPMG) sequences. The objectives of this work were to study the influence of both water and temperature on mobility and hydration of AX and BG within films, in relation to their organization 1 in cell-walls of wheat grains.