Effectors of the Citrus Huanglongbing-Associated Bacteria

Gram-negative bacterial pathogens, like the citrus Huanglongbing (HLB)-associated bacteria, possess secretion systems that deliver virulence proteins into the host. These “effector” proteins can aid infection by manipulating plant physiology and subverting host immunity, thereby promoting bacterial colonization and disease progression. We analyzed Sec-delivered effectors (SDEs) from the genome sequences of Candidatus Liberibacter asiaticus (CLas), the species currently present in the US. One of the core effectors, named SDE1, has 40-folds higher expression in citrus relative to in the insect vector, implicating a possible role in HLB disease progression. To identify the interacting proteins of SDE1 in citrus, we performed yeast two-hybrid (Y2H) screening and found that SDE1 directly interacts with several members belonging to a specific family of proteases known as papain-like cysteine proteases (PLCPs). PLCPs are upregulated during CLas infection; homologs of PLCPs in other plant-pathogen systems have also been shown to contribute to defense against microbial pathogens. Our results advanced understanding of HLB pathogenesis and discovered possible gene targets for genetic manipulation in order to enhance citrus resistance to this devastating disease. Furthermore, we will discuss the application of SDE1 as a biomarker for HLB detection using antibody-based methods.