Transgenic Citrus sinensis Trees Expressing the Arabidopsis NPR1 Systemic Acquired Resistance Gene Demonstrates Enhanced Tolerance to Huanglongbing (HLB)

Systemic acquired resistance (SAR) is the activation of a plant's defense mechanism leading to induction of systemic resistance to subsequent pathogen infection. In efforts to develop a tolerant Citrus sinensis tree to the usually fatal phloem restricted Huanglongbing (HLB) disease, we overexpressed the SAR inducing Arabidopsis NPR1 (non-expresser of PR genes) gene in the two important Citrus sinensis cultivars ‘Hamlin’ and ‘Valencia’. Two constructs, one containing a constitutive 35s-NPR1 transgene and the other a phloem restricted AtSUC2-NPR1 transgene were incorporated into citrus via Agrobacterium mediated transformation. Disease screening for HLB both in the greenhouse and in field with 26 independent 35s-NPR1 lines and 18 independent AtSUC2-NPR1 lines revealed that six of the 35s-NPR1 and seven of the AtSUC2-NPR1 tested lines conferred a significant level of tolerance to HLB. Gene expression analysis suggested a direct correlation between NPR1 production in the transgenic lines and their response to the disease. Plants growing in the field under high inoculum pressure for 3 years have consistently tested negative for the presence of the bacterium. Our results suggest that the NPR1 gene could potentially provide enhanced tolerance to HLB. These transgenic lines can potentially also provide tolerance to other diseases affecting citrus.