Manipulating NHP-mediated systemic immunity confers resistance to Huanglongbing in citrus

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), poses a catastrophic threat to global citrus production. This study investigates the role of N-hydroxy-pipecolic acid (NHP), a systemic acquired resistance (SAR) inducer, in enhancing citrus resistance to HLB. Exogenous application of NHP via root irrigation or trunk injection significantly suppressed CLas proliferation in HLB-susceptible Citrus sinensis cv. Wanjincheng, alleviated disease symptoms, and promoted root growth. NHP triggered systemic accumulation of SAR markers (PR1, PR2) and primed defense responses in phloem tissues, critical for restricting CLas spread. Girdling experiments confirmed phloem-dependent systemic transport of NHP and its role in SAR establishment. Comparative analyses between HLB-tolerant kaffir lime (Citrus hystrix) and susceptible Wanjincheng revealed higher basal and CLas-induced NHP levels in tolerant varieties. Transgenic root-specific overexpression of NHP biosynthesis genes (CsALD1, CsSARD4, CsFMO1) enhanced endogenous NHP production, reduced CLas titers, and delayed systemic infection. Integrated transcriptomic and metabolomic profiling demonstrated NHP-mediated reprogramming of defense pathways, including phenylpropanoid biosynthesis, phytohormone signaling (JA, SA), and accumulation of antimicrobial metabolites (xanthotoxin, medicarpin). Our findings establish NHP as a potent SAR activator in citrus, providing a strategic framework for engineering HLB resistance through metabolic and genetic manipulation of NHP biosynthesis. This study highlights the potential of phloem-targeted SAR inducers for sustainable HLB management.