Companion Cell and Sieve Elements

Abstract

Companion cells and sieve elements are two cell types that are key components of the phloem – a tissue responsible for the translocation of inorganic and organic compounds as well as signals from the leaves to the sites of use. Companion cells form a functional unit with the sieve elements for long‐distance translocation of assimilates, nutrients and signalling molecules and play indispensable roles throughout the life cycle of plants. Key Concepts Sieve elements are cells specialised in translocation of phloem sap. Sieve elements degrade their nucleus and other organelles during development to produce a conduit Enucleate sieve elements are kept alive by the tightly connected companion cells. Plasmodesmata connect companion cells and sieve elements, likely to provide conduits for the exchange of ions, metabolites, hormones, RNAs and proteins. The sieve element companion cell complex is specialised in phloem loading, in the case of sucrose via SUT sucrose/H + symporters, in the case of amino acids via AAP amino acid/H + symporters. Other specialised forms of companion cells include transfer cells and intermediary cells, characterised either by increased plasma membrane surface due to reticulate cell wall invaginations or specialised metabolism for raffinose translocation. Companion cells play crucial roles in hosting diverse metabolic and signalling processes.