A Pathogen Secreted Protein as a Detection Marker for Citrus Huanglongbing

The citrus industry is facing an unprecedented crisis due to Huanglongbing (HLB, aka citrus greening disease), a bacterial disease associated with the pathogen Liberibacter asiaticus ( Las) that affects all commercial varieties. Transmitted by the Asian citrus psyllid (ACP), Las colonizes citrus phloem, leading to reduced yield and fruit quality, and eventually tree decline and death. Since adequate curative measures are not available, a key step in HLB management is to restrict the spread of the disease by identifying infected trees and removing them in a timely manner. However, uneven distribution of Las cells in infected trees and the long latency for disease symptom development makes sampling of trees for Las detection challenging. Here, we report that a Las secreted protein can be used as a biomarker for detecting HLB infected citrus. Proteins secreted from Las cells can presumably move along the phloem, beyond the site of ACP inoculation and Las colonized plant cells, thereby increasing the chance of detecting infected trees. We generated a polyclonal antibody that effectively binds to the secreted protein and developed serological assays that can successfully detect Las infection. This work demonstrates that antibody-based diagnosis using a Las secreted protein as the detection marker for infected trees offers a high-throughput and economic approach that complements the approved quantitative polymerase chain reaction-based methods to enhance HLB management programs.