The significance of matter coupling in f(R) gravity
  The way one chooses to couple gravity to matter is an essential characteristic of any gravitational theory. In theories where the gravitational field is allowed to have more degrees of freedom than those of General Relativity (e.g. scalar-tensor theory, f(R) gravity) this issue often becomes even more important. We concentrate here on f(R) gravity treated within the Palatini variational principle and discuss how the coupling between matter and the extra degrees of freedom of gravity (the independent connections in our case) affects not only the resulting phenomenology but even the geometrical meaning of fundamental fields. 
Thomas P. Sotiriou