England were 247 for six at close of play on Sunday, chasing an unlikely 531 runs to win the second Ashes Test against Australia in Adelaide.
Matt Prior was not out 31 with Stuart Broad on 22 heading into Monday's final day.
Earlier, Australia declared on its overnight 132-3 about 10 minutes before play on Sunday, setting England an improbable 531 to win the Adelaide test and level the series at 1-1.
But after captain Alastair Cook (1) was out hooking Mitchell Johnson in the second over and Michael Carberry was out hooking Peter Siddle with the total at 20, only a 111-run partnership between Root and Pietersen prolonged England's resistance.
Root, other than flashing outside off stump twice in consecutive overs from Johnson, played with composure belying his age.
Promoted to bat at No. 3 after Jonathan Trott quit the tour with a stress-related illness following the 381-run first test defeat, the 22-year-old Root faced 165 balls and shared a defiant partnership with Pietersen that temporarily revived the innings and gave the impression that both could bat all day.
But the pressure eventually was telling, with Pietersen dismissed by Siddle for the ninth time in his career - more than any other bowler in test cricket. Bell, who scored a chanceless, unbeaten 72 to prop up England's first innings of 172, could barely believe he hit a rank full toss from part-time legspinner Steve Smith directly to Johnson just minutes before the tea interval.
The England middle and lower order has wilted in the face of some fearful fast bowling from Johnson in this series, including a spell of 5-12 off 18 balls from the Australian quick on Saturday. With four sessions remaining in Adelaide, and with no more proven batsmen after Root, Australia was extremely likely to take a 2-0 lead to Perth.
After picking up Cook cheaply, Johnson didn't really trouble Root or Pietersen, who played with unusual restraint for most of his innings.
He clubbed a full toss from Smith over long-on at the start of an over and hit two sixes - pulling a long-hop over mid-wicket to bring up the 100 partnership and a drive down the ground to reach his 50 - on the first and last balls of another over late in middle session.
It was the first 100-run partnership of the series for England, which didn't pass 180 in its first three innings.
After Smith's bowling was mauled, Siddle and Shane Watson dried up the runs with five consecutive maidens in a spell that included the wicket of Pietersen.
The 101-test veteran dragged an inside edge from a Siddle ball that pitched well outside off stump onto the bails to end his 99-ball innings.
Root got the jitters slightly, scampering through for a risky single after 35 balls without a run and getting hit by Ryan Harris' shy at the stumps.
England hadn't been expecting to bat Sunday morning but Australia declared - leaving David Warner unbeaten on 83 - with rain or showers forecast for the last two days of the Adelaide Oval match.