Cluster structures on Cox rings

We construct a graded cluster algebra structure on the Cox ring of a smooth complex variety $Z$, depending on a base cluster structure on the ring of regular functions of an open subset $Y$ of $Z$. After considering some elementary examples of our construction, including toric varieties, we discuss the two main applications. First: if $Z$ is a flag variety and $Y$ is the open Schubert cell, we prove that our results recover, by geometric methods, a well known construction of Geiss, Leclerc and Schr\"oer. Second: we define the diagonal partial compactification of a (finite type) cluster variety, and prove that its Cox ring is a graded upper cluster algebra. Along the way, we explain how similar constructions can be done if we replace the Cox ring with a ring of global sections of a sheaf of divisorial algebras on $Z$.