Graduate Student The University of Tennessee-Knoxville
Plasmodesmata (PD) are membrane-bound cytoplasmic bridges that span the cell wall and form direct connections between neighboring plant cells. They are essential for plant growth, development and defense, allowing metabolite exchange and signaling between cells. PD can form in already existing cell walls, allowing communication across tissues, as in between cell files in roots and the epidermis and mesophyll in leaves. The mechanisms mediating the addition of these PD (secondary PD) to existing cell walls remain elusive. Current models for secondary PD formation posit localized loosening of the wall followed by the gradual insertion of membranes and cytoplasm to generate the new pore. There is evidence that this process occurs near existing PD, giving rise to twinned PD and eventually to clusters of PD called pitfields. Existing models also propose that both cells to be connected by a new pore initiate pore formation, and the ‘half PD’ from neighboring cells fuse in the center of the cell wall to form a new pore. Using TEM tomography, we have identified intermediates in the formation of new PD at different stages of insertion. Tomogram reconstruction and examination of resulting three-dimensional models have allowed us to elucidate new details of secondary PD formation. First, we confirm the previous observation that secondary PD are often initiated near existing PD. Second, we observe that new pores are initiated on a single face of the cell wall, suggesting that PD formation is directed by a single cell rather than through a concerted effort by both neighboring cells. Third, the cell wall at initiation sites is distinct from the bulk cell wall, as evidenced by a lack of staining and the presence of ‘ghost’ PD-like structures that span the cell wall. These findings illuminate the mechanism of the essential process of secondary PD formation.
Coauthors: Tessa Burch-Smith – The University of Tennessee-Knoxville