Sieve Element Plastids: An Elusive Organelle.

Vascular plants use the phloem to transport photoassimilates via sieve elements, which are cells with modified anatomy to accommodate long distance transport. Mature sieve elements lack a majority of cell organelles to reduce friction in the tube system, but some organelles like the endoplasmic reticulum, mitochondria, phloem-proteins and sieve element plastids remain. Most of the functions of sieve element organelles are poorly characterized. This leads to the fact that sieve element plastids are often omitted from textbooks as a plastid type, although they apparently derive from proplastids. As they are ultrastructurally diverse, sieve element plastids have been used for taxonomical purposes, but functional information is scarce. Here we review the currently available literature on the structure and function of sieve element plastids.