Plant Xyloglucan Xyloglucosyl Transferases and the Cell Wall Structure: Subtle but Significant.

Barbora Stratilová,Eva Stratilová,Stanislav Kozmon,Maria Hrmova

doi:10.3390/molecules25235619

Abstract

Plant xyloglucan xyloglucosyl transferases or xyloglucan endo-transglycosylases (XET; EC 2.4.1.207) catalogued in the glycoside hydrolase family 16 constitute cell wall-modifying enzymes that play a fundamental role in the cell wall expansion and re-modelling. Over the past thirty years, it has been established that XET enzymes catalyse homo-transglycosylation reactions with xyloglucan (XG)-derived substrates and hetero-transglycosylation reactions with neutral and charged donor and acceptor substrates other than XG-derived. This broad specificity in XET isoforms is credited to a high degree of structural and catalytic plasticity that has evolved ubiquitously in algal, moss, fern, basic Angiosperm, monocot, and eudicot enzymes. These XET isoforms constitute gene families that are differentially expressed in tissues in time- and space-dependent manners during plant growth and development, and in response to biotic and abiotic stresses. Here, we discuss the current state of knowledge of broad specific plant XET enzymes and how their inherently carbohydrate-based transglycosylation reactions tightly link with structural diversity that underlies the complexity of plant cell walls and their mechanics. Based on this knowledge, we conclude that multi- or poly-specific XET enzymes are widespread in plants to allow for modifications of the cell wall structure in muro, a feature that implements the multifaceted roles in plant cells.

Highlights

Implications of transglycosylation reactions catalysed by xyloglucan xyloglucosyl transferases in the cell wall structure, function and dynamics
All land plants classified in Embryophytes evolved from Charophytes green algae (Figure 1A) contain cell walls (CWs) assumed to be one of the most decisive factors that allowed for terrestrialisation [7,8,9,10]
This was reflected by the phylogenetic analyses, which showed that the distribution of xyloglucan transglycosylase/hydrolase (XTH) gene products was segregated into three groups, with XEHs belonging to the XTH III clade, while xyloglucan endo-transglycosylases (XET) were placed in both XTH I and XTH II clades (Figure 2C, Supplementary Data Set S1) [121,123,138,144]

Summary

Plant Cell Walls and Structure and Key Components

The presence of polysaccharide-rich cell walls (CWs) is a characteristic feature of plants and fungi. Minor differences in structures lead to stronger binding of a donor substrate combined with larger loop flexibility at the acceptor binding site, and the higher conformational flexibility of specific residues underlined the hydrolytic preference [140,143] (Figure 2B) This was reflected by the phylogenetic analyses, which showed that the distribution of XTH gene products was segregated into three groups, with XEHs belonging to the XTH III clade, while XETs were placed in both XTH I and XTH II clades (Figure 2C, Supplementary Data Set S1) [121,123,138,144]. Other XET activity assays include viscometry and colorimetry [156]

Substrate Specificity of Plant Xyloglucan Xyloglucosyl Transferases

Findings

Assay Method c Reference