Pressure gradient in sieve tube is maintained by  

Companion cells load sugars to sieve tube element cells through plasmodesmata. Companion cells play a crucial role in maintaining the metabolic activities necessary for the loading and unloading of sugars, since sieve tube elements lack many cell organelles. When sugars are loaded into sieve tube elements, it increases the hypertonic condition where water from xylem is drawn into sieve tube elements, thus increasing pressure.