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Mechanisms influencing the polar distribution of cell wall components in seed coat epidermal cells of Arabidopsis Lee, Yi-Chen


Compositions of plant cell walls are important for morphogenesis and cellular function. Cell wall components are often distributed asymmetrically in the cell wall, but the mechanism for the polar distribution remains unknown. In my research, I used Arabidopsis seed coat epidermal cells as a model system because they deposit large amounts of pectin-rich mucilage in a polar manner to the outer periclinal side of the cell forming a large apoplastic pocket. MUM2 is a β-galactosidase modifying pectin in the mucilage. Using an engineered version of MUM2 fused to a Citrine fluorescent protein (Citrine), distribution pattern of MUM2 in the epidermal cell was determined. MUM2-Citrine is found to preferentially accumulate in the mucilage pocket concomitantly with pectin deposition. The amino acid sequence of MUM2 is not required for the secretion to the pocket. Rather, the polar distribution of MUM2-Citrine is caused by a rearrangement of the secretory pathway that appears to target all secretion to the outer periclinal side of the cell. At the end of mucilage synthesis, the fluorescence of MUM2-Citrine rapidly disappears from the mucilage pocket. The results of western blot analyses shows that the amount of MUM2-Citrine decreases, suggesting that the disappearance of MUM2-Citrine signal is due to protein degradation. Loss-of-function mutations in the genes encoding ASPG1 and RD21A, two proteases that have been detected in mucilage from mature seeds, resulted in a delay of MUM2-Citrine degradation. Also, RD21A, tagged with red fluorescent protein (RFP), accumulated in the vacuole during MUM2 secretion and was translocated to the mucilage pocket at the end of secretion. Taken together, these data suggest that ASPG1 and RD21A are responsible for MUM2 degradation. Mucilage properties were changed in the protease mutants, suggesting that the regulation of distribution of cell wall-modifying enzymes by proteases plays an important role in determining cell wall properties. It has been found that PER36, a peroxidase required for mucilage extrusion, was deposited in a pattern distinct from other mucilage proteins in seed coats. I showed the amino acid sequences of PER36 was required for the unique distribution pattern, suggesting that an unknown distribution mechanism may be involved.

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