Sterol Glucosyltransferases Tailor Polysaccharide Accumulation in Arabidopsis Seed Coat Epidermal Cells.

Adeline Berger,Elodie Akary,Helen M North,Isabelle Debeaujon,Christine Sallé,Marie-Christine Ralet,Olivier Grandjean

doi:10.3390/cells10102546

Abstract

The conjugation of sterols with a Glc moiety is catalyzed by sterol glucosyltransferases (SGTs). A portion of the resulting steryl glucosides (SG) are then esterified with a long-chain fatty acid to form acyl-SG (ASG). SG and ASG are prevalent components of plant cellular membranes and influence their organization and functional properties. Mutant analysis had previously inferred that two Arabidopsis SGTs, UGT80A2 and UGT80B1/TT15, could have specialized roles in the production of SG in seeds, despite an overlap in their enzymatic activity. Here, we establish new roles for both enzymes in the accumulation of polysaccharides in seed coat epidermal cells (SCEs). The rhamnogalacturonan-I (RG-I) content of the inner layer of seed mucilage was higher in ugt80A2, whereas RG-I accumulation was lower in mutants of UGT80B1, with double mutant phenotypes indicating that UGT80A2 acts independently from UGT80B1. In contrast, an additive phenotype was observed in double mutants for increased galactoglucomannan (GGM) content. Double mutants also exhibited increased polymer density within the inner mucilage layer. In contrast, cell wall defects were only observed in mutants defective for UGT80B1, while more mucilage cellulose was only observed when UGT80A2 was mutated. The generation of a range of phenotypic effects, simultaneously within a single cell type, demonstrates that the adjustment of the SG and ASG composition of cellular membranes by UGT80A2 and UGT80B1 tailors polysaccharide accumulation in Arabidopsis seeds.

Highlights

Phytosterols are integral structural components of cell membranes and influence their physicochemical properties and the activity of their associated proteins
The present study has extended the roles for conjugated sterols in polymer accumulation in seeds for both UGT80A2 and UGT80B1 enzymes through the demonstration of additional mutant phenotypes related to polysaccharide deposition in seed coat epidermal cells (SCEs)
This suggests that steryl glucosides (SG) produced by UGT80A2 fulfill a role in synthesis or secretion processes that lead to inner mucilage RG-I and cellulose accumulation, and that this is independent of the function of SG and acyl steryl glucosides (ASG) generated by

Summary

Introduction

Phytosterols are integral structural components of cell membranes and influence their physicochemical properties and the activity of their associated proteins. A diverse range of sterols are produced in plants, and a proportion of these are derivatized by conjugation of the C3-hydroxyl group to a sugar moiety, most frequently glucose (Glc), to form steryl glucosides (SG) [1]. SG can be further modified by the acylation of the sugar to form acyl steryl glucosides (ASG). SG and ASG are integrated into membranes in a differential manner, and this can contribute to their functional organization through the formation of ordered domains, termed ‘lipid rafts’ [2]. The conjugation of sterols with Glc is mediated by sterol glucosyltransferases (SGTs), and this activity has been demonstrated for two enzymes in Arabidopsis termed UGT80A2 and UGT80B1 [3,4]. The gene encoding UGT80B1 was found to be allelic to TRANSPARENT

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