Organization of Xylan Production in the Golgi During Secondary Cell Wall Biosynthesis

Abstract

Secondary cell wall (SCW) production during xylem development requires massive up-regulation of hemicellulose (e.g. glucuronoxylan) biosynthesis in the Golgi. Although mutant studies have revealed much of the xylan biosynthetic machinery, the precise arrangement of these proteins and their products in the Golgi apparatus is largely unknown. We used a fluorescently tagged xylan backbone biosynthetic protein (IRREGULAR XYLEM9; IRX9) as a marker of xylan production in the Golgi of developing protoxylem tracheary elements in Arabidopsis (Arabidopsis thaliana). Both live-cell confocal and transmission electron microscopy (TEM) revealed SCW deposition is accompanied by a significant proliferation of Golgi stacks. Furthermore, although Golgi stacks were randomly distributed, the organization of the cytoplasm ensured their close proximity to developing SCWs. Quantitative immuno-TEM revealed IRX9 is present in a specific subdomain of the Golgi stack and was most abundant in the ring of the inner margins of medial cisternae where fenestrations are abundant. Conversely, the xylan product accumulated in swollen trans cisternal margins and the Trans-Golgi network (TGN). The irx9 mutant lacked this expansion for both the cisternal margins and the TGN, whereas Golgi stack proliferation was unaffected. Golgi in irx9 also displayed dramatic changes in their structure, with increases in cisternal fenestration and tubulation. Our data support a new model where xylan biosynthesis and packaging into secretory vesicles are localized in distinct structural and functional domains of the Golgi. Rather than polysaccharide biosynthesis occurring in the center of the cisternae, IRX9 and the xylan product are arranged in successive concentric rings in Golgi cisternae. 1 This work was supported by the Gouvernement of Canada j Natural Sciences and Engineering Research Council of Canada (Conseil de recherches en sciences naturelles et en génie du Canada) (discovery grants to S.D.M. and A.L.S.); Canada postgraduate scholarships (to M.J.M.); and the Faculty of Graduate Studies, University of British Columbia (four-year fellowship to M.J.M.

The mysterious life of the plant trans‐Golgi network: advances and tools to understand it better

“…A recent work on the biosynthesis and packaging of xylan, a major hemicellulose component of the plant cell wall, has described how various xylan forms move from one Golgi cisterna to the next one by following a presumed cisternal maturation process while xylan biosynthetic enzymes, like IRX9 for instance, are retrieved back from the TGN/EE to the Golgi where they perform their activity. Indeed, a subcellular localization of IRX9‐GFP to the Golgi and TGN/EE (Meents et al ., 2019) is in support of the cisternal maturation model that hypothesizes a maturation of the TGN/EE from the membrane of the trans‐cisternae of the Golgi (Glick & Luini, 2011).…”

Plant Golgi ultrastructure

“…The basic unit of the Golgi apparatus is the cisterna, a flattened disc‐like structure, of which there are three different types: the cis ‐cisternae facing the ER and being the point of entry into the Golgi apparatus for secretory cargo, the median‐cisternae, the trans ‐cisternae and, as a product of the trans ‐cisternae, the trans ‐Golgi network (TGN), with a tubular‐fenestrated morphology. Reflecting their different functions in the processing of oligosaccharide side chains of secretory/vacuolar glycoproteins (for an excellent review see (Strasser, 2016)), and in the synthesis of cell wall polysaccharides (Zhang & Staehelin, 1992; Meents et al ., 2019), the cisternae are structurally and biochemically different. In most eukaryotes with the exception of baker's yeast ( Saccharomyces cerevisiae) , the cisternae are arranged into a polarly structured stack (Fig.…”

Immunohistochemical analyses on two distinct internodes of stinging nettle show different distribution of polysaccharides and proteins in the cell walls of bast fibers

“…However, it should be considered that chemical analysis of the cell wall (Xu et al 2019 ) suggested that the main hemicellulose of nettle bast fibers sampled from older internodes are xyloglucans, given the high amounts of glucose, fucose, and galactose, in addition to xylose. The reason for the lack of LM10 signal could be due to the non-accessibility of the non-reducing ends of xylans (possibly due to masking caused by interaction with other cell wall components) (Ruprecht et al 2017 ; Meents et al 2019 ). Therefore, the combined LM10 and LM11 results could provide insight into the chemical nature of the xylans possibly present in nettle bast fibers (substitution, modification of the non-reducing ends, or their masking by interactions).…”