Foliar Applications of Microbe-Associated Molecular Pattern Molecules Enhance Citrus Resistance Against Huanglongbing

Citrus Huanglongbing (HLB) associated with 'Candidatus Liberibacter asiaticus' (CLas), a biotrophic pathogen, is a devastating citrus diseases worldwide. Enhancing host plant resistance through elicitor treatments has been exploited for crop disease management. HLB was suggested to be a pathogen-triggered immune disease in HLB susceptible varieties. Here, we hypothesized that boosting citrus immunity to CLas with biotic elicitors including microbe-associated molecular patterns (MAMPs) can increase citrus resistance to HLB. In greenhouse assays, foliar spray of chitosan, harpins, or lipopolysaccharides (LPS) activated sweet orange plant immunity in a dosage-dependent manner. Peak expression levels of citrus defense genes including FRK1, MKK4, PAL1, WRKY22, and WRKY29 were recorded at 12 to 72 hours post treatment, and chitosan and harpins induced stronger gene expression than LPS. Additionally, all the three MAMPs elicited reactive oxygen species (ROS) burst in treated leaves, and both chitosan and harpins induced a higher level of ROS than LPS. Moreover, chitosan and harpins, sprayed weekly at 1.0 and 0.25 mg/mL respectively, significantly reduced CLas growth in sweet orange trees and suppressed plant growth. In a 2-year field trial under natural CLas infection conditions, foliar spray of chitosan, harpins, or salicylic acid monthly delayed CLas infection and slowed down the increase of CLas titer, without suppressing plant growth. This study demonstrated that promoting immunity can be used as a useful strategy in controlling a pathogen-triggered immune disease caused by a biotrophic pathogen.