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.

Promotion of Testa Rupture during Garden Cress Germination Involves Seed Compartment-Specific Expression and Activity of Pectin Methylesterases

“…We observed the highest PME activity in the garden cress CAP in the first hours after the start of imbibition (Fig. 5A), which possibly prevents preterm CAP weakening, similar to the observation by Müller et al (2013) that high PME activity in whole seeds was associated with a delay of ER in Arabidopsis. Contrary to the CAP, the RAD showed an increase of PME activity only after TR.…”

PECTIN METHYLESTERASE INHIBITOR6 Promotes Arabidopsis Mucilage Release by Limiting Methylesterification of Homogalacturonan in Seed Coat Epidermal Cells

“…In effect, a comparison of pmei6 seed coat epidermal cell morphology and development did not reveal any difference with the wild type (see Supplemental Figure 5 online). Plants overexpressing PMEI6 showed no obvious difference in seed coat epidermal cell size compared with the wild type (Figure 3), in contrast with 35S: PMEI5-overexpressing lines (Müller et al, 2013). This suggests that the PMEs inhibited by PMEI5 intervene in different seed coat epidermal cell processes to the PME partners of PMEI6.…”

HIGHLY METHYL ESTERIFIED SEEDS Is a Pectin Methyl Esterase Involved in Embryo Development

“…Ectopic expression of PMEI5 driven by the 35S promoter has been shown to produce larger seeds with a bigger embryo and faster germination rate (Müller et al, 2013). To verify if PMEI5 impacts HMS, we crossed a PMEI5 overexpression (OE) line with the hms-1 mutant to generate an hms-1/PMEI5 OE line.…”

“…Cell Wall Loosening Enzymes Present in the Seed Other Than HMS May Act to Limit the Growth of the Embryo OE PMEI5 plants have bigger seeds with bigger embryos (Müller et al, 2013), a phenotype contrasting with that of hms-1. Plants homozygous for hms-1 having the OE PMEI5 construct have larger seeds, similar to those with OE PMEI5 alone (Fig.…”