Abstract
Pectins are major components of plant primary cell walls. They include homogalacturonans (HGs), which are the most abundant pectin and can be the target of apoplastic enzymes like pectin methylesterases (PMEs) that control their methylesterification level. Several PMEs are expressed in the seed coat of Arabidopsis thaliana, particularly in mucilage secretory cells (MSCs). On the basis of public transcriptomic data, seven PME genes were selected and checked for their seed-specific expression by quantitative reverse transcription PCR. Of these, PME58 presented the highest level of expression and was specifically expressed in MSCs at the early stages of seed development. pme58 mutants presented two discrete phenotypes: (i) their adherent mucilage was less stained by ruthenium red when compared to wild-type seeds, but only in the presence of EDTA, a Ca(2+)chelator; and (ii) the MSC surface area was decreased. These phenotypes are the consequence of an increase in the degree of HG methylesterification connected to a decrease in PME activity. Analysis of the sugar composition of soluble and adherent mucilage showed that, in the presence of EDTA, sugars of adherent mucilage were more readily extracted in pme58 mutants. Immunolabelling with LM19, an antibody that preferentially recognizes unesterified HGs, also showed that molecular interactions with HGs were modified in the adherent mucilage of pme58 mutants, suggesting a role of PME58 in mucilage structure and organization. In conclusion, PME58 is the first PME identified to play a direct role in seed mucilage structure.
Highlights
The cell wall plays a key role in numerous aspects of plant biology
PME58 presented the highest level of expression and was expressed in mucilage secretory cells (MSCs) at the early stages of seed development. pme58 mutants presented two discrete phenotypes: (i) their adherent mucilage was less stained by ruthenium red when compared to wild-type seeds, but only in the presence of EDTA, a Ca2+ chelator; and (ii) the MSC surface area was decreased
The expression of these genes was analysed by qRT-PCR on total RNA extracted from Arabidopsis siliques harvested at various developmental stages and from vegetative organs
Summary
The cell wall plays a key role in numerous aspects of plant biology. From a macroscopic point of view, it is responsible for the plant stature, and its structural modifications throughout evolution have enabled the plant kingdom to conquer the large diversity of terrestrial biomes. Despite the fact that the structure and composition of the plant primary cell wall are well described (Cosgrove, 2005; Caffall and Mohnen, 2009), understanding the molecular mechanisms underlying its modification during cell differentiation, as well as their consequences, is not an easy task. This is because is the cell wall a complex dynamic matrix and the interactions between its various components are difficult to evidence
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