Ubiquitin receptor that functions as a positive regulator of grain size and weight (PubMed:34323980). Functions in the same genetic pathway as GW6A to regulate grain size (PubMed:34323980). Modulates grain size in a similar manner to GW6A, by altering cell proliferation in spikelet hulls (PubMed:34323980). Interacts with and enhances the ubiquitination of GW6A (PubMed:34323980). This stabilizes GW6A, delays protein degradation by the 26S proteasome and enhances GW6A histone acetyltransferase activity (PubMed:34323980). Interacts (via N-terminus) with GW6A (via C-terminus). The UIM domains bind molecules modified by monoubiquitin or ubiquitin chains and promote coupled monoubiquitination. Mutant plants produce grains with reduced size. Plants overexpressing HDR3 produces enlarged grains.