Quantification of plasmodesmata frequency under three-dimensional view using focused ion beam-scanning electron microscopy and image analysis.

The quantitative study of plasmodesmata (PD) frequency is routine in plant science for providing information on the potential of intercellular transportation. Here, we report quantification of plasmodesmatal frequency in virus-infected tobacco vascular tissues using serial sectioning and image analysis. The image datasets were collected by focused ion beam-scanning electron microscopy (FIB-SEM), and the measurements of plasmodesmatal frequency were performed after image analysis with commercial computational programs. With a 5-nm step size (less than half the diameter of PD) during FIB sectioning, exhaustive PD sampling was performed in regions of interest. Segmentation of cell wall (CW) and PD from the background densities was performed manually, and PD were assigned automatically to individual CW interfaces by image analysis and then quantified. The PD quantification results were used to compare the plamodesmatal frequencies among different CW interfaces of individual cells and the average frequencies among different cell types were calculated. CWs lacking PD distribution were found in several cellular types, and the PD frequency were used to determine the possible pathways of PD-based symplasmic transportation. The method enables imaging of samples of several cells containing multiple CW interfaces and minimizes PD omission during sectioning and imaging.