800 million years of co-evolution in the green plant lineage: the case of LEUNIG and SEUSS transcriptional co-regulators.

The evolution of land plants has involved significant restructuring and expansion of gene networks responsible for developmental processes, leading to the emergence of new gene expression and protein interaction patterns. The LEUNIG (LUG) and SEUSS (SEU) families of transcriptional co-regulators play crucial roles in angiosperm sexual reproduction, and in developmental and environmental response pathways. These proteins often function together in widely pleiotropic actions and are central members of larger transcription-regulating complexes, interacting with various proteins, such as floral homeotic MADS-box transcription factors. However, the origins and evolution of interaction networks of these gene families remain poorly understood. To systematically address this knowledge gap, we conducted a comprehensive analysis integrating phylogeny reconstruction, protein domain analysis, and protein interaction studies across a broad range of streptophyte algae and land plants. Our findings demonstrate that the LUG and SEU genes have existed for at least 800 million years, with specific domains remaining nearly invariant. Protein interaction analyses reveal that LUG- and SEU-like proteins physically interact in streptophyte algae and across all land plant lineages. Notably, the origin of their interactions with MADS-box proteins also dates to at least the streptophyte algae, highlighting the ancient and conserved roles of LUG and SEU proteins as essential components of transcriptional regulation and hubs for protein interactions. Our analysis reveals insights into the ancient origins and conserved roles of LUG and SEU family members in transcriptional regulation in algal and land plant lineages.