Proteins in the sieve element-companion cell complexes: their detection, localization and possible functions

Many kinds of proteins have been found in the sieve element-companion cell complexes by the analyses of phloem sap and microscopic observations. The c DNAs, which encode some of these sieve-tube proteins, have already been clo ned. As mature sieve elements lack nuclei and most ribosomes, sieve-tube pr oteins have been hypothesized to be synthesized in the companion cells and then transported to the lumina of the functional sieve tubes through the pl asmodesmata connecting the companion cells and sieve elements. Soluble prot eins present in the sieve tubes can be collected by several techniques, suc h as incision or the aphid technique. The composition of the proteins in th e phloem sap is unique compared with that of tissue extract, suggesting the se proteins have important roles for the development and functions of sieve tubes.