Selecting main math font in LuaTeX?

Simply setting \usemathfont{Minion Pro} with unicode-math will not work since it expects an OpenType math font, but you can use range option to use normal text fonts for certain math alphabets (or any math symbol), check documentation for details.

\documentclass{minimal}

\usepackage{fontspec}
\usepackage{unicode-math}
\setmainfont{Minion Pro}
\setmathfont{Asana Math} % for math symbols, can be any other OpenType math font
\setmathfont[range=\mathup]  {Minion Pro}
\setmathfont[range=\mathbfup]{Minion Pro Bold}
\setmathfont[range=\mathbfit]{Minion Pro Bold Italic}
\setmathfont[range=\mathit]  {Minion Pro Italic}

\begin{document}
Here’s some text.  Now some math: $x^2 + y^2 = z^2$, $\mathrm{x^2 + y^2 = z^2}$, $\mathbfit{x^2 + y^2 = z^2}$, $\mathbf{x^2 + y^2 = z^2}$.
\end{document}

You can use the mathastext package:

\documentclass{article}

\usepackage[no-math]{fontspec}
\setmainfont{Minion Pro}
\usepackage[italic]{mathastext}

\begin{document}
Here’s some text.  Now some math: $x^2 + y^2 = z^2$.
\end{document}