Seed coat mucilage cells of Arabidopsis thaliana as a model for plant cell wall research

Abstract

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. This dynamic structure is under continual modification during plant development, and its synthesis and modification require the activity of a myriad of enzymes. The mucilage secretory cells (MSCs) of the Arabidopsis thaliana seed coat provide a model for the discovery of novel genes involved in the synthesis, secretion and modification of cell wall components, particularly pectin. These cells synthesize copious amounts of pectinaceous mucilage during development and, upon hydration of the desiccated seed, the mucilage rapidly swells, bursts from the MSCs and surrounds the seed in a gelatinous capsule. Several genes affecting MSC differentiation, pectin synthesis, and mucilage release have been identified and additional genes involved in these and related processes including pectin secretion and the mechanical alteration of cell walls await to be discovered.

Keywords: arabidopsis; cell wall; mucilage; pectin; seed coat.

Figure 1

(A) Arabidopsis seed with released mucilage. Upon hydration the hydrophilic mucilage breaks out of the MSCs and surrounds the seed. Staining with ruthenium red reveals a dense mucilage layer associated with the seed (arrowhead) surrounded by a more diffuse, less cohesive layer. Bar = 100 µm. (B) Proposed regulatory pathway of mucilage production. During MSC development, both AP2 and TTG1 regulate mucilage production through the upregulation of downstream transcription factors (GL2 and TTG2). TTG1 acts as a complex with the bHLH proteins EGL3 and TT8, and the R2R3 MYBs MYB5 and TT2. GL2 upregulates the rhamnose synthase MUM4, while the downstream targets of TTG2 remain unknown. A third, independent pathway of mucilage production exists under the control of MYB61. (C) The Arabidopsis mucilage secretory cell during mucilage production. At approximately 6–7 dpa, pectin synthesis begins in the MSC. At this stage, the Golgi apparatus and derivative secretory vesicles are distributed throughout the cytoplasm. nucleotide sugar interconversion enzymes embedded in the Golgi membrane or in the cytoplasm synthesize NDP-sugars, which are then transported into the Golgi lumen (sugar transporters not shown). A rhamnose synthase (MUM4/RHM2) is shown synthesizing UDP-L-rhamnose. As yet unidentified GTs in the Golgi lumen assemble pectins (RG I shown; GAUT11 is possible candidate). These polysaccharides are packaged into vesicles and secreted into the extracellular space as part of the mucilage. The apoplastic, mucilage-containing pocket forms a ring around cytoplasm in the apical portion of the cell, leading to a volcano-shaped cytoplasm constrained to a central column and basal portion of the cell. Hydrolytic enzymes (BXL1 and MUM2) modify RG I sidechains in the apoplast. it is possible that this modification occurs in the mucilage and the cell wall, facilitating mucilage release.