  We construct a family of viable scalar-tensor models of dark energy (DE)
which possess a phase of late-time acceleration preceded by a standard matter
era, while at the same time satisfying the local gravity constraints (LGC). The
coupling Q between the scalar field and the non-relativistic matter in the
Einstein frame is assumed to be constant in our scenario, which is a
generalization of f(R) gravity theories corresponding to the coupling
Q=-1/sqrt{6}. We find that these models can be made compatible with local
gravity constraints even when |Q| is of the order of unity through a chameleon
mechanism, if the scalar-field potential is chosen to have a sufficiently large
mass in the high-curvature regions. We show that these models generally lead to
the divergence of the equation of state of DE, which occur at smaller redshifts
as the deviation from the LambdaCDM model become more significant. We also
study the evolution of matter density perturbations and employ them to place
bounds on the coupling |Q| as well as model parameters of the field potential
from observations of the matter power spectrum and the CMB anisotropies. We
find that, as long as |Q| is smaller than the order of unity, there exist
allowed parameter regions that are consistent with both observational and local
gravity constraints.
