Superrigid subgroups of solvable Lie groups

Let $\Gamma$ be a discrete subgroup of a simply connected, solvable Lie group $G$, such that $\operatorname {Ad}_G\Gamma$ has the same Zariski closure as $\operatorname {Ad}G$. If $\alpha \colon \Gamma \to \operatorname {GL}_n(\mathord {\mathbb R})$ is any finite-dimensional representation of $\Gamma$, we show that $\alpha$ virtually extends to a continuous representation $\sigma$ of $G$. Furthermore, the image of $\sigma$ is contained in the Zariski closure of the image of $\alpha$. When $\Gamma$ is not discrete, the same conclusions are true if we make the additional assumption that the closure of $[\Gamma , \Gamma ]$ is a finite-index subgroup of $[G,G] \cap \Gamma$ (and $\Gamma$ is closed and $\alpha$ is continuous).