Evolutionary variation in MADS-box dimerization affects floral development and protein degradation dynamics

The B-class MADS-box transcription factors STERILE TASSEL SILKY EAR1 (STS1) and SILKY1 (SI1) specify floral organ identity in the grass Zea mays (maize). STS1 and SI1 bind DNA as obligate heterodimers. Obligate heterodimerization between STS1 and SI1 homologs, although common in flowering plants, arose very recently in the grass family. This recent emergence of obligate heterodimerization from STS1 homodimerization provided an opportunity to test the consequences of evolutionary shifts in MADS-box protein-protein interactions. We tested the ability of evolutionary variation in STS1 dimerization to impact floral development, downstream gene regulation, and protein complex formation in maize. We found that STS1 hetero- vs. homodimerization had subtle effects on protein localization and stamen development. In contrast, differential STS1 dimerization resulted in large- scale changes to gene expression and protein complex composition. We identified kinases and proteins involved in ubiquitylation as candidate interactors with MADS-box proteins, and found that STS1 was phosphorylated, and in a complex with ubiquitylated proteins. In addition, we found that STS1 homodimers were more abundant than STS1-SI1 heterodimers, independent of RNA levels. Thus, differential dimerization can affect both protein degradation dynamics and combinatorial assembly of MADS-box protein complexes. Our results highlight the robustness of floral development to some molecular change, which may contribute to the evolvability of floral form.