title: Demethylesterification of cell wall pectins in Arabidopsis plays a role in seed germination.
abstract: The methylesterification status of cell wall homogalacturonans, mediated through the action of pectin methylesterases (PMEs), influences the biophysical properties of plant cell walls such as elasticity and porosity, important parameters for cell elongation and water uptake. The completion of seed germination requires cell wall extensibility changes in both the radicle itself and in the micropylar tissues surrounding the radicle. In wild-type seeds of Arabidopsis (Arabidopsis thaliana), PME activities peaked around the time of testa rupture but declined just before the completion of germination (endosperm weakening and rupture). We overexpressed an Arabidopsis PME inhibitor to investigate PME involvement in seed germination. Seeds of the resultant lines showed a denser methylesterification status of their cell wall homogalacturonans, but there were no changes in the neutral sugar and uronic acid composition of the cell walls. As compared with wild-type seeds, the PME activities of the overexpressing lines were greatly reduced throughout germination, and the low steady-state levels neither increased nor decreased. The most striking phenotype was a significantly faster rate of germination, which was not connected to altered testa rupture morphology but to alterations of the micropylar endosperm cells, evident by environmental scanning electron microscopy. The transgenic seeds also exhibited an apparent reduced sensitivity to abscisic acid with respect to its inhibitory effects on germination. We speculate that PME activity contributes to the temporal regulation of radicle emergence in endospermic seeds by altering the mechanical properties of the cell walls and thereby the balance between the two opposing forces of radicle elongation and mechanical resistance of the endosperm.
authors.affiliation: Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
authors.forename: Kerstin
authors.initials: K
authors.lastname: Müller
authors.forename: Gabriel
authors.initials: G
authors.lastname: Levesque-Tremblay
authors.forename: Sebastian
authors.initials: S
authors.lastname: Bartels
authors.forename: Karin
authors.initials: K
authors.lastname: Weitbrecht
authors.forename: Alexandra
authors.initials: A
authors.lastname: Wormit
authors.forename: Bjoern
authors.initials: B
authors.lastname: Usadel
authors.forename: George
authors.initials: G
authors.lastname: Haughn
authors.forename: Allison R
authors.initials: AR
authors.lastname: Kermode
mesh_terms: D000040:Abscisic Acid / Q000378:metabolism; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D001696:Biomechanical Phenomena; D002265:Carboxylic Ester Hydrolases / Q000235:genetics / Q000378:metabolism*; D048429:Cell Size; D002473:Cell Wall / Q000378:metabolism*; D004789:Enzyme Activation; D004951:Esterification; D035264:Flowers / Q000201:enzymology; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D018525:Germination; D008855:Microscopy, Electron, Scanning; D010368:Pectins / Q000378:metabolism*; D010641:Phenotype; D030821:Plants, Genetically Modified / Q000201:enzymology / Q000235:genetics; D012639:Seeds / Q000201:enzymology / Q000254:growth & development* / Q000648:ultrastructure; D014574:Uronic Acids / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D014574:Uronic Acids; D000040:Abscisic Acid; D010368:Pectins; D002265:Carboxylic Ester Hydrolases; C022323:pectinesterase; C003181:polygalacturonic acid