Lights off - Role of bioluminescence for the biology of the biocontrol agent Photorhabdus luminescens

Bioluminescence is found across various organisms having crucial functions for biotic interactions and stress adaptation. The only known terrestrial bioluminescent bacteria are entomopathogenic bacteria of the genus <i>Photorhabdus</i>. However, the reason why these bacteria produce light is not understood. <i>P. luminescens</i> exists in two cell forms called primary (1°) and secondary (2°) cells. The 1° cells colonize the nematode symbiosis partner and produce bright light, whereas 2° cells colonize plant roots only emitting weak light. Here we show that bioluminescence is important but not essential for the biology of the bacteria. Deletion of the <i>luxCDABE</i> operon in 1° cells impaired insect pathogenicity and nematode interaction. The complete loss of light of 2° cells resulted in enhanced plant root colonization, enhanced haemolysis, and reduced oxidative stress adaptation. Since bioluminescence is not essential for the survival of the bacteria, <i>P. luminescens</i> Δ<i>lux</i> 1° and 2° emerged as useful tools for bioluminescence-based reporter assays.