Homogalacturonan Accumulation in Cell Walls of the Green Alga Zygnema sp. (Charophyta) Increases Desiccation Resistance.

Klaus Herburger,Anzhou Xin,Andreas Holzinger

doi:10.3389/fpls.2019.00540

Klaus Herburger, Anzhou Xin + Show 1 more

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https://doi.org/10.3389/fpls.2019.00540

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Abstract

Land plants inherited several traits from their green algal ancestors (Zygnematophyceae), including a polysaccharide-rich cell wall, which is a prerequisite for terrestrial survival. A major component of both land plant and Zygnematophyceaen cell walls is the pectin homogalacturonan (HG), and its high water holding capacity may have helped algae to colonize terrestrial habitats, characterized by water scarcity. To test this, HG was removed from the cell walls of Zygnema filaments by pectate lyase (PL), and their effective quantum yield of photosystem II (YII) as a proxy for photosynthetic performance was measured in response to desiccation stress by pulse amplitude modulation (PAM). Old filaments were found to contain more HG and are more resistant against desiccation stress but relatively lose more desiccation resistance after HG removal than young filaments. After rehydration, the photosynthetic performance recovered less efficiently in filaments with a HG content reduced by PL, independently of filament age. Immunolabeling showed that partial or un-methylesterified HG occurs throughout the longitudinal cell walls of both young and old filaments, while no labeling signal occurred when filaments were treated with PL prior labeling. This confirmed that most HG can be removed from the cell walls by PL. The initial labeling pattern was restored after ~3 days. A different form of methylesterified HG was restricted to cell poles and cross cell walls. In conclusion, it was shown that the accumulation of HG in Zygnema filaments increases their resistance against desiccation stress. This trait might have played an important role during the colonization of land by Zygnematophyceae, which founded the evolution of all land plants.

Highlights

The earliest land plant fossils date to the Mid-Ordovician (Steemans et al, 2009), and it was concluded that the algal ancestors of land plants started colonizing terrestrial habits at least ~450 Myr ago (Kenrick et al, 2012)
pectate lyase (PL) treatment of old filaments resulted in a patchy labeling pattern in longitudinal but not cross cell walls (Figure 2E)
HG is the predominant polysaccharide in the pectin fraction of late diverged charophyte green algae (CGA) (Sørensen et al, 2011), and correspondingly, we found GalA2─ΔUA oligosaccharides released by PL from Zygnema alcohol insoluble residue (AIR)

Summary

Introduction

The earliest land plant fossils date to the Mid-Ordovician (Steemans et al, 2009), and it was concluded that the algal ancestors of land plants started colonizing terrestrial habits at least ~450 Myr ago (Kenrick et al, 2012). Genomic, and transcriptomic data (Wodniok et al, 2011; Timme et al, 2012; Mikkelsen et al, 2014; Rippin et al, 2017; De Vries et al, 2018; Jensen et al, 2018), it was concluded that the most important core cell wall components of land plants—including cellulose, homogalacturonan (HG), xyloglucan, mannans, and xylans—were already present in some streptophyte green algae, and that a polysaccharide rich cell wall can be considered a prerequisite for terrestrial survival (Harholt et al, 2016) In their natural habitats, vegetative cells of Zygnema filaments form specialized resistant cells termed “pre-akinets” (McLean and Pessoney, 1971; Pichrtová et al, 2014a), which differ from young cells in their accumulation of starch and lipid bodies rich in C18 fatty acids (Pichrtová et al, 2016b), a reduction of chloroplast size and cell diameter and an increased resistance against osmotic and desiccation stress (Kaplan et al, 2013; Pichrtová et al, 2014a; Herburger et al, 2015). HG, which is still an important component of land plant cell walls, is a linear homopolymer consisting of α-1,4-linked d-galacturonic acid (GalA) and synthesized in a highly methylesterified form (methylation at C6 carboxyl of GalA); upon secretion into the cell wall, the ester-bonds are enzymatically broken by apoplastic pectin methylesterase (Wolf et al, 2009)

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