Analysis of xyloglucan metabolism mutants highlights the prominent role of xylose cleavage in seed dormancy

Abstract

Seed dormancy is an adaptive trait that delays germination until environmental conditions become favorable for seedling survival and growth. Germination has been shown to depend on the mechanical resistance strength of the covering layers (testa and endosperm) that counteracts the growth force of the embryo. Cell wall remodeling is essential in the regulation of germination processes. In Arabidopsis thaliana, the side chain trimming of xyloglucans (XyG), the major hemicellulose in cell walls, by the apoplastic XYLOSIDASE1 (XYL1), has been previously shown to regulate XyG side chain length and seed dormancy. To investigate to what extent side chain complexity impacts on cell wall mechanical properties and regulates seed germination, xyl1 mutations were combined here with mutations in the two other glycosidases, the fucosidase AXY8 and the beta-galactosidase BGAL10. Analysis of germination phenotypes in axy8 bgal10 xyl1 and in several XyG biosynthesis mutants did not show any link between dormancy depth and side chain length. The very specific effect of xyl1 on seed dormancy in single and multiple mutants was clearly correlated with alterations in XyG intracellular localization, together with release and oxidation of free oligosaccharides (XGO). Accumulation of oxidized XGO could negatively impact cell wall remodeling by impairing remobilization and polarized secretion in cell walls, thus reducing growth anisotropy in elongating organs and modifying mechanical characteristics in seed tissues.