A lipopolysaccharide-dependent phage infects a pseudomonad phytopathogen and can evolve to evade phage resistance

Bacterial pathogens are major causes of crop diseases, leading to significant production losses. For instance, kiwifruit canker, caused by the phytopathogen Pseudomonas syringae pv. actinidiae ( Psa ), has posed a global challenge to kiwifruit production. Treatment with copper and antibiotics, whilst initially effective, is leading to the rise of bacterial resistance, requiring new biocontrol approaches. Previously, we isolated a group of closely related Psa phages with biocontrol potential, which represent environmentally sustainable antimicrobials. However, their deployment as antimicrobials requires further insight into their properties and infection strategy. Here, we provide an in-depth examination of the genome of ΦPsa374-like phages and show that they use lipopolysaccharides (LPS) as their main receptor. Through proteomics and cryo-electron microscopy of ΦPsa374, we revealed the structural proteome and that this phage possess a T=9 capsid triangulation, unusual for myoviruses. Furthermore, we show that ΦPsa374 phage resistance arises in planta through mutations in a glycosyltransferase involved in LPS synthesis. Lastly, through in vitro evolution experiments we showed that phage-resistance is overcome by mutations in a tail fiber and structural protein of unknown function in ΦPsa374. This study provides new insight into the properties of ΦPsa374-like phages that informs their use as antimicrobials against Psa . Originality-Significance Statement The rise of phytopathogen resistance to agrichemicals poses a significant threat to crop production, and requires urgent attention. The work presented here examines a phage genus, members of which utilize LPS as a receptor, and show potential as biocontrol agents of Psa . Studies in planta showed the development of Psa resistance to the phage, whilst maintaining bacterial virulence. This finding underscores the importance of examining phages in their ecological context for the informed design of phage cocktails that are effective in phytopathogen control. ### Competing Interest Statement The authors have declared no competing interest.