New insights into bryophyte arabinogalactan-proteins from a hornwort and a moss model organism
Two bryophyte models, the hornwort Anthoceros agrestis (Anthoceros) and the moss Physcomitrium patens (Physcomitrium), were analysed for presence of arabinogalactan-proteins (AGPs), as emergence of these signalling glycoproteins in evolution is still under debate. AGPs of both species had a galactan core structure similar to that of other bryophyte and fern AGPs, but different to angiosperm AGPs, as 1,6-linked pyranosidic galactose was almost absent. In the Physcomitrium AGP, furanosidic arabinose (Araf) linkages were mainly terminal (10 %) or 5-linked (13 %), while in Anthoceros, terminal Araf dominated (26 %) and was accompanied by very low amounts of 1,3-Araf and pyranosidic terminal Ara. Unusual 3-O-methylated pyranosidic rhamnose, which has never been detected in cell walls of angiosperms, occurred in both bryophyte AGPs (5 % in Anthoceros, 10 % in Physcomitrium AGP), This was comparable to AGPs of other spore-producing land plants. Bioinformatic search in genomes of 14 bryophyte species revealed that most hornworts lack sequences encoding GPI-anchored classical AGPs. Generally, hornworts contained less sequences for AGP protein backbones compared to the liverwort Marchantia polymorpha and the moss Physcomitrium patens. All of them comprise sequences for chimeric AGPs, and among those surprisingly xylogen-like AGPs. Homologous sequences encoding glycosyltransferases and other enzymes involved in the synthesis and decoration of the AGP galactan framework were present in all bryophyte genomes. Immunocytochemistry of Anthoceros tissue detected AGPs at the plasma membrane/cell wall interface but also at vacuolar and vesicle membranes, suggesting new functions of AGPs in bryophytes. SIGNIFICANCE STATEMENTExtant bryophytes are key to infer evolution of the most recent common ancestor of all land plants. As cell walls were important for adaptation to life on land, we analysed arabinogalactan-proteins from the hornwort Anthoceros agrestis and the moss Physcomitrium patens and detected terminal 3-O-methylrhamnose residues, which also occur in fern AGPs but not in angiosperms. Bioinformatic search for AGP protein backbones and glycosyltransferases in bryophyte genomes further strengthens understanding of AGP evolution during terrestrialization.