• companion cell;
  • macromolecular signalling;
  • mass flow;
  • phloem evolution;
  • phloem loading;
  • phloem-specific proteins;
  • phloem transport;
  • phloem unloading;
  • plasma-membrane-associated proteins;
  • plasmodesmata;
  • sieve-element plastids


This review deals with aspects of the cellular and molecular biology of the sieve element/companion cell complex, the functional unit of sieve tubes in angiosperms. It includes the following issues: (a) evolution of the sieve elements; (b) the specific structural outfit of sieve elements and its functional significance; (c) modes of cellular and molecular interaction between sieve element and companion cell; (d) plasmodesmal trafficking between sieve element and companion cell as the basis for macromolecular long-distance signalling in the phloem; (e) diversity of sieve element/companion cell complexes in the respective phloem zones (collection phloem, transport phloem, release phloem); (f) deployment of carriers, pumps and channels on the plasma membrane of sieve element/companion cell complexes in various phloem zones; and (g) implications of the molecular-cellular equipment of sieve element/companion cells complexes for mass flow of water and solutes in a whole-plant frame.