The development of primary and secondary PD

Primary PD are laid down at cytokinesis when the new cell wall is laid down. These PD are formed by entrapment of cortical ER strands within the developing phragmoplast. During organ growth, secondary PD arise de novo across existing cell walls to keep pace with cell expansion and to provide additional cell-cell communication. We are studying the formation of primary and secondary PD using a combination of techniques, including transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM; collaboration with Chris Jeffree, University of Edinburgh, non-invasive fluorescence imaging, and simulation modelling (collaboration with Ozgur Akman, Centre for Systems Biology, University of Edinburgh Recently, we showed that in the basal cell walls of trichomes secondary PD arise immediately adjacent to existing primary PD using the latter as a templates for their formation.

Tobacco trichomes are a simple model for studying PD development

Freeze fracturing of the trichomes reveals the trichome basal cell wall (CW) in surface view. In places the plasma membrane (PM) is exposed. Confocal imaging of the exposed cell wall reveals extensive surface views of PD (green). The wall was stained with the cellulose-specific probe, Calcofluor. Note that PD appear in cellulose-depleted pit fields

As the trichome wall extends without division, we were able to trace the insertions of new secondary PD during wall development. In our current model, secondary PD arise from longitudinal fissions of existing PD. Using FESEM, we were able to map the positions of several thousand PD on the extending basal trichome wall and used these data to derive a simulation model (‘Plasmodesmap’) that accurately reproduced the patterns seen in the microscope. For details see Faulkner et al. (2008).

At higher magnification, individual PD are now visible in the exposed cell wall and plasma membrane. The inset shows the positions of each PD pore marked in green

PD at high resolution (FESEM). Each PD pore is about 40nm diameter. The ‘creases’ on the PM are the imprints of cellulose microfibrils. The small particles (arrows) may be cellulose synthase complexes

In the twinning model of secondary PD formation, new PD arise in contact with existing PD. Each new pore, in turn, is capable of acting as a template for subsequent PD, giving rise to several paired PD within a pit field

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By analysing the positions of several thousand PD in expanding cell walls, we derived a simulation model (‘Plasmodesmap’) that accurately reproduced the distribution of PD seen in fractured cell walls (click play to see simulation)

Current model of secondary PD formation. Simple primary PD pores act as templates for the formation of secondary PD. New pores may arise from a fission of the original primary PD (A), or by the de novo formation of two new channels that are ‘drilled’ through the adjoining cell walls (B)