At lunch, England were 183 for two in their second innings -- a lead of 80 runs -- after losing openers Alastair Cook and Alex Hales.
Root dropped on 25 and struggling with a longstanding back problem, was 34 not out, with James Vince 20 not out in an unbroken third-wicket partnership of 57.
With the four-match series all square at 1-1, England resumed on 120 without loss, 17 runs in front. Cook, the England captain, was 64 not out and Hales 50 not out after they had erased a first-innings deficit of 103 with their maiden century stand in 18 innings as a Test-match opening pair.
Sohail had bowled a lacklustre spell Friday after marking his return to Test cricket following a five-year absence with five for 96 in England's first-innings 297.
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But, publicly upbraided by Pakistan coach Mickey Arthur after Friday's stumps, Sohail now whistled a delivery past new batsman Root's nose.
But it was left-arm quick Mohammad Amir who turned England's 126 for one into 126 for two when Hales (54) edged a seaming delivery and Younis Khan, held a difficult low chance going to his right at second slip.
But it is also one of the Hampshire batsman's best strokes and it enabled him to get off the mark with a stylish four off Amir.
Root, however, had a reprieve on 25 when he edged Rahat Ali only for Mohammad Hafeez to drop the low, two-handed, chance at first slip -- a rare blemish in an otherwise much-improved Pakistan catching effort this match.