Studies on the cell walls of selected monocotyledons

Abstract

In this thesis I have examined aspects of the composition and structure of the cell walls of selected species of monocotyledons. Included is a study of the monomeric and dimeric phenolic acids in the unlignified cell walls of pineapple fruit as well as the characterization of a major feruloylated oligosaccharide isolated from these cell walls. Also included is a study of the polysaccharide compositions of the unlignified and lignified cell walls of members of the order Poales. Cross polarisation/ magic angle spinning 13C nuclear magnetic resonance (CP/MAS 13C NMR) was used to examine the molecular ordering of the polysaccharides in the unlignified cell walls of several monocotyledons. Trans ferulic acid was shown to be the major monomeric phenolic acid that was ester-linked to the unlignified cell walls of pineapple fruit. Smaller amounts of cis ferulic acid and trans and cis ρ-coumaric acid were also found. In addition, preliminary evidence was obtained for the presence of dehydrodimers of ferulic acid, including 5-5'dehydrodiferulic acid, and cyclobutane derivatives of ferulic acid. Treatment of the unlignified cell walls of pineapple fruit with 'Driselase' released a series of feruloylated oligosaccharides which were separated by paper and thin layer chromatography and located by their fluorescence in UV radiation. The structure of the most abundant of these feruloylated oligosaccharides(Compound 1) was determined using partial acid hydrolysis, monosaccharide and methylation analysis, and fast atom bombardment-mass spectroscopy. Compound 1 was found to be FAXX (O-[5-O-(tras-feruloyl)-α-L-arabinofuranosyl](l→3)-O-ß-D- xylopyranosyl-(1→4)-D-xylopyranose) which had previously been characterized from similar digests of the unlignified cell walls of the Poaceae. This indicated that in the unlignified cell walls of pineapple fruit, ferulic acid was linked to glucuronoarabinoxylans (GAXs) in the same way as it is linked to GAXs in the cell walls of the Poaceae. The polysaccharide compositions of unlignified cell walls of species in the following families of the order Poales were examined: Anarthriaceae, Centrolepidaceae, Ecdeiocoleaceae, Flagellariaceae, Poaceae, and Restionaceae. The results showed that the unlignified cell walls contained cellulose, large amounts of highly substituted GAXs, variable amounts of xyloglucans, and small amounts of pectic polysaccharides. Some of these unlignified cell walls also contained (1→3,1→4)-β-D-glucans in varying amounts but some contained none. This is the first evidence of (1→3,1→4)-β-D-glucans in cell walls of angiosperm families other than the family Poaceae. The polysaccharide compositions of the lignified cell walls of species in the following families were also examined: Flagellariaceae, Poaceae, and Restionaceae. The results showed that cellulose and GAXs were the major polysaccharides, but the GAXs in the lignified cell walls contained fewer arabinofuranosyl substitutions than those in the unlignified cell walls of the same species. Thus, the compositions of the unlignified and lignified cell walls of the other families of the order Poales examined were similar to those of the Poaceae. Solid-state CP/MAS 13C NMR spectroscopy was used to characterize the molecular ordering of the polysaccharides in the unlignified cell walls of three monocotyledon species, Italian ryegrass, pineapple fruit, and onion and one dicotyledon species, cabbage. These species were chosen because their unlignified cell walls had a range of non-cellulosic polysaccharide compositions. The proton spin relaxation time constant T1ρ (H) (rotating-frame) showed that in the cell walls of all four species the cellulose was in a crystalline rather than a 'paracrystalline' or amorphous state. The percentages of cellulose molecules assigned to the crystal-interior cellulose were 44%, 36%, 34%, and 32% for the unlignified cell walls of Italian ryegrass, pineapple, onion, and cabbage. respectively. These percentages corresponded to cellulose crystallite cross-sectional dimensions of 3.2 x 3.1 nm for Italian ryegrass and 2.7 x 2.4 nm for pineapple, onion, and cabbage. A resolution enhancement procedure showed that both the triclinic (Iα) and the monoclinic (Iβ) crystal forms of cellulose were present in similar proportions in the cell walls of all four species examined. There was no evidence that the non-cellulosic polysaccharide compositions of the cell walls had any influence on the structure and forms of cellulose present in the unlignified cell walls. CP/MAS 13C NMR was also used to examine the mobilities of the non-cellulosic polysaccharides in the same cell walls. Experiments using T1ρ (H) alone showed that the polysaccharides of the cell walls could be separated into two domains, one rigid and containing cellulose, and the other mobile and containing non-cellulosic polysaccharides. Experiments using the proton relaxation time constant T2(H)(spin-spin) showed that while most of the non-cellulosic polysaccharides occupied a domain that was highly mobile (domain C), some occupied a domain that was more rigid (domain B), but not as rigid as that of cellulose (domain A). In the cell walls of all the species examined domain A contained highly crystalline cellulose. In the cell walls of Italian ryegrass and pineapple, domain B contained small amounts of polysaccharides, including xyloglucans, and domain C contained mainly GAXs and small amounts of rhamnogalacturonans. In comparison, domain B of the cell walls of onion and cabbage contained galacturonans and xyloglucans and domain C contained mainly rhamnogalacturonans with galactan (onion) or arabinan (cabbage) sidechains. NMR experiments using single pulse excitation were also carried out on the cell walls of Italian ryegrass and cabbage to investigate the most mobile polysaccharides in the cell walls. These experiments showed that GAXs and small amounts of pectic polysaccharides were the most mobile polysaccharides in the cell walls of Italian ryegrass and that arabinan side chains of rhamnogalacturonans were the most mobile polysaccharides in the cell walls of cabbage.