What are the control points in plants where quantity and type of solutes reaching the xylem is adjusted ?

Which part in roots has transport proteins that are control points, where a plant adjust the quantity & types of solutes that reach the xylem :

Transport proteins of __________ are control points , where a plant adjusts the quantity and types of solutes that reach the xylem.

Transport proteins of __________ are control points , where a plant adjusts the quantity and types of solutes that reach the xylem.

Transport proteins of __________ are control points , where a plant adjusts the quantity and types of solutes that reach the xylem.

Transport proteins of __________ are control points , where a plant adjusts the quantity and types of solutes that reach the xylem.

Transport proteins of endodermal cells are control point where a plant adjusts the quantity and types of solutes that reach the xylem. Root endodermis is able to actively transport ions in one direction only because of the layer of .