  We consider the effect of modified gravity on the growth of large-scale
structures at second order in perturbation theory. We show that modified
gravity models changing the linear growth rate of fluctuations are also bound
to change, although mildly, the mode coupling amplitude in the density and
reduced velocity fields. We present explicit formulae which describe this
effect. We then focus on models of modified gravity involving a scalar field
coupled to matter, in particular chameleons and dilatons, where it is shown
that there exists a transition scale around which the existence of an extra
scalar degree of freedom induces significant changes in the coupling properties
of the cosmic fields. We obtain the amplitude of this effect for realistic
dilaton models at the tree-order level for the bispectrum, finding them to be
comparable in amplitude to those obtained in the DGP model.
