Assembly and synthesis of the extracellular matrix in brown algae.

In brown algae, the extracellular matrix (ECM) and its constitutive polymers play crucial roles in specialized functions, including algal growth and development. In this review we offer an integrative view of ECM construction in brown algae. We briefly report the chemical composition of its main constituents, and how these are interlinked in a structural model. We examine the ECM assembly at the tissue and cell level, with consideration on its structure in vivo and on the putative subcellular sites for the synthesis of its main constituents. We further discuss the biosynthetic pathways of two major polysaccharides, alginates and sulfated fucans, and the progress made beyond the candidate genes with the biochemical validation of encoded proteins. Key enzymes involved in the elongation of the glycan chains are still unknown and predictions have been made at the gene level. Here, we offer a re-examination of some glycosyltransferases and sulfotransferases from published genomes. Overall, our analysis suggests novel investigations to be performed at both the cellular and biochemical levels. First, to depict the location of polysaccharide structures in tissues. Secondly, to identify putative actors in the ECM synthesis to be functionally studied in the future.

Changes in Cell Wall Structure During Rhizoid Formation of Silvetia babingtonii (Fucales, Phaeophyceae) Zygotes.

We examined the ultrastructure of the cell wall and immunolocalization of alginates using specific antibodies against M-rich alginates and MG blocks during rhizoid formation in fucoid zygotes, Silvetia babingtonii. The thallus region of 24-h-old zygotes had a cell wall made of three layers with different fiber distribution. In the 12-h-old zygotes, three layers in the thallus were observed before rhizoid formation, namely the inner, middle, and outer layers. During rhizoid elongation, only the inner layer was apparent close to the rhizoid tip area. Immunoelectron microscopy detected M-rich blocks of alginate on the inner half of the cell wall, irrespective of the number of layers in the thallus and rhizoid regions. The MG blocks were seen to cover a slightly wider area than M-rich alginate blocks. It was suggested that parts of M in mannuronan would be rapidly converted to G, and MG-blocks are generated. Transcriptome analysis was performed using 3 -, 10 -, and 24-h-old zygotes after fertilization to examine the relationship between gene expression and alginate synthesis over time. The expression of two mannuronan C5-epimerase homologs that convert mannuronic acid into guluronic acid in alginates was upregulated or downregulated over the course of the examination.

Ultrastructural Observation of Cytokinesis and Plasmodesmata Formation in Brown Algae.

Similar to land plant cells, brown algal cells possess plasmodesmata with minute cytoplasmic tunnels, which enable the direct connection between adjacent cells. Plasmodesmata are distributed depending on the association of their formation with cytokinesis. Primary plasmodesmata are formed during cytokinesis, while secondary plasmodesmata appear on the cell wall septum following cytokinesis. Typically, the plasmodesmata of brown algae are cylindrical without the penetration of desmotubules from the endoplasmic reticulum, and there are no morphological differences between primary and secondary plasmodesmata. This present chapter describes the observation of cytokinesis and primary plasmodesmata formation in brown algae using electron microscopy as well as the examination of polysaccharide distribution using antibodies and enzyme-gold probes.