Northern Iowa led 13-7 before the two teams combined for three touchdowns in the final 4:30 to keep the crowd of 54,672 - the largest ever for an Iowa State home opener - guessing until the end.
Jantz threw a 26-yard touchdown pass to Josh Lenz on a fourth-and-10 play to put Iowa State up 14-13. But on the next play, Tirrell Rennie lofted an 80-yard touchdown pass to David Johnson to put the FCS Panthers back on top, 19-14. Johnson slipped out of the backfield, beat linebacker Jake Knott down the left sideline and caught the ball in full stride.
Jantz then took Iowa State back down the field for the game winner. His 32-yard pass to Aaron Horne put the ball at the Northern Iowa 2 and the Cyclones got it into the end zone on three straight sneaks by Jantz.
Jantz, who won the competition to replace three-year starter Austen Arnaud during preseason camp, struggled until coming up big at the end. He often threw wide of his receivers or overthrew them and was intercepted three times, but settled down when the Cyclones needed him the most.
He led drives of 61 and 60 yards for the final two touchdowns and finished 18 of 40 for 187 yards. He also ran 20 times for 80 yards and scored twice.
Until Jantz's clutch play late, Rennie had given the Cyclones fits with his running and scrambling, rushing 18 times for 127 yards. He was at his best while leading a 22-play, 90-yard drive that Johnson capped with a 1-yard run for a 13-7 lead late in the third quarter. The Panthers went 6-for-6 on third-down conversions on that march, which ran 10:36 off the clock.
Neither team did much after that until the flurry at the end.
Rennie, who last year became the first Northern Iowa player to rush for more than 1,000 yards and pass for more than 1,000 in the same season, completed 15 of 32 passes for 181 yards and no interceptions. The Panthers didn't have a turnover until Iowa State's Willie Scott grabbed an errant lateral with 21 seconds left to secure the victory for the Cyclones.
The game had more than its share of season-opening miscues and got a little chippy at times.
Both teams were flagged for multiple personal fouls and Iowa State's C.J. Morgan was ejected. Northern Iowa was flagged for 10 penalties in the first half, twice drawing three penalties on the same play, and had 16 for the game. Iowa State had eight penalties and four turnovers.
Even on their long scoring drive, the Panthers weren't always sharp. They were flagged for a false start on consecutive plays and later were called for holding, though that penalty was offset by the personal foul that resulted in Morgan's ejection.
Rennie kept that drive going with a 16-yard scramble on third-and-8 from his own 38 and third-down completions to Jarred Herring and Austin Wells. Wells' catch put the ball at the Iowa State 3 and Johnson scored three plays later, on yet another third-down snap.
Iowa State's first five possessions with Jantz under center ended with two three-and-outs, two interceptions and a fumble. The fumble, by running back Shontrelle Johnson, was the most disheartening of the turnovers, coming on the 12th play of a drive that carried the Cyclones to the Northern Iowa 24.
But the defense forced a quick three-and-out on the Panthers, Jantz took ISU down the field and he appeared to throw a 10-yard touchdown pass to Reid Branderhorst, who thought he had made a diving one-handed grab in the back of the end zone.
The catch was disallowed after a review, but the Cyclones got the touchdown anyway - with the help of four penalties on Northern Iowa, three on the same play when the Cyclones faced third-and-goal from the 15. Another UNI penalty, this one for pass interference, gave Iowa State the ball inside the 1-yard line and Jantz, with a push from running back Jeff Woody, bulled into the end zone on the final play of the half.
On the play from the 15, Northern Iowa was called for pass interference, roughing the passer and unsportsmanlike conduct.
The Panthers also were penalized three times on the same play earlier in the first half. They were flagged for jumping offsides and two personal fouls, resulting in a 35-yard walk-off that moved Iowa State to the UNI 25. After the Cyclones drove to the 9, Jantz's pass glanced of Darius Reynolds' hands and Tre'Darrius Canady picked it off for the Panthers.
Rennie hit Terrell Sinkfield with a 30-yard pass on the game's first play and the Panthers drove to the Iowa State 25 before settling for Tyler Sievertsen's 42-yard field goal. Rennie completed three passes and scrambled 9 yards for a first down during an 80-yard drive that Sievertsen finished with a 24-yard field goal, making it 6-0.