Hemicellulose-remodelling transglycanase activities from charophytes: towards the evolution of the land-plant cell wall.

Lenka Franková,Stephen C Fry

doi:10.1111/tpj.15500

Abstract

Transglycanases remodel cell-wall polymers, having a critical impact on many physiological processes. Unlike xyloglucan endotransglucosylase (XET) activity, widely studied in land plants, very little is known about charophyte wall-modifying enzymes - information that would promote our understanding of the 'primordial' wall, revealing how the wall matrix is remodelled in the closest living algal relatives of land plants, and what changed during terrestrialisation. We conducted various in-vitro assays for wall-remodelling transglycosylases, monitoring either (a) polysaccharide-to-[3 H]oligosaccharide transglycosylation or (b) non-radioactive oligosaccharide-to-oligosaccharide transglycosylation. We screened a wide collection of enzyme extracts from charophytes (and early-diverging land plants for comparison) and discovered several homo- and hetero-transglycanase activities. In contrast to most land plants, charophytes possess high trans-β-1,4-mannanase activity, suggesting that land plants' algal ancestors prioritised mannan remodelling. Trans-β-1,4-xylanase activity was also found, most abundantly in Chara, Nitella and Klebsormidium. Exo-acting transglycosidase activities (trans-β-1,4-xylosidase and trans-β-1,4-mannosidase) were also detected. In addition, charophytes exhibited homo- and hetero-trans-β-glucanase activities (XET, mixed-linkage glucan [MLG]:xyloglucan endotransglucosylase and cellulose:xyloglucan endotransglucosylase) despite the paucity or lack of land-plant-like xyloglucan and MLG as potential donor substrates in their cell walls. However, trans-α-xylosidase activity (which remodels xyloglucan in angiosperms) was absent in charophytes and early-diverging land plants. Transglycanase action was also found insitu, acting on endogenous algal polysaccharides as donor substrates and fluorescent xyloglucan oligosaccharides as acceptor substrates. We conclude that trans-β-mannanase and trans-β-xylanase activities are present and thus may play key roles in charophyte walls (most of which possess little or no xyloglucan and MLG, but often contain abundant β-mannans and β-xylans), comparable to the roles of XET in xyloglucan-rich land plants.

Highlights

The cells of all land plants, from bryophytes to the tallest angiosperm trees, are surrounded by cell walls that define the shape and size of plant, act as barriers to pathogens, resist turgor pressure and constrain the rate and direction of cell expansion (Albersheim et al, 2010)
To discover how hemicelluloses may have been remodelled in ancient (e.g. Cambrian) charophytes compared with early-diverging land plants, we focused on wallremodelling enzymes that post-synthetically modify modern algal and land-plant walls
Based on the discovery of numerous non-mechanistic transglycosylases, we provide evidence that charophytic algae are equipped with enzymic toolkits which covalently modify β-mannans, β-xylans and β-glucans, potentially enabling the emergence of well-known land-plant wall processes

Summary

Introduction

The cells of all land plants (embryophytes), from bryophytes (mosses, liverworts and hornworts) to the tallest angiosperm trees, are surrounded by cell walls that define the shape and size of plant, act as barriers to pathogens, resist turgor pressure and constrain the rate and direction of cell expansion (Albersheim et al, 2010). A pivotal event in plant evolution was the emergence of a charophytic green alga from a freshwater habitat and adaptation to terrestrial habitats with harsh atmospheric conditions. Charophytes plus land plants constitute a well-supported clade in plant evolution: the Streptophyta (Leliaert et al, 2012; Morris et al, 2018). It is presumed that dramatic evolutionary changes befell the cell wall during the never-to-be-repeated invasion of the land by an alga roughly 470–500 million years ago (Gensel et al, 1990; Leliaert et al, 2012; Morris et al, 2018). Since the first land plants and their immediate algal ancestors are extinct, to explore the transitions in cell-wall biochemistry which took place during colonisation of the land we necessarily focus on modern streptophytic green algae (charophytes), the closest extant relatives of land plants. To understand how land-plant cell walls came into existence, we are comparing wall structure and remodelling mechanisms in land plants (relatively well studied) with those in their charophytic relatives (relatively unexplored)

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