Elongating maize root: zone-specific combinations of polysaccharides from type I and type II primary cell walls.

The dynamics of cell wall polysaccharides may modulate the cell wall mechanics and thus control the expansion growth of plant cells. The unique composition of type II primary cell wall characteristic of grasses suggests that they employ specific mechanisms for cell enlargement. We characterized the transcriptomes in five zones along maize root, clustered the expression of genes for numerous glycosyltransferases and performed extensive immunohistochemical analysis to relate the changes in cell wall polysaccharides to critical stages of cell development in Poaceae. Specific patterns of cell wall formation differentiate the initiation, realization and cessation of elongation growth. Cell walls of meristem and early elongation zone represent a mixture of type I and type II specific polysaccharides. Xyloglucans and homogalacturonans are synthesized there actively together with mixed-linkage glucans and glucuronoarabinoxylans. Rhamnogalacturonans-I with the side-chains of branched 1,4-galactan and arabinan persisted in cell walls throughout the development. Thus, the machinery to generate the type I primary cell wall constituents is completely established and operates. The expression of glycosyltransferases responsible for mixed-linkage glucan and glucuronoarabinoxylan synthesis peaks at active or late elongation. These findings widen the number of jigsaw pieces which should be put together to solve the puzzle of grass cell growth.

Flax rhamnogalacturonan lyases: phylogeny, differential expression and modeling of protein structure.

Rhamnogalacturonan lyases (RGLs; EC 4.2.2.23) degrade the rhamnogalacturonan I (RG-I) backbone of pectins present in the plant cell wall. These enzymes belong to polysaccharide lyase family 4, members of which are mainly from plants and plant pathogens. RGLs are investigated, as a rule, as pathogen 'weapons' for plant cell wall degradation and subsequent infection. Despite the presence of genes annotated as RGLs in plant genomes and the presence of substrates for enzyme activity in plant cells, evidence supporting the involvement of this enzyme in certain processes is limited. The differential expression of some RGL genes in flax (Linum usitatissimum L.) tissues, revealed in our previous work, prompted us to carry out a total revision (phylogenetic analysis, analysis of expression and protein structure modeling) of all the sequences of flax predicted as coding for RGLs. Comparison of the expressions of LusRGL in various tissues of flax stem revealed that LusRGLs belong to distinct phylogenetic clades, which correspond to two co-expression groups. One of these groups comprised LusRGL6-A and LusRGL6-B genes and was specifically upregulated in flax fibers during deposition of the tertiary cell wall, which has complex RG-I as a key noncellulosic component. The results of homology modeling and docking demonstrated that the topology of the LusRGL6-A catalytic site allowed binding to the RG-I ligand. These findings lead us to suggest the presence of RGL activity in planta and the involvement of special isoforms of RGLs in the modification of RG-I of the tertiary cell wall in plant fibers.

Transcriptome portrait of cellulose-enriched flax fibres at advanced stage of specialization.

Functional specialization of cells is among the most fundamental processes of higher organism ontogenesis. The major obstacle to studying this phenomenon in plants is the difficulty of isolating certain types of cells at defined stages of in planta development for in-depth analysis. A rare opportunity is given by the developed model system of flax (Linum usitatissimum L.) phloem fibres that can be purified from the surrounding tissues at the stage of the tertiary cell wall deposition. The performed comparison of the whole transcriptome profile in isolated fibres and other portions of the flax stem, together with fibre metabolism characterization, helped to elucidate the general picture of the advanced stage of plant cell specialization and to reveal novel participants potentially involved in fibre metabolism regulation and cell wall formation. Down-regulation of all genes encoding proteins involved in xylan and lignin synthesis and up-regulation of genes for the specific set of transcription factors transcribed during tertiary cell wall formation were revealed. The increased abundance of transcripts for several glycosyltransferases indicated the enzymes that may be involved in synthesis of fibre-specific version of rhamnogalacturonan I.