But the legendary former Led Zeppelin singer and the heavenly voiced bluegrass queen are happily engaged in a musical affair of the heart that began with their chart-topping Grammy Award-winning 2007 album, "Raising Sand," and is continuing nightly on their first concert tour together.

"It's a great experience; I'll take it with me forever," Krauss, 36, said during an April interview from her Nashville home.

Plant, 58, voiced similar sentiments this month in a joint interview with Krauss and their producer and musical director, T-Bone Burnett, 60.

"What a delight it is to sit next to this woman," Plant said, prompting her to giggle.

"Not only does she have ... this beautiful, sonorous voice that creates its own resonance, but she hit a note recently (during one of our concerts) that created a frequency where the whole room just resonated. It was spectacular."

A laughing Krauss tried to deflect Plant's praise.

"I couldn't find it again," she joked about that frequency-shifting note. "We have to go back to that room!
The song in question is "Trampled Rose" by Tom Waits and Kathleen Brennan, one of many highlights on

"Raising Sand." The transcendent note Krauss hit in concert also made an impression on Burnett, who selected the musicians and vintage recording gear featured on the album, along with some of the songs, and plays rhythm guitar on it.

"That (note) was in B-minor," Burnett recalled, a touch of awe still audible in his voice.

"Yeah," said Plant. "The room was in B-minor and that Tom Waits song was brought to the party. I had no idea how intense it would be and how it would manifest itself with us three."

This heady sense of artistic discovery is precisely what makes this unlikely collaboration such a pleasure for performer and listener alike.

A delight from start to finish, the 13-song "Raising Sand" exudes rustic charm and an almost otherworldly ambience, yet the music sounds fresh and vital at the same time. With its heady blend of blues, backwoods country, rockabilly, Celtic-tinged folk and vintage New Orleans R&B, the album defines and transcends American roots music.

"There's a lot of blue in whatever this music is," Plant said. "And even though Alison comes from bluegrass roots, what has been created between the three of us has (become) its own genre, really."
Apart from a new version of "Please Read the Letter," which he wrote with former Led Zeppelin partner Jimmy Page for their 1998 album, "Walking Into Clarksdale," the rest of "Raising Sand" finds Plant and Krauss reinventing songs by a wide array of artists. These range from the Everly Brothers' "Gone Gone Gone (Done Moved On)" and Allen Toussaint's "Fortune Teller" to Doc and Rosa Lee Watson's "Your Long Journey" and two gems by former Byrds' member Gene Clark.

"We went in to do four songs in a week," Burnett recalled by phone from Nashville. "I think we got four done the first day."

A veteran solo artist himself, Burnett has produced standout albums by everyone from Elvis Costello and Cassandra Wilson to A.J. Croce and the late Roy Orbison. He regards working with Plant and the violin-playing Krauss as a career highlight.

"I'd like to work with Robert for the rest of my life, and Alison, too," said Burnett, who has produced upcoming albums by Costello, B.B. King and John Mellencamp. He first worked with Krauss in 2000, when he produced the multiple-Grammy-winning "O Brother, Where Art Thou?" film soundtrack album.

"The two things 'Raising Sand' and 'O Brother' have in common is they both feature some extraordinary musicians, playing really good songs with some of the best singers in the world," he continued. "The recklessness of bluegrass and rock 'n' roll certainly brings Alison and Robert together."

Plant and Krauss will return to their own bands after this tour - she leads Union Station and he heads The Strange Sensation. But both expressed enthusiasm about collaborating more in the future.

"I'm in no hurry to go anywhere. This is a font of knowledge and I'll stay as close as I can," Plant said. "Alison and I are telepathically looped when we work on vocal intensity. That was something I didn't know we could do, or that it even existed."

Their partnership dates back to 2004 when the two teamed up in Cleveland at a tribute concert sponsored by the Rock and Roll Hall of Fame and Museum. But the three songs they performed by folk-blues pioneer Leadbelly - "Black Girl," "Poor Howard" and "Green Corn" - sounded tentative.

"That Leadbelly stuff wasn't in the right range for us," Krauss said. "But I knew it would be a very interesting thing to get into - me with King Kong!"

The two later reconvened. When they hit a snag, Krauss suggested bringing in Burnett to help their artistic vision reach fruition. The three immediately hit it off, musically and personally.

"It was great," she said. "As we were sitting - Robert, T-Bone and myself - in my kitchen, working on this thing, I was like: 'Wow! I can talk about this song, or this person, or this solo, as much as I want, and nobody's rolling their eyes.'"

Krauss chuckled when asked if she ever teases Plant, on or off stage, by singing a Zeppelin-styled refrain of Ooohh, baby, baby, baby!

"No, he does that himself," she said, laughing. "He's a very funny person."

Plant reunited with Led Zeppelin's two other surviving members, Page and John Paul Jones, last December for a sold-out concert in London, which drew more than 20 million e-mail requests to buy tickets.

Speculation was high that a full-fledged tour would follow. But Plant made it clear his already-planned summer concert trek with Krauss was his priority. This led some disgruntled fans to liken Plant's new musical partner to the famously divisive wife of a former Beatle.

"It's not true," Krauss said. "I'm not Yoko Ono!"

Plant agreed.

"You know, singing in this revue is ... the most challenging thing I can remember," he said. "I'm learning so much about American music from Alison, T-Bone and the whole band.

"My whole deal about singing is that I don't go into remote-control to satisfy my ego. ... Sometimes (on this tour), I'm pinching myself: 'Am I really in the middle of this?' It's such a great cacophony - I couldn't wish for anything better."

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