Secondary cell-wall assembly in flax phloem fibres: role of galactans

Homofusion of Golgi secretory vesicles in flax phloem fibers during formation of the gelatinous secondary cell wall

The gelatinous type of secondary cell wall is present in tension wood and in phloem fibers of many plants. It is characterized by the absence of xylan and lignin, a high cellulose content and axially orientated microfibrils in the huge S2 layer. In flax phloem fiber, the major non-cellulosic component of such cell walls is tissue-specific galactan, which is tightly bound to cellulose. Ultrastructural analysis of flax fiber revealed that initiation of gelatinous secondary cell wall formation was accompanied by the accumulation of specific Golgi vesicles, which had a characteristic bicolor (dark-light) appearance and were easily distinguishable from vesicles made in different tissues and during the other stages of fiber development. Many of the bicolor vesicles appeared to fuse with each other, forming large vacuoles. The largest observed was 4 mum in diameter. Bicolor vesicles and vacuoles fused with the plasma membrane and spread their content in a characteristic "syringe-like" manner, covering a significant area of periplasm and forming "dark" stripes on the inner wall surface. Both Golgi derivatives and cell wall layers were labeled by LM5 antibody, indicating the presence of tissue- and stage-specific (1-->4)-beta-galactan. We suggest that this specific type of galactan secretion, which allows coverage of a large area of periplasm, is designed to increase the chance of the galactan meeting the cellulose microfibrils while they are still in the process of construction. The membrane fusion machinery of flax fiber must possess special components, which may be crucial for the formation of the gelatinous type cell wall.

Microarray analysis of developing flax hypocotyls identifies novel transcripts correlated with specific stages of phloem fibre differentiation

BACKGROUND AND AIMS

Hypocotyls are a commonly used model to study primary growth in plants, since post-germinative hypocotyls increase in size by cell elongation rather than cell division. Flax hypocotyls produce phloem fibres in bundles one to two cell layers thick, parallel to the protoxylem poles of the stele. Cell wall deposition within these cells occurs rapidly at a well-defined stage of development. The aim was to identify transcripts associated with distinct stages of hypocotyl and phloem fibre development.

METHODS

Stages of flax hypocotyl development were defined by analysing hypocotyl length in relation to fibre secondary wall deposition. Selected stages of development were used in microarray analyses to identify transcripts involved in the transition from elongation to secondary cell wall deposition in fibres. Expression of specific genes was confirmed by qRT-PCR and by enzymatic assays.

KEY RESULTS

Genes enriched in the elongation phase included transcripts related to cell-wall modification or primary-wall deposition. Transcripts specifically enriched at the transition between elongation and secondary wall deposition included beta-galactosidase and arabinogalactan proteins. Later stages of wall development showed an increase in secondary metabolism-related transcripts, chitinases and glycosyl hydrolases including KORRIGAN. Microarray analysis also identified groups of transcription factors enriched at one or more stages of fibre development. Subsequent analysis of a differentially expressed beta-galactosidase confirmed that the post-elongation increase in beta-galactosidase enzyme activity was localized to phloem fibres.

CONCLUSIONS

Transcripts were identified associated with specific stages of hypocotyl development, in which phloem fibre cells were undergoing thickening of secondary walls. Temporal and spatial regulation of beta-galactosidase activity suggests a role for this enzyme in remodelling of flax bast fibre cell walls during secondary cell wall deposition.

In situ analysis of cell wall polymers associated with phloem fibre cells in stems of hemp, Cannabis sativa L

A study of stem anatomy and the sclerenchyma fibre cells associated with the phloem tissues of hemp (Cannabis sativa L.) plants is of interest for both understanding the formation of secondary cell walls and for the enhancement of fibre utility as industrial fibres and textiles. Using a range of molecular probes for cell wall polysaccharides we have surveyed the presence of cell wall components in stems of hemp in conjunction with an anatomical survey of stem and phloem fibre development. The only polysaccharide detected to occur abundantly throughout the secondary cell walls of phloem fibres was cellulose. Pectic homogalacturonan epitopes were detected in the primary cell walls/intercellular matrices between the phloem fibres although these epitopes were present at a lower level than in the surrounding parenchyma cell walls. Arabinogalactan-protein glycan epitopes displayed a diversity of occurrence in relation to fibre development and the JIM14 epitope was specific to fibre cells, binding to the inner surface of secondary cell walls, throughout development. Xylan epitopes were found to be present in the fibre cells (and xylem secondary cell walls) and absent from adjacent parenchyma cell walls. Analysis of xylan occurrence in the phloem fibre cells of hemp and flax indicated that xylan epitopes were restricted to the primary cell walls of fibre cells and were not present in the secondary cell walls of these cells.

A flax fibre proteome: identification of proteins enriched in bast fibres

BACKGROUND

Bast fibres from the phloem tissues of flax are scientifically interesting and economically useful due in part to a dynamic system of secondary cell wall deposition. To better understand the molecular mechanisms underlying the process of cell wall development in flax, we extracted proteins from individually dissected phloem fibres (i.e. individual cells) at an early stage of secondary cell wall development, and compared these extracts to protein extracts from surrounding, non-fibre cells of the cortex, using fluorescent (DiGE) labels and 2D-gel electrophoresis, with identities assigned to some proteins by mass spectrometry.

