title: Evidence for land plant cell wall biosynthetic mechanisms in charophyte green algae.
abstract: OBJECTIVE
  The charophyte green algae (CGA) are thought to be the closest living relatives to the land plants, and ancestral CGA were unique in giving rise to the land plant lineage. The cell wall has been suggested to be a defining structure that enabled the green algal ancestor to colonize land. These cell walls provide support and protection, are a source of signalling molecules, and provide developmental cues for cell differentiation and elongation. The cell wall of land plants is a highly complex fibre composite, characterized by cellulose cross-linked by non-cellulosic polysaccharides, such as xyloglucan, embedded in a matrix of pectic polysaccharides. How the land plant cell wall evolved is currently unknown: early-divergent chlorophyte and prasinophyte algae genomes contain a low number of glycosyl transferases (GTs), while land plants contain hundreds. The number of GTs in CGA is currently unknown, as no genomes are available, so this study sought to give insight into the evolution of the biosynthetic machinery of CGA through an analysis of available transcriptomes.
  
  
  METHODS
  Available CGA transcriptomes were mined for cell wall biosynthesis GTs and compared with GTs characterized in land plants. In addition, gene cloning was employed in two cases to answer important evolutionary questions.
  
  
  RESULTS
  Genetic evidence was obtained indicating that many of the most important core cell wall polysaccharides have their evolutionary origins in the CGA, including cellulose, mannan, xyloglucan, xylan and pectin, as well as arabino-galactan protein. Moreover, two putative cellulose synthase-like D family genes (CSLDs) from the CGA species Coleochaete orbicularis and a fragment of a putative CSLA/K-like sequence from a CGA Spirogyra species were cloned, providing the first evidence that all the cellulose synthase/-like genes present in early-divergent land plants were already present in CGA.
  
  
  CONCLUSIONS
  The results provide new insights into the evolution of cell walls and support the notion that the CGA were pre-adapted to life on land by virtue of the their cell wall biosynthetic capacity. These findings are highly significant for understanding plant cell wall evolution as they imply that some features of land plant cell walls evolved prior to the transition to land, rather than having evolved as a result of selection pressures inherent in this transition.
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark mdmi@plen.ku.dk.
authors.forename: Maria D
authors.initials: MD
authors.lastname: Mikkelsen
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark.
authors.forename: Jesper
authors.initials: J
authors.lastname: Harholt
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Ulvskov
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark.
authors.forename: Ida E
authors.initials: IE
authors.lastname: Johansen
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark.
authors.forename: Jonatan U
authors.initials: JU
authors.lastname: Fangel
authors.affiliation: ARC Centre of Excellence in Plant Cell Walls, School of Botany, University of Melbourne, Victoria 3010, Australia.
authors.forename: Monika S
authors.initials: MS
authors.lastname: Doblin
authors.affiliation: ARC Centre of Excellence in Plant Cell Walls, School of Botany, University of Melbourne, Victoria 3010, Australia.
authors.forename: Antony
authors.initials: A
authors.lastname: Bacic
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark.
authors.forename: William G T
authors.initials: WG
authors.lastname: Willats
mesh_terms: D001483:Base Sequence; D005075:Biological Evolution; D002473:Cell Wall / Q000737:chemistry / Q000378:metabolism*; D057946:Charophyceae / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019669:Embryophyta / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005964:Glucosyltransferases / Q000235:genetics / Q000378:metabolism; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D005810:Multigene Family; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D011134:Polysaccharides / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D058131:Spirogyra / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D059467:Transcriptome
chemical_list: D006023:Glycoproteins; D010940:Plant Proteins; D011134:Polysaccharides; D016695:Glycosyltransferases; D005964:Glucosyltransferases; C478648:cellulose synthase
keywords: CAZy; CSLA; CSLD; CSLK; Coleochaete orbicularis; Plant cell wall; Spirogyra; cellulose synthase-like; charophyte green algae; evolution; glycosyl transferases; polysaccharides; transcriptomes