  Cosmological simulations of disc galaxy formation, when compared to the
observed Tully-Fisher relation, suggest a low Mass-to-Light (M/L) ratio for the
stellar component in spirals. We show that a number of "bottom-light" Initial
Mass Functions (IMFs) suggested independently in literature, do imply M/L
ratios as low as required, at least for late type spirals (Sbc/Sc). However the
typical M/L ratio, and correspondingly the zero-point of the Tully-Fisher
relation, is expected to vary considerably with Hubble type.
  Bottom-light IMFs tend to have a metal production in excess of what is
tipically estimated for spiral galaxies. Suitable tuning of the IMF slope and
mass limits, post-supernova fallback of metals onto black holes or metal
outflows must then be invoked, to reproduce the observed chemical properties of
disc galaxies.
