How companion cells help in maintaining the pressure gradient in the sieve tubes?

The sucrose produced in the leafs is sent into the sieve tubes through the plasmodesmatal connections there by increasing the osmotic potential of the sieve tube due to the concentration gradient there develops a pressure gradient(negative pressure in the sieve tubes)due to which water from xylem vessels near by flows into the sieve tubes and pushes the mass of sucrose to other parts of the plant body