Biological characterization and host-phage interaction of a novel Pectobacterium phage

Abstract

Taro (Colocasia esculenta (L.) Schott) is an important vegetable and food crop in China, but in recent years, soft rot disease has frequently occurred during its cultivation and production. This disease damages the underground corms and petiole bases of taro, causing decay in the affected parts and emitting a foul odor, leading to wilting and lodging of the entire plant. This has resulted in significant economic losses to taro production in China, along with food safety issues and ecological problems caused by excessive pesticide use, making it urgent to find a green and efficient control method. Due to its specificity and environmental safety, phage therapy exhibits advantages that chemical pesticides cannot match, representing a promising alternative to chemical pesticides for controlling pathogenic bacteria. In the preliminary work of this study, a bacterial strain was isolated from taro soft rot in Shaoguan, Guangdong, and initially identified as Pectobacterium colocasium ZXC0623. Using this strain as the host bacterium, a Pectobacterium phage was screened and named QJphage. We analyzed its physicochemical properties and obtained its biological characteristics, including optimal titer, optimal infection latency period, optimal infection multiplicity, optimal storage solvent, and resistance to ultraviolet light, pH, and chloroform. Through homologous alignment analysis, eight tail fiber proteins encoded in the QJphage genome were predicted as putative receptor-binding proteins (RBPs). To validate this prediction, the corresponding genes were cloned downstream of the egfp gene via homologous recombination, and the resulting recombinant plasmids were transformed into a prokaryotic host to express EGFP-tagged tail fiber fusion proteins. Fluorescence detection and confocal laser scanning microscopy confirmed that the protein encoded by ORF04 functions as the RBP. Furthermore, lipopolysaccharide (LPS) was knocked out in the host strain P. colocasium ZXC0623. Both{Delta} LPS1 and{Delta} LPS2 mutants formed smaller plaques compared to the wild-type strain, and the{Delta} LPS1 mutant additionally exhibited a significant reduction in plaque number, indicating that LPS serves as a receptor involved in QJphage adsorption. Finally, transcriptomic analysis during the latent period of infection focused on 20 genes predicted to be associated with phage-host receptor binding and anti-phage immune systems. The results revealed that pilin proteins act as potential reversible adsorption receptors for QJphage, while the host strain ZXC0623 also possesses a diverse repertoire of anti-phage defense systems. Collectively, QJphage exhibits stable physicochemical properties, a well-defined LPS-dependent infection mechanism, and a host with diverse defense systems, providing a foundation for the control of taro soft rot and future phage-related research. ImportancePhage therapy has emerged as a highly effective biocontrol strategy against Pectobacterium, with its specificity making it particularly valuable. A critical aspect of this approach is the identification of phage receptors. The initial step in the phage life cycle involves adsorption to the bacterial host, beginning with reversible contact followed by irreversible binding between phage receptor-binding proteins and specific bacterial surface receptors. Potential receptors include glycolipids in the Gram-negative outer membrane, capsular polysaccharides, and various membrane proteins or appendages. In this study, we first characterized the physicochemical properties of the isolated QJphage. Through integrated transcriptomic and whole-genome analyses, we demonstrated that the LPS of Pectobacterium specifically interact with the tail fiber proteins of QJphage. This research provides the first evidence revealing the molecular mechanism of interaction between Pectobacterium and its phage, establishing a foundation for developing phage-based control strategies against soft rot diseases.