A stable antimicrobial peptide with dual functions of treating and preventing citrus Huanglongbing

Abstract

Citrus Huanglongbing (HLB), caused by a vector-transmitted phloem-limited bacterium <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las), is the most devastating citrus disease worldwide. Currently, there are no effective strategies to prevent infection or to cure HLB-positive trees. Here, using comparative analysis between HLB-sensitive citrus cultivars and HLB-tolerant citrus hybrids and relatives, we identified a novel class of stable antimicrobial peptides (SAMPs). The SAMP from <i>Microcitrus</i><i>australiasica</i> can rapidly kill <i>Liberibacter crescens</i> (<i>Lcr</i>), a culturable Liberibacter strain, and inhibit infections of <i>C</i>Las and <i>C</i>L. solanacearum in plants. In controlled greenhouse trials, SAMP not only effectively reduced <i>C</i>Las titer and disease symptoms in HLB-positive trees but also induced innate immunity to prevent and inhibit infections. Importantly, unlike antibiotics, SAMP is heat stable, making it better suited for field applications. Spray-applied SAMP was taken up by citrus leaves, stayed stable inside the plants for at least a week, and moved systemically through the vascular system where <i>C</i>Las is located. We further demonstrate that SAMP is most effective on α-proteobacteria and causes rapid cytosol leakage and cell lysis. The α-helix-2 domain of SAMP is sufficient to kill <i>Lcr</i> Future field trials will help determine the efficacy of SAMP in controlling HLB and the ideal mode of application.