The Living Fossil Psilotum nudum Has Cortical Fibers With Mannan-Based Cell Wall Matrix

Characterization of the fiber-like cortical cells in moss gametophytes

MAIN CONCLUSION

Fiber-like cells with thickened cell walls of specific structure and polymer composition that includes (1 → 4)-β-galactans develop in the outer stem cortex of several moss species gametophytes. The early land plants evolved several specialized cell types and tissues that did not exist in their aquatic ancestors. Of these, water-conducting elements and reproductive organs have received most of the research attention. The evolution of tissues specialized to fulfill a mechanical function is by far less studied despite their wide distribution in land plants. For vascular plants following a homoiohydric trajectory, the evolutionary emergence of mechanical tissues is mainly discussed starting with the fern-like plants with their hypodermal sterome or sclerified fibers that have xylan and lignin-based cell walls. However, mechanical challenges were also faced by bryophytes, which lack lignified cell-walls. To characterize mechanical tissues in the bryophyte lineage, following a poikilohydric trajectory, we used six wild moss species (Polytrichum juniperinum, Dicranum sp., Rhodobryum roseum, Eurhynchiadelphus sp., Climacium dendroides, and Hylocomium splendens) and analyzed the structure and composition of their cell walls. In all of them, the outer stem cortex of the leafy gametophytic generation had fiber-like cells with a thickened but non-lignified cell wall. Such cells have a spindle-like shape with pointed tips. The additional thick cell wall layer in those fiber-like cells is composed of sublayers with structural evidence for different cellulose microfibril orientation, and with specific polymer composition that includes (1 → 4)-β-galactans. Thus, the basic cellular characters of the cells that provide mechanical support in vascular plant taxa (elongated cell shape, location at the periphery of a primary organ, the thickened cell wall and its peculiar composition and structure) also exist in mosses.

Fern cell walls and the evolution of arabinogalactan proteins in streptophytes

Significant changes have occurred in plant cell wall composition during evolution and diversification of tracheophytes. As the sister lineage to seed plants, knowledge on the cell wall of ferns is key to track evolutionary changes across tracheophytes and to understand seed plant-specific evolutionary innovations. Fern cell wall composition is not fully understood, including limited knowledge of glycoproteins such as the fern arabinogalactan proteins (AGPs). Here, we characterize the AGPs from the leptosporangiate fern genera Azolla, Salvinia, and Ceratopteris. The carbohydrate moiety of seed plant AGPs consists of a galactan backbone including mainly 1,3- and 1,3,6-linked pyranosidic galactose, which is conserved across the investigated fern AGPs. Yet, unlike AGPs of angiosperms, those of ferns contained the unusual sugar 3-O-methylrhamnose. Besides terminal furanosidic arabinose, Ara (Araf), the main linkage type of Araf in the ferns was 1,2-linked Araf, whereas in seed plants 1,5-linked Araf is often dominating. Antibodies directed against carbohydrate epitopes of AGPs supported the structural differences between AGPs of ferns and seed plants. Comparison of AGP linkage types across the streptophyte lineage showed that angiosperms have rather conserved monosaccharide linkage types; by contrast bryophytes, ferns, and gymnosperms showed more variability. Phylogenetic analyses of glycosyltransferases involved in AGP biosynthesis and bioinformatic search for AGP protein backbones revealed a versatile genetic toolkit for AGP complexity in ferns. Our data reveal important differences across AGP diversity of which the functional significance is unknown. This diversity sheds light on the evolution of the hallmark feature of tracheophytes: their elaborate cell walls.

Deep origin and gradual evolution of transporting tissues: Perspectives from across the land plants

The evolution of transporting tissues was an important innovation in terrestrial plants that allowed them to adapt to almost all nonaquatic environments. These tissues consist of water-conducting cells and food-conducting cells and bridge plant-soil and plant-air interfaces over long distances. The largest group of land plants, representing about 95% of all known plant species, is associated with morphologically complex transporting tissue in plants with a range of additional traits. Therefore, this entire clade was named tracheophytes, or vascular plants. However, some nonvascular plants possess conductive tissues that closely resemble vascular tissue in their organization, structure, and function. Recent molecular studies also point to a highly conserved toolbox of molecular regulators for transporting tissues. Here, we reflect on the distinguishing features of conductive and vascular tissues and their evolutionary history. Rather than sudden emergence of complex, vascular tissues, plant transporting tissues likely evolved gradually, building on pre-existing developmental mechanisms and genetic components. Improved knowledge of the intimate structure and developmental regulation of transporting tissues across the entire taxonomic breadth of extant plant lineages, combined with more comprehensive documentation of the fossil record of transporting tissues, is required for a full understanding of the evolutionary trajectory of transporting tissues.

Modern mannan: a hemicellulose's journey

Hemicellulosic polysaccharides built of β-1,4-linked mannose units have been found throughout the plant kingdom and have numerous industrial applications. Here, I review recent advances in the biosynthesis and modification of plant β-mannans. These matrix polymers can associate with cellulose bundles to impact the mechanical properties of plant fibers or biocomposites. In certain algae, mannan microfibrils even replace cellulose as the dominant structural component of the cell wall. Conversely, patterned galactoglucomannan found in Arabidopsis thaliana seed mucilage significantly modulates cell wall architecture and abiotic stress tolerance despite its relatively low content. I also discuss the subcellular requirements for β-mannan biosynthesis, the increasing number of carbohydrate-active enzymes involved in this process, and the players that continue to be puzzling. I discuss how cellulose synthase-like enzymes elongate (gluco)mannans in orthogonal hosts and highlight the discoveries of plant enzymes that add specific galactosyl or acetyl decorations. Hydrolytic enzymes such as endo-β-1,4-mannanases have recently been involved in a wide range of biological contexts including seed germination, wood formation, heavy metal tolerance, and defense responses. Synthetic biology tools now provide faster tracks to modulate the increasingly-relevant mannan structures for improved plant traits and bioproducts.

Cell Wall Layer Induced in Xylem Fibers of Flax Upon Gravistimulation Is Similar to Constitutively Formed Cell Walls of Bast Fibers

In the fibers of many plant species after the formation of secondary cell walls, cellulose-enriched cell wall layers (often named G-layers or tertiary cell walls) are deposited which are important in many physiological situations. Flax (Linum usitatissimum L.) phloem fibers constitutively develop tertiary cell walls during normal plant growth. During the gravitropic response after plant inclination, the deposition of a cellulose-enriched cell wall layer is induced in xylem fibers on one side of the stem, providing a system similar to that of tension wood in angiosperm trees. Atomic force microscopy (AFM), immunochemistry, and transcriptomic analyses demonstrated that the G-layer induced in flax xylem fibers was similar to the constitutively formed tertiary cell wall of bast (phloem) fibers but different from the secondary cell wall. The tertiary cell walls, independent of tissue of origin and inducibility, were twice as stiff as the secondary cell walls. In the gravitropic response, the tertiary cell wall deposition rate in xylem was higher than that of the secondary cell wall. Rhamnogalacturonan I (RG-I) with galactan side chains was a prominent component in cellulose-rich layers of both phloem and xylem flax fibers. Transcriptomic events underlying G-layer deposition in phloem and xylem fibers had much in common. At the induction of tertiary cell wall deposition, several genes for rhamnosyltransferases of the GT106 family were activated in xylem samples. The same genes were expressed in the isolated phloem fibers depositing the tertiary cell wall. The comparison of transcriptomes in fibers with both inducible and constitutive tertiary cell wall deposition and xylem tissues that formed the secondary cell walls is an effective system that revealed important molecular players involved in the formation of cellulose-enriched cell walls.