Research Fellow The University of Western Australia KEWDALE, Western Australia, Australia
Body of Abstract: The structure–function relationship of proteins is a guiding principle in understanding their role and mode of action. In plants, two homologous proteins, DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2), perceive bioactive butenolide compounds, such as strigolactones and karrikins, respectively. These compounds influence several aspects of plant growth and development, as well as interactions with fungi and parasitic plants. Both D14 and KAI2 are α/β hydrolases that share similar tertiary structures and ligand-binding domains, as well as an essential Ser-His-Asp catalytic triad held in structurally correspondent positions. The mode of action of D14 and KAI2 involves similar ligand hydrolysis and binding mechanisms that initiate signal transduction.
The α/β hydrolase Regulator-of-SigmaB-Q (RsbQ), found in the plant growth-promoting rhizobacterium Bacillus subtilis, shares significant structural similarity with the plant proteins and regulates responses to nutritional stress. Although RsbQ is thought to perceive an unknown ligand, we hypothesised that it could perceive butenolide compounds that are active in plants. To test this, we investigated the response of RsbQ to the plant strigolactone analogues GR24 and desmethyl-GR24.
We observed in vitro interaction and hydrolysis of butenolide compounds by RsbQ, and mass spectrometry revealed covalent interaction of butenolides with RsbQ with location of binding suggesting a reaction mechanism similar to the plant proteins. Moreover, we observed that these butenolides inhibit the nutritional stress response pathway of B. subtilisin vivo. Our results suggest a functional similarity between bacterial and plant proteins, raising the intriguing possibility of plant-bacterial interaction mediated by butenolide signals.