  We study the boundedness of Riesz transforms in $L^p$ for $p>2$ on a doubling
metric measure space endowed with a gradient operator and an injective,
$\omega$-accretive operator $L$ satisfying Davies-Gaffney estimates. If $L$ is
non-negative self-adjoint, we show that under a reverse H\"older inequality,
the Riesz transform is always bounded on $L^p$ for $p$ in some interval
$[2,2+\varepsilon)$, and that $L^p$ gradient estimates for the semigroup imply
boundedness of the Riesz transform in $L^q$ for $q \in [2,p)$. This improves
results of \cite{ACDH} and \cite{AC}, where the stronger assumption of a
Poincar\'e inequality and the assumption $e^{-tL}(1)=1$ were made. The
Poincar\'e inequality assumption is also weakened in the setting of a sectorial
operator $L$. In the last section, we study elliptic perturbations of Riesz
transforms.
