Abstract
The extensin (EXT) network is elaborated by the covalent intermolecular crosslinking of EXT glycoprotein monomers, and its proper assembly is important for numerous aspects of basic wall architecture and cellular defense. In this review, we discuss new advances in the secretion of EXT monomers and the molecular drivers of EXT network self-assembly. Many of the functions of EXTs are conferred through covalent crosslinking into the wall, so we also discuss the different types of known intermolecular crosslinks, the enzymes that are involved, as well as the potential for additional crosslinks that are yet to be identified. EXTs also function in wall architecture independent of crosslinking status, and therefore, we explore the role of non-crosslinking EXTs. As EXT crosslinking is upregulated in response to wounding and pathogen infection, we discuss a potential regulatory mechanism to control covalent crosslinking and its relationship to the subcellular localization of the crosslinking enzymes.
Highlights
Plant cell walls form the basis of plant stature and morphology
The association of glucan chains by hydrogen bonding (Atalla and Vanderhart, 1988; Delmer and Amor, 1995; Haigler et al, 2016), the crosslinking of hemicellulose polymers by xyloglucan endotransglucosylase/hydrolases (Rose et al, 2002), Ca+2 crosslinking of homogalacturonans, and extensin (EXT) crosslinking by Extensin Network Assembly and Crosslinking covalent tyrosine linkages (Held et al, 2004) may all be considered as examples of homopolymeric crosslinking
Structural glycoproteins of the plant cell wall are both structurally and functionally diverse and most belong to the hydroxyproline-rich glycoproteins (HRGPs) superfamily, which consists of a broad continuum of glycoproteins
Summary
Plant cell walls form the basis of plant stature and morphology. Cell walls are complex supramolecular structures composed mostly of carbohydrate, lignin, and glycoprotein polymers. Plant cells produce two types of cell walls: a thinner primary wall that is deposited as plant cells grow and elongate, and a thicker secondary wall that accumulates after cell growth ceases As these walls are produced, individual cell wall polymers undergo molecular assembly to form three-dimensional composite networks with specific physiochemical properties tailored to individual cell types. Having multiply interconnected polymeric networks (hemicellulose, pectin, and structural glycoproteins) may facilitate load sharing in response to turgor-driven wall extension by increasing stress distribution (Park and Cosgrove, 2012). Identification of these crosslinks has been difficult, in part due to their relatively low abundance within the wall, making them proverbial “needles in a haystack.”. After providing a brief discussion of EXT structure and composition, we take a look at recent advances in cell wall glycoprotein analyses and the enzymes responsible for the covalent crosslinking of EXTs
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