Interaction between sieve element and companion cell and the consequences for photoassimilate distribution. Two structural hardware frames with associated physiological software packages in dicotyledons

The unique properties of the sieve element/companion cell complex (SE/CC complex) are based on the concerted action of SE and CC. A prominent role in this interaction is played by the branched plasmodesmata (pore-plasmodesma units, PPUs) between SE and CC. The large molecular exclusion limit of the PPUs enables an intensive exchange of macromolecular materials and messengers. Further, the degree of symplasmic autonomy of the SE/CC complex and the structural/functional properties of the SE/CC modules may differ between the dicotyledonous families. The speculation is advanced that the difference in building blocks produces two distinct phloem frames ('hardware') with associated physiological programmes ('software'). The implications of two phloem types for photosynthate distribution and relative growth rate are discussed.