  By using a set of different SFR indicators, including WISE mid-infrared and
Halpha emission, we study the slope of the Main Sequence (MS) of local star
forming galaxies at stellar masses larger than 10^{10} M_{\odot}. The slope of
the relation strongly depends on the SFR indicator used. In all cases, the
local MS shows a bending at high stellar masses with respect to the slope
obtained in the low mass regime. While the distribution of galaxies in the
upper envelope of the MS is consistent with a log-normal distribution, the
lower envelope shows an excess of galaxies, which increases as a function of
the stellar mass but varies as a function of the SFR indicator used. The
scatter of the best log-normal distribution increases with stellar mass from
~0.3 dex at 10^{10} M_{\odot} to ~0.45 at 10^{11} M_{\odot}. The MS high-mass
end is dominated by central galaxies of group sized halos with a red bulge and
a disk redder than the lower mass counterparts. We argue that the MS bending in
this region is due to two processes: i) the formation of a bulge component as a
consequence of the increased merger activity in groups, and ii) the cold gas
starvation induced by the hot halo environment, which cuts off the gas inflow
onto the disk. Similarly, the increase of the MS scatter at high stellar masses
would be explained by the larger spread of star formation histories of central
group and cluster galaxies with respect to lower mass systems.
