2019 ASHS Annual Conference

Huanglongbing (HLB) or “citrus greening” is the most devastating disease ever encountered by citrus growers. The bacteria colonize phloem of the trees resulting in an uneven blotchy mottle symptom in the leaves, aborted seeds and production of salty and bitter fruits. Because of its potential of destruction, the most desirable form of long-term management for HLB is the use of genetic resistance. The expression of transgenes involved with plant defense activation pathways is an effective approach to increase tolerance to HLB. The SALICYLIC ACID BINDING PROTEIN 2 (SABP2), an enzyme that mediates the conversion of methyl salicylic acid into salicylic acid, is a well-known signal involved with the activation of plant defense. To obtain durable HLB tolerance in the otherwise susceptible sweet orange, transgenic trees were produced that overexpressed the Nicotiana tabacum SABP2 (NtSABP2). These sweet orange (cvs. ‘Hamlin’ and ‘Valencia’) were produced to express the NtSABP2 gene, either under the control of a constitutive CaMV 35S promoter (35S-SABP2) or a phloem specific Arabidopsis thaliana SUC2 promoter (AtSUC2-SABP2). The transcript and protein analysis of transgenics assayed by qPCR and western blot enabled the selection of eleven lines. Plant defense related gene expression assay revealed an increase in PR-2 but not of PR-1. The transgenic lines with the highest SABP2 transgene expression also had the highest accumulation of PR-2. All transgenic lines were planted in our USDA approved transgenic field site. Under field conditions, two of the 35S-SABP2 lines and one of the AtSUC2-SABP2 line had enhanced tolerance to HLB, remaining PCR negative even after several years in the field. Altogether, the results indicate that NtSABP2 expressed either constitutively or in the phloem tissues can confer tolerance to HLB. The increase in resistance might be mediated by the activation of plant defense pathways, however, the specific pathway and biochemical mechanism orchestrating this process remain to be investigated.