Do symplasmic networks in cambial zones correspond with secondary growth patterns?

The plasmodesmal (PD) network in the cambial zone of Arabidopsis thaliana hypocotyls was analysed using electron microscopy and dye-coupling studies and compared to those of internodes of Populus nigra and Solanum lycopersicum. In all species, PD densities and frequencies undergo alterations in topologically successive cambial walls reflecting species-specific patterns of PD degradation and PD insertion during cell development. Longitudinal PD fission is responsible for an abrupt increment of PD numbers in specific walls of the youngest derivatives at the xylem and/or phloem side. Here, PDs seem to mediate positional signalling to control tissue fate and early cell determination. PD numbers at all cambial interfaces of A. thaliana correspond to those of the herbaceous tomato, but are higher with the woody poplar. This suggests a positive correlation between PD frequencies and the rapidity of cell division activity. Photoactivated green fluorescent protein (26 kDa) did not diffuse through cambial PDs of A. thaliana. This is in keeping with the common size exclusion limit (SEL) of 8-10 kDa observed for PDs at the youngest interfaces of tomato and poplar which may mediate diffusive exchange of developmental signals of equal molecular size. The regular growth patterns in internodal cambial zones of poplar and tomato result from synchronized cell division activity of neighbouring initials. A. thaliana hypocotyls have an irregular mode of secondary growth. Here, signalling through PDs in misaligned radial walls between non-homologous derivatives may control tissue development. The observed organizational differences between the cambia cast doubts on the suitability of A. thaliana as a model plant for cambial research.