Sieve element and companion cell: the story of the comatose patient and the hyperactive nurse

Sieve elements and companion cells constitute the modules of the conducting elements in the phloem of Angiosperms. Consequently, phloem transport basically relies on the concerted action of the sieve element/companion cell complexes. Sieve elements and companion cells are highly interactive units and show an extreme division of labour as exemplified by their state of life. Whereas the sieve element is almost 'clinically' dead, the companion cell is a paragon of bubbling activity. In the course of evolution, the sieve element has sacrificed all of its genetic and most of its metabolic equipment to serve photoassimilate translocation. A small part of the structural and metabolic outfit has been retained for a proper accomplishment of its function. In contrast, the cells bordering the sieve element have gained metabolic weight during evolution. With reference to their evolutionary descent, the peculiarities of sieve elements and companion cells are discussed in the light of recent cell-biological and molecularbiological findings. Emphasis is focused on their interaction, which is the secret of the success of the sieve element/companion cell complex.