A Pathogen Secreted Protein as a Detection Marker for Citrus Huanglongbing

Abstract

The citrus industry is facing an unprecedented crisis due to Huanglongbing (HLB, aka citrus greening disease), a bacterial disease associated with the pathogen <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las) that affects all commercial varieties. Transmitted by the Asian citrus psyllid (ACP), <i>C</i>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 <i>C</i>Las cells in infected trees and the long latency for disease symptom development makes sampling of trees for <i>C</i>Las detection challenging. Here, we report that a <i>C</i>Las secreted protein can be used as a biomarker for detecting HLB infected citrus. Proteins secreted from <i>C</i>Las cells can presumably move along the phloem, beyond the site of ACP inoculation and <i>C</i>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 <i>C</i>Las infection. This work demonstrates that antibody-based diagnosis using a <i>C</i>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.