Chitosan Nanoparticles Encapsulate Antimicrobial Peptides for the Prevention and Control of <i>Citrus reticulata</i> Huanglongbing and Their Distribution in Plants

Huanglongbing (HLB), caused by <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las), is a serious bacterial disease affecting citrus. This study found that the expression of long stable antimicrobial peptide (<i>LSAMP</i>) increases with citrus immune genes <i>CsPR1</i>, <i>CsPR2</i>, and <i>CsNPR1</i>, indicating a link to citrus immunity. Additionally, cut antimicrobial peptide (CAMP) functions similarly to stable antimicrobial peptide (SAMP) in inhibiting <i>C</i>Las proliferation and boosting immunity in periwinkle (<i>Catharanthus roseus</i>). Chitosan nanoparticles (CS-NPs) were synthesized to encapsulate CAMP (CAMP-NPs) and SAMP (SAMP-NPs), which improved their stability and bioavailability. Both CAMP-NPs and SAMP-NPs outperformed their unencapsulated forms in inhibiting <i>C</i>Las proliferation and enhancing immunity in HLB-infected periwinkle and citrus; they also enhance citrus photosynthesis. Confocal microscopy revealed that CS-NPs mainly localized in the leaf apoplast but also penetrated chloroplasts. This study offers a novel biological control strategy for HLB and provides a practical framework for utilizing CS-NPs loaded with antimicrobial peptides (AMPs) for plant disease resistance.