# Arabinogalactan-proteins of the eusporangiate fern Psilotum nudum show atypical structural features compared to other ferns

## TL;DR

This study compares the structure of arabinogalactan-proteins in the fern Psilotum nudum to other plants, revealing unique features that may reflect evolutionary adaptations in cell walls.

## Contribution

The paper presents the first detailed structural and immunolocalization analysis of AGPs in the eusporangiate fern Psilotum nudum.

## Key findings

- P. nudum AGPs have lower 1,6-linkage in the galactan core compared to A. thaliana AGPs.
- P. nudum AGPs show a distinct pattern of arabinose residues, including terminal pyranosidic rhamnose not found in A. thaliana.
- AGPs in P. nudum are localized in various tissues, excluding mature protoxylem, and are found along the plasma membrane and tonoplast.

## Abstract

During the colonization of land, abiotic and biotic challenges induced strong adaptations in plants, including changes of their cell walls. Up to now, cell walls of spore-bearing plants have been only sparsely investigated. As ferns are the sister group to seed plants, studying their cell walls is key to understanding the evolutionary development of the cell wall in the tracheophyte lineage. Beside polysaccharides, especially arabinogalactan-proteins (AGPs) are important cell wall components involved in structural but also signalling functions. We chose a member of the eusporangiate ferns, Psilotum nudum, to characterize and immunolocalize AGPs. Biochemical analyses identified special structural features of P. nudum AGPs in comparison to A. thaliana AGPs, including lower amounts of 1,6-linkage in the galactan core. With regard to the arabinose (Ara) residues, A. thaliana AGP was dominated by terminal furanosidic Ara, followed by 1,5-Ara, whereas in P. nudum AGPs, 1,3-Ara, terminal furanosidic Ara and terminal pyranosidic Ara occurred in comparable amounts. Terminal pyranosidic rhamnose was only found in the fern AGP. Immunofluorescence analysis using JIM13 antibody detected AGPs in all tissues of the P. nudum stem with the exception of the mature protoxylem. Localization was observed along the plasma membrane as well as tonoplast.