Money is symbolic, of I'd say power but what exactly its symbolic of is debatable. And, as a symbol I don't think it can be the root of anything, in the very least what it represented would the root. However I think its even further then that. Money or power is just no all inclusive enough to be cause of all stress.
I would instead define stress as a discrepancy between our perception of our actual selves and or lives, and our perception of a perfect person and or our lives. Money appears to be the cause of all stress as it would seem to most people to be the easiest route to making ourselves and or our lives into our perceived perfection. But, if a person had all the money he desires or if a person lived in an environment was money was not used I believe he would still feel stress because the flaws of our human nature prevent us from reaching any state of true perfection.