CHOLESTERIC-LIKE CRYSTAL ANALOGS IN GLUCURONOXYLAN-RICH CELL-WALL COMPOSITES - EXPERIMENTAL APPROACH OF ACELLULAR RE-ASSEMBLY FROM NATIVE CELLULOSIC SUSPENSION

Many plant cell walls are constructed according to a helicoidal patter n that is analog to a cholesteric liquid crystal order. This raises th e question whether the wall assembly passes through a true but tempora ry liquid crystal state. The paper focuses on experiments performed fr om aqueous suspensions of extracted quince slime, i.e., a cellulose/gl ucuronoxylan wall composite that presents a helicoidal order when obse rved in situ, within the enlarged periplasm of the seed epidermal cell s. Experiments carried out in acellular conditions showed that a spont aneous reassociation into a helicoidal order can be obtained from tota lly dispersed suspensions. The ultrastructural aspect of the reassembl ed mucilage suspension was different according to the resin used (LR W hite or nanoplast, a water-soluble melamin resin). It was always typic ally polydomain, and when an order was visible it was cholesteric-like and similar to the in situ native organization. Transition states wit h many imperfections expressed the difficulty of the system to reassem ble in the absence of constraining surfaces. The possible intervention of glucuronoxylan (GX) in the ordered assembly of the microfibrils wa s checked by: (1) progressive extraction of GX by trifluoroacetic acid (TFA). The extraction was associated to a control of the fraction by analysis of uronic acid contents and observation at the electron micro scope level. Extraction of GX provoked the formation of a flocculent m ass, the flocculation being more intense when the TFA was more concent rated; (2) progressive change of pH in order to analyze the influence of pH on flocculation. Low pH (ca. pH 3) led also to a flocculation of the suspension, but the flee was reversibly lost after dialysis again st distilled water. The results indicate the antifloc role of the GX d ue to the anionic charges carried by the side-chains. However, the fun ction of GX as helper twisting agent in the cholesteric-like reassembl y must not be ruled out.