Kompanjoncellene

Kompanjoncellene (companion cells) are specialized parenchyma cells in the phloem of vascular plants that accompany sieve tube elements and are connected to them by numerous plasmodesmata. They are typically located adjacent to sieve elements and share cytoplasm through the plasmodesmal connections, forming a functional unit that supports phloem transport.

Structurally, companion cells contain a prominent nucleus, abundant cytoplasm, and rich organellar content, including mitochondria, endoplasmic reticulum, and Golgi apparatus. The plasma membrane often forms infoldings that increase surface area to facilitate transport. This cellular organization enables companion cells to be highly metabolically active, supplying the energy and biosynthetic capacity required for loading and unloading phloem sap.

Functionally, companion cells regulate phloem transport by loading sugars—especially sucrose—from photosynthesizing tissues into the sieve tube elements, and by maintaining their metabolic vigor. They achieve this through active transport mechanisms driven by proton pumps and a range of sugar transport proteins (such as SUC/SUT family transporters). Through their connections with sieve elements, companion cells provide ATP, nucleotides, proteins, and regulatory RNAs, supporting the growth and function of the conducting sieve tubes. In some species, specialized forms called transfer companion cells enhance loading efficiency in particular tissues.

Developmentally, companion cells arise from the same meristematic precursors as sieve elements. When sieve elements mature, they lose most organelles and nuclei, while companion cells remain alive to sustain the function of the phloem. Together, companion cells and sieve tube elements form the functional phloem responsible for long-distance transport of photoassimilates.