Huanglongbing (HLB) is currently the most devastating disease of citrus and has created unperceived challenges to global citrus production. In order to properly manage HLB, robust detection methods must be implemented to prevent pathogen expansion and genetic resources must be explored to develop HLB-tolerance in citrus. To achieve this, much is to be elucidated on the pathogen virulence mechanisms and host defense responses involved during HLB disease progression.
The most prevalent HLB-associated disease agent is Candidatus Liberibacter asiaticus (CLas); a fastidious, gram-negative bacterium. Gram-negative bacterial pathogens possess secretion systems which deliver virulence proteins, known as effectors, into the host. Effectors can modify host physiology and suppress immunity to promote pathogen colonization and subsequent disease. Previous research found that CLas encodes Sec-delivered effectors (SDEs), many of which are highly expressed in CLas-infected citrus tissues, conserved across CLas isolates, and have uncharacterized functions. As such, SDEs can be utilized as molecular probes to better understand pathogen biology.
In this thesis, I employed SDEs of CLas to aid in development of detection technologies and to further knowledge of the CLas-citrus arms race. In Chapter I, I used CLas SDE1 and SDE2 as biomarkers in the development of antibodies for serological-based detection technologies. I evaluated and purified antibodies raised against the two biomarker proteins enhancing antibody efficiency and functionality for subsequent detection platforms. These results demonstrated their potential use for CLas detection.
In Chapter II, I utilized CLas SDE1 as a molecular probe to unveil host targets and characterize effector function. I found SDE1 interacts with and inhibits the activity of citrus papain-like cysteine proteases (PLCPs). PLCPs are known to be involved in defense in many other pathosystems and exhibit increased abundance during CLas infection. Collectively, these results indicated that SDE1 works as a virulence protein of CLas and can promote bacterial infection, likely by inhibiting citrus PLCP activity to suppress host defenses.
In Chapter III, I further explored SDE1 function in HLB disease progression. My results demonstrated that SDE1 protein expression in Arabidopsis thaliana caused yellowing, reminiscent of HLB symptoms. This SDE1-induced yellowing was associated with the induction of leaf senescence signatures, such as reactive oxygen species and senescence-associated genes. Senescence gene induction was also slightly altered in SDE1-transgenic citrus but, only in plants with the additional stress of CLas-infection. Thus, manipulation of senescence could be a possible virulence strategy of CLas in an SDE1-related manner.
