In plants, DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2) perceive the bioactive butenolide compounds strigolactones and karrikins, respectively. These compounds influence several aspects of plant growth and development, as well as interactions with fungi and parasitic plants. D14 and KAI2 are homologous α/β-hydrolases that feature similar ligand binding and hydrolysis activity.
The α/β-hydrolase Regulator-of-SigmaB-Q (RsbQ) from the plant growth-promoting rhizobacterium Bacillus subtilis is a distant homologue of D14 and KAI2 regulating transcriptional responses to nutritional stress via the alternative transcription factor SigmaB (σB). We hypothesised that RsbQ is a bacterial butenolide receptor. To test this, we investigated the capacity of RsbQ to mediate responses to the synthetic butenolides GR24 and desmethyl-GR24, which are agonists for D14 and KAI2, respectively.
We observed in vitro interaction and hydrolysis of butenolide compounds by RsbQ, with a clear substrate preference for desmethyl butenolides. Mass spectrometry revealed a covalent interaction of butenolides with the catalytic histidine of RsbQ, suggesting a reaction mechanism similar to that of plant proteins. Exogenous application of both GR24 and dGR24 inhibited the endogenous signalling function of RsbQ in vivo, with dGR24 being 10-fold more potent. Application of dGR24 to B. subtilis phenocopied loss-of-function rsbQ mutations and led to a significant downregulation of σB-regulated transcripts. Moreover, plant-bioactive butenolides were found to antagonise the nutritional stress response pathway of B. subtilis in vivo, thereby encouraging the formation of biofilms. Our results indicate considerable functional similarity within class of α/β-hydrolases, raising the intriguing possibility of plant-bacterial interaction mediated by butenolide signals.