The production of hydrophilic mucilages by the seed coat or pericarp, which are released upon seed hydration, is a commonly found adaptation in angiosperms, known as myxodiaspory. These are composed primarily of pectins and hemicelluloses that undergo substantive swelling upon hydration. Synthesized in the Golgi apparatus and secreted to an apoplastic space via secretory vesicles, mucilages can also contain cellulose microfibrils or cellulosic fibres that are synthesized at the plasma membrane in association with microtubules. Investigation of mucilage production in, and differentiation of, the mucilage secretory cells of the genetic model plantArabidopsis thalianahas identified a number of regulatory genes and enzymes involved in pectin synthesis and secretion,in muropectin modification and secondary cell wall synthesis. Studies of the role of mucilages in both a number of species and in Arabidopsis mutants affected in its production suggest that they have multiple ecological roles. These include facilitation of seed hydration, mediation of germination under waterlogged conditions, prevention of seed dispersal or predation by adherence to soil, and promotion of seed dispersal by attachment to animals. The precise role of mucilages appears to be dependent on species and their environmental context.

Following pollination, the epidermal cells of the Arabidopsis (Arabidopsis thaliana) ovule undergo a complex differentiation process that includes the synthesis and polar secretion of pectinaceous mucilage followed by the production of a secondary cell wall. Wetting of mature seeds leads to the rapid bursting of these mucilage secretory cells to release a hydrophilic gel that surrounds the seed and is believed to aid in seed hydration and germination. A novel mutant is identified where mucilage release is both patchy and slow and whose seeds display delayed germination. While developmental analysis of mutant seeds reveals no change in mucilage secretory cell morphology, changes in monosaccharide quantities are detected, suggesting the mucilage release defect results from altered mucilage composition. Plasmid rescue and cloning of the mutant locus revealed a T-DNA insertion in AtBXL1, which encodes a putative bifunctional β-d-xylosidase/α-l-arabinofuranosidase that has been implicated as a β-d-xylosidase acting during vascular development. Chemical and immunological analyses of mucilage extracted from bxl1 mutant seeds and antibody staining of developing seed coats reveal an increase in (1→5)-linked arabinans, suggesting that BXL1 is acting as an α-l-arabinofuranosidase in the seed coat. This implication is supported by the ability to rescue mucilage release through treatment of bxl1 seeds with exogenous α-l-arabinofuranosidases. Together, these results suggest that trimming of rhamnogalacturonan I arabinan side chains is required for correct mucilage release and reveal a new role for BXL1 as an α-l-arabinofuranosidase acting in seed coat development.

Summary: The seeds of Arabidopsis thaliana and many other plants are surrounded by a pectinaceous mucilage that aids in seed hydration and germination. Mucilage is synthesized during seed development within maternally derived seed coat mucilage secretory cells (MSCs), and is released to surround the seed upon imbibition. The FEI1/FEI2 receptor‐like kinases and the SOS5 extracellular GPI‐anchored protein were shown previously to act on a pathway that regulates the synthesis of cellulose in Arabidopsis roots. Here, we demonstrate that both FEI2 and SOS5 also play a role in the synthesis of seed mucilage. Disruption of FEI2 or SOS5 leads to a reduction in the rays of cellulose observed across the seed mucilage inner layer, which alters the structure of the mucilage in response to hydration. Mutations in CESA5, which disrupts an isoform of cellulose synthase involved in primary cell wall synthesis, result in a similar seed mucilage phenotype. The data indicate that CESA5‐derived cellulose plays an important role in the synthesis and structure of seed coat mucilage and that the FEI2/SOS5 pathway plays a role in the regulation of cellulose synthesis in MSCs. Moreover, these results establish a novel structural role for cellulose in anchoring the pectic component of seed coat mucilage to the seed surface.

Plant cells are encased within a complex polysaccharide wall that strengthens the cell and has key roles in all aspects of plant cell growth, differentiation, and interaction with the environment. ...

The function of a putative galacturonosyltransferase from Arabidopsis (Arabidopsis thaliana; At1g02720; GALACTURONOSYLTRANSFERASE-LIKE5 [AtGATL5]) was studied using a combination of molecular genetic, chemical, and immunological approaches. AtGATL5 is expressed in all plant tissues, with highest expression levels in siliques 7 DPA. Furthermore, its expression is positively regulated by several transcription factors that are known to regulate seed coat mucilage production. AtGATL5 is localized in both endoplasmic reticulum and Golgi, in comparison with marker proteins resident to these subcellular compartments. A transfer DNA insertion in the AtGATL5 gene generates seed coat epidermal cell defects both in mucilage synthesis and cell adhesion. Transformation of atgatl5-1 mutants with the wild-type AtGATL5 gene results in the complementation of all morphological phenotypes. Compositional analyses of the mucilage isolated from the atgatl5-1 mutant demonstrated that galacturonic acid and rhamnose contents are decreased significantly in atgatl5-1 compared with wild-type mucilage. No changes in structure were observed between soluble mucilage isolated from wild-type and mutant seeds, except that the molecular weight of the mutant mucilage increased 63% compared with that of the wild type. These data provide evidence that AtGATL5 might function in the regulation of the final size of the mucilage rhamnogalacturonan I.