Plant cell wall evolution: exploring changes necessary for life on land with focus on streptophyte algae and bryophytes

• Applicant: Professorin Dr. Birgit Classen
• Subject Area: Plant Cell and Developmental Biology ; Evolution and Systematics of Plants and Fungi
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 440046237

The evolutionary pressures that occured during terrestrialisation most propably brought about cell wall modifications to cope with the new conditions. In a first step, sequential extraction of cell walls of at least all MAdLand model organisms followed by monosaccharide analyses of these fractions will offer an overview on whole cell wall composition and the elucidation of differences over a broader phylogenetic range. Our further work will focus on streptophyte algae and bryophytes, as their ancestors were mainly involved in transition from water to land. We will isolate and characterize arabinogalactan-proteins (AGPs) and pectic polysaccharides, which are the two most complex macromolecule groups present in the plant cell wall. In the first phase, we detected AGPs in bryophytes and ferns, which contained unusual features, especially terminally linked 3-O-methylrhamnose, a monosaccharide not present in angiosperms. Within the streptophyte algae, no AGPs were detected in Chara sp. (Charophyceae), whereas cell walls of Spirogyra pratensis (Zygnematophyceae), contained AGP-like molecules with nearly complete replacement of arabinose by rhamnose. To find more evidence, we will search for AGPs in the new streptophyte algae models Zygnema circumcarinatum and Mesotaenium endlicherianum and additionally Mougeotiopsis calospora and Coleochaete scutata as well as in the liverwort Riccia fluitans and suspension cultures of the hornwort Anthoceros agrestis. Presence and nature of pectic polysaccharides in streptophyte algae and bryophytes is not finally settled. It is generally accepted that homogalacturonan has deep roots in the green lineage, but knowledge on rhamnogalacturonan-I (RG-I) and -II (RG-II) in bryophytes and streptophyte algae is mainly based on reactivity with antibodies directed against glycan motifs present in pectins. Therefore we will isolate and structurally characterize RG-I and RG-II from Chara sp., Spirogyra pratensis, Riccia fluitans and Anthoceros agrestis. Antibodies directed against AGP- and pectic glycan epitopes will be used in ELISA to complement analytical structure elucidation of the molecules. Furthermore, these antibodies will be used for immunolocalization in plant tissue which offers first hints on possible functions of these macromolecules in these organisms. Additionally, bioinformatic searches for genes of AGP protein backbones and glycosyltransferases involved in AGP and pectin biosynthesis will be performed. Thus, our work on biochemical and bioinformatic characterization of cell walls will identify the cell wall key features necessary for the conquest of land and broaden the knowledge on molecular adaptations in early land plant evolution. This will contribute to elucidate the nature of the most common ancestor of all extant land plants and of streptophyte algae and land plants.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2237: MAdLand — Molecular Adaptation to Land: plant evolution to change