Abstract
Homogalacturonan pectin domains are synthesized in a highly methyl-esterified form that later can be differentially demethyl esterified by pectin methyl esterase (PME) to strengthen or loosen plant cell walls that contain pectin, including seed coat mucilage, a specialized secondary cell wall of seed coat epidermal cells. As a means to identify the active PMEs in seed coat mucilage, we identified seven PMEs expressed during seed coat development. One of these, HIGHLY METHYL ESTERIFIED SEEDS (HMS), is abundant during mucilage secretion, peaking at 7 d postanthesis in both the seed coat and the embryo. We have determined that this gene is required for normal levels of PME activity and homogalacturonan methyl esterification in the seed. The hms-1 mutant displays altered embryo morphology and mucilage extrusion, both of which are a consequence of defects in embryo development. A significant decrease in the size of cells in the embryo suggests that the changes in embryo morphology are a consequence of lack of cell expansion. Progeny from a cross between hms-1 and the previously characterized PME inhibitor5 overexpression line suggest that HMS acts independently from other cell wall-modifying enzymes in the embryo. We propose that HMS is required for cell wall loosening in the embryo to facilitate cell expansion during the accumulation of storage reserves and that its role in the seed coat is masked by redundancy.
Highlights
Homogalacturonan pectin domains are synthesized in a highly methyl-esterified form that later can be differentially demethyl esterified by pectin methyl esterase (PME) to strengthen or loosen plant cell walls that contain pectin, including seed coat mucilage, a specialized secondary cell wall of seed coat epidermal cells
Arabidopsis (Arabidopsis thaliana) PMEs and Pectin methyl esterase inhibitors (PMEIs) are encoded by two large gene families consisting of 66 and 69 genes, respectively, which seem to have expanded as a result of a duplication event and evolved to have divergent spatiotemporal specialization (Wang et al, 2013)
Using the eFP browser and a seed coat-specific microarray, we identified seven PMEs expressed in the seed coat (Fig. 1)
Summary
Homogalacturonan pectin domains are synthesized in a highly methyl-esterified form that later can be differentially demethyl esterified by pectin methyl esterase (PME) to strengthen or loosen plant cell walls that contain pectin, including seed coat mucilage, a specialized secondary cell wall of seed coat epidermal cells. HIGHLY METHYL ESTERIFIED SEEDS (HMS), is abundant during mucilage secretion, peaking at 7 d postanthesis in both the seed coat and the embryo. The epidermal cell layer of Arabidopsis seed coat undergoes complex cell differentiation, during which large quantities of pectinaceous mucilage are secreted between the plasma membrane and the outer tangential primary cell wall (Haughn and Western, 2012; North et al, 2014). Seed mucilage is recognized by HG-specific antibodies John Innes Monoclonal Antibody (JIM5), JIM7, PAM1, and 2F4 (Willats et al, 2001; Macquet et al, 2007; Voiniciuc et al, 2013)
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