What is the role of companion cells in phloem?

Companion cells in phloem play a crucial role in supporting the function and survival of sieve-tube elements.

Companion cells are specialised parenchyma cells, found in the phloem of flowering plants. They are closely associated with sieve-tube elements, which are responsible for the transport of sugars and other nutrients throughout the plant. The companion cells and sieve-tube elements are derived from the same mother cell and remain connected through plasmodesmata, small channels that allow for the exchange of substances between the cells.

The primary role of companion cells is to support the function and survival of sieve-tube elements. Sieve-tube elements lack certain organelles, including a nucleus, ribosomes, and a central vacuole, which makes them dependent on companion cells for many metabolic processes. For instance, companion cells are responsible for synthesising proteins and other molecules that are then transported to the sieve-tube elements.

In addition to their metabolic support, companion cells also play a role in the loading and unloading of sugars into the sieve-tube elements. They actively transport sugars from the surrounding parenchyma cells into the sieve-tube elements, where they can then be transported to other parts of the plant. This process is crucial for the distribution of nutrients throughout the plant, supporting growth and development.

Furthermore, companion cells are thought to play a role in the regulation of the sieve-tube elements. They may help to control the pressure flow within the sieve tubes, ensuring that nutrients are transported efficiently and effectively. This regulatory role is still not fully understood and is a topic of ongoing research.

In summary, companion cells in the phloem are essential for the function and survival of sieve-tube elements. They provide metabolic support, assist in the transport of sugars, and may also play a regulatory role.