I will be auditioning for a band that describes itself as 'pop americana' they have a sort of country vibe.

As an aging musician I have come to believe that any genre has its outstanding exemplars, but I have never delved into country or roots music. I know it HAS to be more than 1-5-1-5 ad infinitum. Who are the greats in country and roots music?

Country is far from simple. I know, because I play in a country cover band and I get a lot of greif about it. The lines in today's top 40 are a lot more melodic than you think. True there is a lot of 1-5-1-5, but the guys in Nashville add a lot of nuances in the rhythm and how they walk between the chords.
Here are some of my influences:
Mike Chapman (Garth Brooks)
Glen Worf (Pretty much everything in Nashville: Tim McGraw, Faith Hill, Sara Evens, Alan Jackson, etc...)

Nowadays, country bassists play like any pop bassists you're familiar with. It's not as simple as it used to be, but it's still not brain surgery. Just think of Skynyrd, early 70's blues rock bands from England like Bad Company and Free, and Bob Seger and that'll get you pretty much in the ballpark.