The Genome and Genetics of a High Oxidative Stress Tolerant Serratia sp. LCN16 isolated from the plant parasitic nematode Bursaphelenchus xylophilus

Pine wilt disease (PWD) is a serious devastative disease in East Asia and Europa forests, caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus. Previous research has shown the presence of bacteria in PWD system, gathering in masses around the PWN in the host tree, and hypothesized their influence in PWD. Serratia sp. LCN16 is an PWN associated bacterium, highly resistant to in vitro oxidative stress, which beneficially contributes to the PWN survival in the same conditions. In planta, the oxidative stress is generated as a basal defense mechanism to contain pathogenic invasion. Here, we investigated the potential of the Serratia sp. LCN16 and PWN interaction in the disease development by exploring the LCN16 genome and the genetics behind its impressive oxidative stress resistance. Serratia sp. LCN16 is phylogeneticaly closest to the phytosphere group of Serratia enclosing S. proteamaculans, S. grimessi and S. liquefaciens, and displays ability to survive and colonize a plant-environment. In oxidative stress conditions (i.e. H2O2, hydrogen peroxide, as main stressor), LCN16 expresses katG (hydroperoxide I, HPI) under positive regulation of OxyR. LCN16 kat (HPII) is OxyR-independent. LCN16 ?oxyR was more sensitive to H2O2 than LCN16 ?kat, and both failed to protect the PWN from H2O2-stress exposure. In planta, co-inoculation of LCN16 ?oxyR with PWN resulted in a slower development of disease symptoms. In contrast, the co-inoculation of wild-type LCN16 and PWN led to a faster PWD development than only PWN inoculation. Hence, this study gives new insights into the bacteria-nematode interaction in the natural host of PWD and strengthens our previous hypothesis in the opportunistic helper effect of bacteria in the first stages of PWN infection.