A host-derived chimeric peptide protects citrus against Huanglongbing without threatening the native microbial community of the phyllosphere

The application of host-derived antibacterial peptides has been highlighted as a potential efficacious and safe tool for treatment of Huanglongbing (HLB), the most devastating disease of citrus. However, pathogenic bacteria such HLB-causing Candidatus Liberibacter asiaticus (CLas) often develop resistance against the host antibacterial peptides. We showed that chimeras containing two different host antibacterial peptides not only retain antibacterial activity but also overcome bacterial resistance and enhance plant defense responses. Also, chimeric peptides can have an off-target impact on the structure and function of plant associated microbiome. However, there is a lack of understanding of the impact of the chimeric peptide therapy on the microbial structure in the citrus phyllosphere while reducing the CLas titers. Here, we aim to evaluate the efficacy of a chimeric peptide (UGK17) to reduce CLas titers, inducing plant defense response and impacting the microbiome associated with the citrus phyllosphere. Leaf samples were collected from orange and grapefruit trees in Texas and identified as old and young leaves according to their maturity. We collected three different types of leaves based on their infection and symptoms: healthy, symptomatic (infected with typical symptoms), and asymptomatic (infected s without symptoms). In planta assay was performed by dipping the leaves in the 0, 5, and 25 uM of UGK17 solutions for 48 hours. The quantifications of CLas titer and pathogenesis-related (PR) gene expression were done by qPCR and RT-qPCR, respectively. Amplicon sequencing was done to evaluate the impact of UGK17 on individual bacterial community structure. In addition, we performed an ex planta assay to access the effect of UGK17 on the growth of individual bacterial isolates. The UGK17 treatment reduced the CLas titers in both asymptomatic and symptomatic citrus leaves regardless of the age of the leaves. The UGK17 application augmented the PR gene expression. There is no statistically significant inhibition of bacterial growth or change of microbial community structure upon UGK17 treatment. Our results suggest that the chimeric peptide therapy is a promising solution to combat HLB by targeting gram-negative pathogens and enhancing the plant immune responses without impairing the beneficial microbial community in the citrus phyllosphere.