Australia toiled in the field in the morning session without a breakthrough, as Root and nightwatchman Tim Bresnan put together a formidable partnership.
Bresnan fell following the resumption toe ending an attempted pull to Chris Rogers at mid-wicket.
New batsman Ian Bell should have been back in hutch shortly after edging to Steve Smith at gully. The catch was referred to the third umpire.
It looked as if the ball had carried but bafflingly the decision wasn’t given. Bell went on to notch a quickfire 50, and Root passed his 150 soon after.
The Yorkshireman picked up the scoring rate towards the end of the day, smashing two huge sixes off Steve Smith to put him in view of his double ton.