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 analyzed for the presence of arabinogalactan-proteins (AGPs), as the emergence of these signaling 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 from 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. A bioinformatic search in the genomes of 14 bryophyte species revealed that most hornworts lack sequences encoding GPI-anchored classical AGPs. Generally, hornworts contained fewer sequences for AGP protein backbones compared with the liverwort Marchantia polymorpha and the moss P. 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 the tonoplast, suggesting new functions of AGPs in bryophytes.