Evolution of the interaction of floral homeotic proteins

The flower development of angiosperms is controlled by floral homeotic MIKCC-type MADS-domain transcription factors (MADS-TFs) that activate or repress target genes by forming floral organ specific DNA-bound heterotetrameric complexes termed floral quartets. The ability to form floral quartets highly differs between floral homeotic MADS-TFs of certain subfamilies. However, to date relatively little is known about how these subfamily-specific interaction patterns of floral homeotic proteins evolved during angiosperm evolution and which sequence determinants account for the different interaction capabilities. Based on interaction studies of floral homeotic proteins from early diverging angiosperms I could show that the interactions governing flower development in core eudicots are also present in these distantly related species. However, especially AP3- and PI-like proteins from early diverging angiosperms possess additional interactions compared to their orthologs from core eudicots which form obligate heterodimers only. The more diverse interactions among floral homeotic proteins from early diverging angiosperms suggest a shift from promiscuity to specificity in the protein-protein interaction network during early angiosperm evolution. By comprehensive amino acid sequence analyses of MADS-TFs I demonstrated that the structure of the protein-protein interacting keratin-like domain (K-domain) is most likely highly similar among all subfamilies of floral homeotic proteins. Amino acid substitutions within the K-domain of the floral homeotic hub protein SEP3 revealed that highly conserved leucine residues at interacting sites are essential mediators of floral quartet-like complex formation. The absence of leucine residues at homologous amino acid positions in non-hubs such as AP3- and PI-like proteins probably accounts for their less promiscuous interactions. Beside the highly specific protein-protein interactions among floral homeotic proteins I studied another interaction of the K-domain. The phytoplasma effector protein SAP54 targets the K-domain to specifically bind MADS-TFs of certain subfamilies and destines them for degradation. Based on amino acid sequence analyses and structural predictions I provided preliminary evidence that SAP54 folds into a structure similar to that of the K-domain. Based on my findings I hypothesized that SAP54 evolved via convergent sequence and structural evolution to mimic the K-domain of its MADS-TF targets

Studying the Function of Phytoplasma Effector Proteins Using a Chemical-Inducible Expression System in Transgenic Plants

Phytoplasmas are bacterial pathogens that live mainly in the phloem of their plant hosts. They dramatically manipulate plant development by secreting effector proteins that target developmental proteins of their hosts. Traditionally, the effects of individual effector proteins have been studied by ectopic overexpression using strong, ubiquitously active promoters in transgenic model plants. However, the impact of phytoplasma infection on the host plants depends on the intensity and timing of infection with respect to the developmental stage of the host. To facilitate investigations addressing the timing of effector protein activity, we have established chemical-inducible expression systems for the three most well-characterized phytoplasma effector proteins, SECRETED ASTER YELLOWS WITCHES’ BROOM PROTEIN 11 (SAP11), SAP54 and TENGU in transgenic Arabidopsis thaliana . We induced gene expression either continuously, or at germination stage, seedling stage, or flowering stage. mRNA expression was determined by quantitative reverse transcription PCR, protein accumulation by confocal laser scanning microscopy of GFP fusion proteins. Our data reveal tight regulation of effector gene expression and strong upregulation after induction. Phenotypic analyses showed differences in disease phenotypes depending on the timing of induction. Comparative phenotype analysis revealed so far unreported similarities in disease phenotypes, with all three effector proteins interfering with flower development and shoot branching, indicating a surprising functional redundancy of SAP54, SAP11 and TENGU. However, subtle but mechanistically important differences were also observed, especially affecting the branching pattern of the plants

Genomes of multicellular algal sisters to land plants illuminate signaling network evolution

Zygnematophyceae are the algal sisters of land plants. Here we sequenced four genomes of filamentous Zygnematophyceae, including chromosome-scale assemblies for three strains of Zygnema circumcarinatum. We inferred traits in the ancestor of Zygnematophyceae and land plants that might have ushered in the conquest of land by plants: expanded genes for signaling cascades, environmental response, and multicellular growth. Zygnematophyceae and land plants share all the major enzymes for cell wall synthesis and remodifications, and gene gains shaped this toolkit. Co-expression network analyses uncover gene cohorts that unite environmental signaling with multicellular developmental programs. Our data shed light on a molecular chassis that balances environmental response and growth modulation across more than 600 million years of streptophyte evolution