In myxospermous species such as Arabidopsis (Arabidopsis thaliana), exposure of the mature seed coat to water causes the seed coat to release a polysaccharide-rich gel known as seed coat mucilage. Production of mucilage during seed coat development is governed by a gene regulatory network composed of transcription factors that are arranged into a hierarchy1. Previous work has shown that mutations in two closely related MADS-box transcription factors SHATTERPROOF1 (SHP1) and SHATTERPROOF2 (SHP2) have a mucilage release defect, although the nature of these defects has not been studied2. SHP1 and SHP2 are closely related to SEEDSTICK (STK), which has a well-characterised role in mucilage production3. As STK and SHP1/SHP2 work together during ovule development4, it is possible that these transcription factors perform similar roles in the developing seed coat. Using a combination of phenotypic and molecular analyses, SHP1/SHP2 are identified as regulators of genes associated with mucilage regulation, synthesis, and modification, and it is demonstrated that the function of SHP1/SHP2 differs from STK. Additionally, we present preliminary work suggesting SHP1/SHP2 expression is controlled by the upstream regulators DE1-BINDING FACTOR1 (DF1) and MYB61. These results suggest that the function of SHP1/SHP2 and STK diverges following fertilisation and that they control different aspects of mucilage production/modification. As well as providing insight into the role of MADS-box transcription factors during seed coat development, these results identify new interactions between transcription factors within the gene regulatory network controlling the production of seed coat mucilage.