Sieve Elements and Companion Cells—Traffic Control Centers of the Phloem

Of all the intricate cell–cell interactions in nature, those between the sieve element (SE) and its companion cell (CC) rank among the most complex and mysterious. Mature SEs are enucleate and retain only a highly degenerate cytoplasm, yet they can remain viable and functional for decades (Parthasarathy and Tomlinson, 1967). Generations of school children have been taught that the CC “keeps the SE alive,” an axiom that is only now, with the availability of modern techniques in cell and molecular biology, receiving experimental scrutiny. We will review recent (and some old) evidence that the SE and CC do indeed interact intimately and that the SE-CC complex plays a central role in trafficking a wide range of endogenous and foreign macromolecules, as well as solutes, throughout the plant. A central tenet is that the SE-CC complex must function in several roles, including phloem loading and unloading, long-distance transport, and as a quality control center for monitoring the nature of substances passing into and out of the phloem. In many ways, the day-to-day problems faced by a SE-CC complex can be likened to a busy international airport through which numerous travelers (and their luggage) pass before embarking on short- and long-distance flights. We develop this analogy further in the following sections.