Transport of UDP-rhamnose by URGT2, URGT4, and URGT6 modulates rhamnogalacturonan-I length

Abstract

Rhamnogalacturonan-I biosynthesis occurs in the lumen of the Golgi apparatus, a compartment where UDP-Rhamnose and UDP-Galacturonic Acid are the main substrates for synthesis of the backbone polymer of pectin. Recent studies showed that UDP-Rha is transported from the cytosol into the Golgi apparatus by a family of six UDP-rhamnose/UDP-galactose transporters (URGT1-6). In this study, analysis of adherent and soluble mucilage of Arabidopsis thaliana seeds revealed distinct roles of URGT2, URGT4, and URGT6 in mucilage biosynthesis. Characterization of soluble mucilage polymer size showed shorter chains in the urgt2 urgt4 and urgt2 urgt4 urgt6 mutants, suggesting that URGT2 and URGT4 are mainly involved in RG-I elongation. Meanwhile, mutants in urgt6 exhibited changes only in adherent mucilage. Surprisingly, the estimated number of RG-I polymer chains present in urgt2 urgt4 and urgt2 urgt4 urgt6 mutants was higher than in wild-type. Interestingly, the increased number of shorter RG-I chains was accompanied by an increased amount of xylan. In the urgt mutants, expression analysis of other genes involved in mucilage biosynthesis showed some compensation. Studies of mutants of transcription factors regulating mucilage formation indicated that URGT2, URGT4, and URGT6 are likely part of a gene network controlled by these regulators and involved in RG-I synthesis. These results suggest that URGT2, URGT4, and URGT6 play different roles in the biosynthesis of mucilage, and the lack of all three affects the production of shorter RG-I polymers and longer xylan domains.

Rhamnogalacturonan-I forms mucilage: behind its simplicity, a cutting-edge organization

“…Thanks to the great work realized by Zhang and collaborators, the tangled ‘ball of wool’ which represents the regulation of pectin synthesis is a little bit clearer now. At the mature stage, mucilage has been characterized as a plethora of unbranched RG-I polymers with an average size of 600 kDa; therefore, it is formed by ~1800 units of Rha–GalA disaccharide ( Box 2A ; Macquet et al , 2007a ; Williams et al , 2020 ; Saez-Aguayo et al , 2021 ). Elongation of this ‘giant’ polymer is initiated by the coordinated action of enzymes localized in the Golgi apparatus and in the cytosol ( Fabrissin et al , 2019 ; Saez-Aguayo et al , 2021 ).…”

“…At the mature stage, mucilage has been characterized as a plethora of unbranched RG-I polymers with an average size of 600 kDa; therefore, it is formed by ~1800 units of Rha–GalA disaccharide ( Box 2A ; Macquet et al , 2007a ; Williams et al , 2020 ; Saez-Aguayo et al , 2021 ). Elongation of this ‘giant’ polymer is initiated by the coordinated action of enzymes localized in the Golgi apparatus and in the cytosol ( Fabrissin et al , 2019 ; Saez-Aguayo et al , 2021 ). MUCILAGE-MODIFIED-4 (MUM4) converts UDP-Glc to UDP-Rha in the cytosol before the transport of the latter to the Golgi lumen by URGT2/4/6 transporters ( Western et al , 2004 ; Saez-Aguayo et al , 2021 ).…”

“…In the epidermal cells, the central columella is surrounded by an enriched pectin pocket. C, columella; M, mucilage; dw, distal wall; rw, radial wall ( Macquet et al , 2007a ; Voiniciuc et al , 2015a ; Šola et al , 2018 ; Saez-Aguayo et al , 2021 )…”

Pre-Anthesis Cytokinin Applications Increase Table Grape Berry Firmness by Modulating Cell Wall Polysaccharides

“…Such differences are likely indicative of the early effect of CPPU on inflorescences at the cell wall level, mainly in the genesis of firmer berries, due to calcium bridges correlating with higher cell wall stiffness in grape berries [2]. The differences in the xyloglucan epitope distribution suggest modifications in their structure but we cannot exclude the fact that they could be masked by the other polymer components or changes in the density of the cell wall polysaccharide network [35,45]. The immunohistochemical differences between control and CPPU-treated inflorescences at anthesis propose an early pectin remodeling in response to the exogenous CPPU phytohormone.…”

Sterol Glucosyltransferases Tailor Polysaccharide Accumulation in Arabidopsis Seed Coat Epidermal Cells

“…The macromolecular characteristics of outer mucilage Na 2 CO 3 extracts were analyzed by high-performance size exclusion chromatography (HP-SEC) coupled to a differential refractometer, a dual laser light scattering, and a differential pressure viscometer as detailed by Saez-Aguayo et al [34]. Briefly, 1 mL of extract was boiled for 5 min and filtered through a polyvinylidene difluoride filter (13 mm-diameter, 0.45 µm-pore size, Whatman, Maidstone, UK, https://www.cytivalifesciences.com/ accessed on 21 September 2021).…”