Companion Cells

Companion cells represent a major component of the phloem, the plant vascular tissue system responsible for long-distance transport of organic nutrients from the sites of synthesis (source organs) to sites of utilization (sink organs). The companion cells are intimately associated with phloem sieve elements, which make up the actual transport vessel (sieve tube) through which mass flow of assimilates occurs. Sieve elements are usually enucleate, although they are living cells, and are thus completely dependent upon the associated companion cells leading to the concept of the sieve element–companion cell complex (SE/CC). The companion cell is necessary not only for metabolic maintenance of sieve elements but also for the selective accumulation (phloem loading) of assimilates and macromolecules present in sieve elements and to be transported by the phloem.