He set a single-season franchise record for Double-A Altoona with 178 hits last year. He led the Eastern League in batting (.332), hits, doubles (38) and outfield assists (18). And he was named MLB.com's No. 35 overall prospect heading into this season.

But he had never hit a grand slam.

That changed Sunday.

Marte slugged a grand slam to cap a seven-run ninth inning as the Triple-A Indianapolis Indians rallied for a 9-4 victory over the Louisville Bats.

Trailing, 4-2, the Indians had six straight batters reach base to start the ninth. Jordy Mercer hit a two-run homer before Marte slugged his third longball of the season and first since April 29.

"He's got unbelievable power," Mercer said. "You saw a glimpse of it right there. That ball still hasn't landed yet. We strung together four, five, six at-bats in a row, and I think that's what it takes to come back. Then you get a big knock like Marte does and it puts the game out of reach."

The slam gave Marte his fourth multi-hit effort in five games, a span during which the 23-year-old center fielder is 9-for-23 to raise his batting average 22 points to .261.

That streak follows a week-long stretch in which Marte hit .115 (3-for-26) with eight strikeouts in seven games. The Pirates' No. 3 prospect also missed four games with a hand injury after getting hit by a pitch on May 6.

The ninth-inning rally got Indians starter Jeff Locke off the hook. The Bucs' No. 9 prospect recorded a season-high 11 strikeouts over 6 1/3 innings and allowed three runs on four hits.

Reds' No. 6 prospect Neftali Soto went 2-for-3 with a solo homer for the Bats.