The alkane 1-monooxigenase gene alkB of Pseudomonas sp. FF2 is upregulated during colonisation of Arabidopsis thaliana leaves

Bacteria on leaf surfaces encounter highly variable access to nutrients and water. This oligotrophic environment is partly due to the presence of cuticular waxes that render the leaf surface hydrophobic, reducing the leaching of nutrients and water loss from inside the leaf. Bacteria have evolved adaptations to survive under these conditions. While alkB, an alkane hydroxylase gene, is widely prevalent in leaf-associated bacteria, its role and activity is unclear. Here, we developed a bioreporter in Pseudomonas sp. FF2 (PFF2) to investigate alkB promoter activity in diesel and on Arabidopsis thaliana leaves. In general, alkB promoter activity is highly heterogeneous, with a subpopulation exhibiting strong activation, suggesting bet-hedging in alkane metabolism. In planta, the promoter remained active over the course of seven days, indicating constant access of alkanes over time. Single-cell fluorescence intensity was heterogeneous, reflecting differences in microhabitats on the leaf surface or bet-hedging. While our results support a role of alkB in bacterial adaptation to the phyllosphere, direct evidence of cuticular wax degradation is missing. Future studies should trace the incorporation of plant-derived aliphatic compounds to elucidate the potential use of alkanes and other aliphatic compounds as resources for bacteria in the leaf environment.