RESULTS

The abundance of many proteins in fibres was notably different from the surrounding non-fibre cells of the cortex, with approximately 13% of the 1,850 detectable spots being significantly (> 1.5 fold, p < or = 0.05) enriched in fibres. Following mass spectrometry, we assigned identity to 114 spots, of which 51 were significantly enriched in fibres. We observed that a K+ channel subunit, annexins, porins, secretory pathway components, beta-amylase, beta-galactosidase and pectin and galactan biosynthetic enzymes were among the most highly enriched proteins detected in developing flax fibres, with many of these proteins showing electrophoretic patterns consistent with post-translational modifications.

CONCLUSION

The fibre-enriched proteins we identified are consistent with the dynamic process of secondary wall deposition previously suggested by histological and biochemical analyses, and particularly the importance of galactans and the secretory pathway in this process. The apparent abundance of beta-amylase suggests that starch may be an unappreciated source of materials for cell wall biogenesis in flax bast fibres. Furthermore, our observations confirm previous reports that correlate accumulation proteins such as annexins, and specific heat shock proteins with secondary cell wall deposition.

Microarray analysis of bast fibre producing tissues of Cannabis sativa identifies transcripts associated with conserved and specialised processes of secondary wall development

The mechanisms underlying bast fibre differentiation in hemp (Cannabis sativa L.) are largely unknown. We hybridised a cDNA microarray with RNA from fibre enriched tissues extracted at three different positions along the stem axis. Accordingly, we identified transcripts that were enriched in tissues in which phloem fibres were elongating or undergoing secondary wall thickening. These results were consistent with a dynamic pattern of cell wall deposition involving tissue specific expression of a large set of distinct glycosyltransferases and glycosylhydrolases apparently acting on polymers containing galactans, mannans, xylans, and glucans, as well as raffinose-series disaccharides. Putative arabinogalactan proteins and lipid transfer proteins were among the most highly enriched transcripts in various stem segments, with different complements of each expressed at each stage of development. We also detected stage-specific expression of brassinosteroid-related transcripts, various transporters, polyamine and phenylpropanoid related genes, and seven putative transcription factors. Finally, we observed enrichment of many transcripts with unknown biochemical function, some of which had been previously implicated in fibre development in poplar or cotton. Together these data complement and extend existing biochemical models of bast fibre development and secondary wall deposition and highlight uncharacterised, but conserved, components of these processes.

Biomimetic engineering of cellulose-based materials

Biomimetics is a field of science that investigates biological structures and processes for their use as models for the development of artificial systems. Biomimetic approaches have considerable potential in the development of new high-performance materials with low environmental impact. The cell walls of different plant species represent complex and highly sophisticated composite materials that can provide inspiration on how to design and fabricate lightweight materials with unique properties. Such materials can provide environmentally compatible solutions in advanced packaging, electronic devices, vehicles and sports equipment. This review gives an overview of the structures and interactions in natural plant cell walls and describes the first attempts towards mimicking them to develop novel biomaterials.

Microarray analysis of flax (Linum usitatissimum L.) stems identifies transcripts enriched in fibre-bearing phloem tissues

To better understand the molecular processes associated with the development of the unusually long (> 30 mm) and strong bast fibre cells within the phloem of flax stems, we conducted a gene discovery experiment to identify transcripts enriched in fibre-bearing tissues, with the intention that these transcripts would serve as future targets for crop improvement and research in phloem development and cell wall deposition. We produced a library of 9,600 cDNA clones from the peels of flax stems, and selected tissue-specific cDNAs for sequencing based on two series of microarray experiments. In the first microarray series, we compared transcript abundance in stem-peels and leaves, and identified stem-enriched transcripts putatively involved in the processes of polysaccharide and cell wall metabolism. In the second microarray series, we compared gene expression in three segments of the vertical stem axis, which constituted a developmental series for phloem fibres and other cell types. The expression of specific LTP and AGP transcripts was particularly well-correlated with stem segments during either the elongation phase or cell-wall thickening phase of phloem fibre development, and the phloem-specific enrichment of these transcripts was confirmed by qRT-PCR. Transcripts representing multiple, distinct chitinases, beta-galactosidases, arabinogalactan proteins (AGP), and lipid transfer proteins (LTPs) were among the interesting transcripts enriched in specific stages of the developing stem. Considered together, the results of our analyses suggest similarity between the molecular mechanisms underlying phloem fibre development and the gelatinous fibres of tension wood in trees.

Cadmium-induced alterations of the structural features of pectins in flax hypocotyl

In the course of our studies on the putative role of pectins in the control of cell growth, we have investigated the effect of cadmium on their composition, remodelling and distribution within the epidermis and fibre tissues of flax hypocotyl (Linum usitatissimum L.). Cadmium-stressed seedlings showed a significant inhibition of growth whereas the hypocotyl volume did not significantly change, due to the swelling of most tissues. The structural alterations consisted of significant increase of the thickness of all cell walls and the marked collapse of the sub-epidermal layer. The pectic epitopes recognized by the anti-PGA/RGI and JIM5 antibodies increased in the outer parts of the epidermis (external tangential wall and junctions) and fibres (primary wall and junctions). Concomitantly, there was a remarkable decrease of JIM7 antibody labelling and consequently an increase of the ratio JIM5/JIM7. Conversely, the ratio JIM7/JIM5 increased in the wall domains closest to the plasmalemma, which would expel the cadmium ions from the cytoplasm. The hydrolysis of cell walls revealed a cadmium-induced increase of uronic acid in the pectic matrix. Sequential extractions showed a remodelling of both homogalacturonan and rhamnogalacturonan I. In fractions enriched in primary walls, the main part of the pectins became cross-linked and could be extracted only with alkali. In fractions enriched in secondary walls, the homogalacturonan moieties were found more abundantly in the calcium-chelator extract while the rhamnogacturonan level increased in the boiling water extract.