The plant cell wall.

Research on the many aspects of the plant cell wall has experienced rejuvenation during the past few years. This is perhaps mainly due to the commercial interest in the chemical components of the cell wall that have potential for industrial use: Cellulose for fibers and together with hemicelluloses for bioethanol, lignin for plastics or biofuel, pectins as gel agents, let alone woody cell wall material for construction or pulp production and for intelligent materials for electronics – the new uses are mindboggling! As cell wall materials are the hype of the time, pressure on the basic knowledge of the cell wall materials is increasing. This emphasizes basic research results that can be used, for example, in bioenergy applications, in the production of synthetic and semi-synthetic polymers, and in new carbon fibers or even as polymers with altogether new and intelligent properties. Without the fundamental information on the structure and functions of the cell wall components, it is not possible to invent ingenious new uses for this old material. Wood cell walls contain lignin, a phenolic polymer, up to approximately 32% of cell wall dry material. Lignin is, after cellulose, the second most abundant biopolymer on earth, and it plays a crucial role for structural integrity of cell walls and for stiffness and strength of the plant body. The biosynthetic route for monolignols is well known but their transport into the cell wall and polymerization in vivo are not that clear, and hence, the research efforts on the laccases and peroxidases functioning in the formation of lignin polymer are presented in this special issue in the articles by Koutaniemi et al. (2015) on laccases in lignin-forming tissues of Norway spruce and by Shigeto et al. (2015) on the effect of peroxidase double mutants on lignin contents in Arabidopsis. Plant cell walls have also admirable plasticity. This is made evident in the two articles in this special issue by M elida et al. (2015) on ‘Ectopic lignification in primary cellulose-deficient cell walls of maize cell suspension cultures’, and by de Castro et al. (2015) in their article titled ‘The biosynthesis and wall-binding of hemicelluloses in cellulose-deficient maize cells: An example of metabolic plasticity’. Wood formation is an important process with both biological and economical aspects. Trees and wood are of considerable importance in many countries where a large part of the gross national income comes from the forest sector. To be able to produce wood products with targeted characteristics, we needmore information on the basic structures in the xylem cell walls and on the biosynthesis of their components. This information is vital for directed changes in these structures through conventional breeding methods and through genetic modification with potential high economic impact. The article by Andersson et al. (2015) concentrates on changes in the cell wall structures in compression wood, i.e. wood that forms in leaning stems of gymnosperms, and alters the properties of sawn timber – not always for the best! Hence, it is important to know what has happened in the microstructure of wood and how this could be remedied or – on the other hand – made better use of. Furthermore, the paper of Andersson et al. (2015) is interesting in an evolutionary point of view, as the studied species, Gingko biloba, can be described as a living fossil. The biosynthesis of the many cell wall polymers is fascinating but there is still room for more knowledge, especially on the regulation of the synthetic processes. This research area is reviewed by Liu et al. (2015) in their article titled ‘The connection of cytoskeletal network with plasma membrane and the cell wall’. Cell wall is also known to be modified during and after its formation. Glass et al. (2015) report in their article that endoglucanases can have an impact on cellulose crystallization; the changes in this can lead into considerable changes even in the morphology of Arabidopsis plants. High-throughput techniques, such as that described by Frankov a and Fry (2015) for transglycanases, will reveal novel enzyme activities that contribute to cell wall assembly and/or restructuring during plant development. As the sequencing ofwhole plant genomes is gettingmore and more common, the use of genetic and bioinformatics analyses have given new ideas and reveal new possibilities in the use of plant material and breeding of new crop varieties. This kind of information is made use of in the article by Ermawar et al. (2015) on the biosynthetic gene families in sorghum for (1,3;1,4)-b-glucan, important in human health, animal feed and biofuel applications. As cell wall related issues are very important in fundamental biological understanding of plant development, new articles on the plant cell wall are emerging daily. The articles presented here in this issue give a glimpse into some of the vital compounds in the making of a functional and versatile plant cell wall.