Differences in the frequency and disposition of plasmodesmata resulting from root cell elongation.

Differences in plasmodesmatal organization and frequency between cells which have and have not undergone wall expansion, were studied in four plant species (Trifolium repens L., Raphanus sativus L., Zea mays L., Sorghum vulgare L.). Plasmodesmatal disposition ranged from single dispersed to clustered into primary pit-fields. As the result of wall expansion (i.e. by comparing meristematic and elongate cells) there is a general shift from dispersed to clustered plasmodesmata. Plasmodesmatal frequency over the cell surface (number/μm(2)) does not decline as the result of wall expansion and cell elongation, thus indicating that secondary formation of plasmodesmata is a naturally occurring process during cell elongation, its extent being highest in Zea mays and lowest in Trifolium repens. The concurrent increases in frequency and clustering of plasmodesmata indicate that secondary formation may primarily occur in close proximity to pre-existing plasmodesmata.