New insights into bryophyte arabinogalactan-proteins from a hornwort and a moss model organism

Two bryophyte models, the hornwortAnthoceros agrestis(Anthoceros) and the mossPhyscomitrium 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 thePhyscomitriumAGP, furanosidic arabinose (Araf) linkages were mainly terminal (10 %) or 5-linked (13 %), while inAnthoceros, terminal Arafdominated (26 %) and was accompanied by very low amounts of 1,3-Arafand 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 % inAnthoceros, 10 % inPhyscomitriumAGP), 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 liverwortMarchantia polymorphaand the mossPhyscomitrium 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 ofAnthocerostissue detected AGPs at the plasma membrane/cell wall interface but also at vacuolar and vesicle membranes, suggesting new functions of AGPs in bryophytes.