Tear down that wall - cell wall remodeling in charophyte algae.

Abstract

Why do plants have cell walls? This could be a typical exam question. One of the correct answers is that walls allow cells to develop turgor pressure, which keeps the plant from wilting. And if a plant experiences drought and turgor pressure is lost, the cell walls help to maintain structural integrity. Although cell walls are essential for a plant, they also pose challenges. The cell wall consists of two layers: the primary cell wall that surrounds the growing cells and the secondary cell wall that is developed after the cell stops expanding. The secondary cell wall is more rigid, while the primary cell wall is thinner and more stretchable, but still tough. The plant cell wall contains many enzymes that constantly remodel the polysaccharides that make up the wall. Transglycosylases are one class of such cell wall-remodeling enzymes. Transglycosylases participate in ‘cutting and pasting’ of sugar residues: they cleave off part of the backbone of a polysaccharide (donor) and graft it onto another (acceptor) (Franková and Fry, 2013). These molecular rearrangements turn the cell wall in a flexible compartment that allows the cell to grow. The cell wall and its remodeling enzymes have been well studied in land plants. It is thought that the cell wall underwent substantial changes during colonization of the land plants, roughly 470–500 million years ago. This is because a terrestrial lifestyle has different demands than an aquatic one. Underwater, plants are supported by their buoyancy, whereas on land, they have to stand up against gravity and resist the wind. To understand how the cell wall changed during terrestrialization, some researchers study charophyte algae, the closest extant relatives of the algal lineage from which land plants evolved. The cell walls of charophytes exhibit major chemical differences from their land plant counterparts. For example, xyloglucan is present in the cell walls of all land plants, whereas most charophytes contain only low levels or completely lack this polysaccharide (Mikkelsen et al., 2021; Popper and Fry, 2003). Little was known about the transglycosylase enzymes that remodel the cell walls of charophytes. In this issue, Franková and Fry (2021) investigated transglycosylases in charophytes to get insight into the enzymatic cell wall machinery that allowed plants to colonize the land. To do this, they used a collection of algal cultures with members of the Coleochaetales, Klebsormidiales, Zygnematales, Chlorokybales, and Charales. From each algal culture they extracted cell wall enzymes, and then they applied the enzyme extracts to a mixture of polymers that can serve as donor substrates, mixed with radioactively labeled polymers that can serve as acceptor substrates. When donor segments get grafted to the acceptor substrates, new radioactive polysaccharides are formed that often have a different molecular weight than the supplied donor substrate, making the product easily distinguishable (Figure). The authors found that transglycosylase activity in charophyte cell walls is different from that in land plants; most notably charophytes possess a particularly high level of trans-β-1,4-mannanase activity, a transglycosylase that acts on mannan substrates. Charophytes contain abundant β-mannans in their cell walls, and this finding suggests that charophytes might prioritize mannan remodeling. By contrast, land plants mainly remodel the hemicellulose xyloglucan, which is carried out by transglucanases. However, the most striking finding was that the charophyte enzyme extracts also possessed trans-β-glucanase activity. Except for a few species, charophytes lack xyloglucan in their cell walls. Therefore, the authors decided to test if they could also find glucanase activity in situ. They fed fluorescently labeled oligoglucans and found that they were incorporated into the cell walls of most of the tested algal cultures (Figure), indicating the presence of xyloglucan transglycanase activity. Fry and Franková are not sure why charophyte algae possess glucanase activity; possibly charophytes might have an uncharacterized β-glucan-based polysaccharide that serves as a substrate. Alternatively, the observed enzymatic activity might not be specialized for glucan-based substrates. Whatever the answer, the finding of glucanase activity in charophyte algae suggests that enzymes with glucanase activity were present in the ancient charophytes from which land plants evolved. Hence, although the polysaccharide composition of the cell wall changed fundamentally during the transition to land, the cell wall remodeling machinery might have remained conserved. In the future, Fry and his group want to identify mannan and xylan substrates on which transglycosylases act in the cell walls of charophytes. Moreover, they would like to know if these substrates were highly conserved in extinct charophytes. Fry notes that although the main sugar composition of algal hemicelluloses has been studied (O’Rourke et al., 2015), the linkages and sugar residue sequences in the polysaccharide backbones and/or their side chains are still unknown.