The phloem, a miracle of ingenuity
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.
Keywords: companion cell; macromolecular signalling; mass flow; phloem evolution; phloem loading; phloem transport; phloem unloading; phloem-specific proteins; plasma-membrane-associated proteins; plasmodesmata; sieve-element plastids
Document Type: Original Article
Publication date: January 1, 2003