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

Abstract

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 largescale 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. Significance StatementInteractions between transcription factors can alter downstream gene expression patterns and, in turn, organismal development. In plants, MADS-box transcription factors specify floral organ identity as part of large complexes. Shifting interactions between MADS-box proteins have been proposed as drivers in the evolution of the flower, and in the diversification of floral form. However, the functional consequences of changes to individual MADS-box protein-protein interactions are unknown. Here, we show that floral development is subtly affected by altered transcription factor protein-protein interactions. We also show that shifting transcription factor interactions can contribute to differential protein degradation. This reveals an important consequence for evolutionary variation in transcription factor interactions, and adds a new layer of complexity to the evolution of developmental gene networks.

Interspecies transfer of syntenic RAMOSA1 orthologs and promoter cis sequences impacts maize inflorescence architecture

“…Elegant genetic studies on the spatiotemporal regulation and function of transcription factors have shed important light on the mechanisms governing inflorescence branching patterns. Genetic variation in distal regulatory elements (Clark et al, 2006;, proximal or intronic cis regulatory elements Wills et al, 2013;Kusters et al, 2015), coding sequences that alter protein function , protein-protein interactions Abraham-Juarez et al, 2020) or protein-DNA interactions (Maizel et al, 2005; are critical drivers of inflorescence branching. Our data leveraging interspecies gene transfer and chimeric transgene expression suggest that cis-encoded regulation of RA1 expression is a key factor in modulating meristem determinacy that ultimately impacts grass inflorescence architecture.…”