  Recent observations show that the Milky Way's metallicity distribution
function (MDF) changes its shape as a function of radius. This new evidence of
radial migration within the stellar disc sets additional constraints on
Galactic models. By performing controlled test particle simulations in a very
detailed, observationally motivated model of the Milky Way, we demonstrate
that, in the inner region of the disc, the MDF is shaped by the joint action of
the bar and spiral arms, while at outer radii the MDF is mainly shaped by the
spiral arms. We show that the spiral arms are able to imprint their signature
in the radial migration, shaping the MDF in the outskirts of the Galactic disc
with a minimal participation of the bar. Conversely, this work has the
potential to characterise some structural and dynamical parameters of the
spiral arms based on radial migration and the shape of the MDF. Finally, the
resemblance obtained with this approximation to the MDF curves of the Galaxy as
seen by APOGEE, show that a fundamental factor influencing their shape is the
Galactic potential.
