Cell Biology of Sieve Element-Companion Cell Complexes

This chapter reviews the fundamentals of phloem development and structure with particular focus on their intercellular connections. One of the most intriguing aspects of the phloem is the connection between living sieve tubes formed from interconnected sieve elements (SEs) and the sieve-tube control system provided by the companion cells (CCs). In angiosperms, the only structures within the lumen of a SE are the phloem-specific proteins. The major component of the remaining cytoplasm is the sieve element reticulum (SER); a peripheral meshwork of endoplasmic reticulum (ER) that is free of ribosomes. At maturity, the SE-CCs are linked by highly specialized plasmodesmata (PD) connections to surrounding tissues. During differentiation and maturation of the SE-CC, the simple PD connections between CCs and SEs become modified to form pore-plasmodesma units (PPUs). The highly asymmetric ultrastructure of PPUs presumably reflects a compositional and functional asymmetry in molecular exchange between CCs and SEs.