Publications

3807

Huanglongbing (HLB) is currently the largest threat to global citrus production. We examined the effect of HLB pathogen ‘Candidatus Liberibacter asiaticus’ infection or infestation by its vector, Diaphorina citri, on ‘Valencia’ sweet orange leaf pigments using high-performance liquid chromatography, followed by gene expression analysis for 46 involved genes in carotenoid and chlorophyll biosynthesis pathways. Both ‘Ca. L. asiaticus’ and D. citri alter the total citrus leaf pigment balance with a greater impact by ‘Ca. L. asiaticus’. Although zeaxanthin was accumulated in ‘Ca. L. asiaticus’-infected leaves, chlorophyllide a was increased in D. citri-infested plants. Our findings support the idea that both ‘Ca. L. asiaticus’ and D. citri affect the citrus pigments and promote symptom development but using two different mechanisms. ‘Ca. L. asiaticus’ promotes chlorophyll degradation but accelerates the biosynthesis of carotenoid pigments, resulting in accumulation of abscisic acid and its precursor, zeaxanthin. Zeaxanthin also has a photoprotective role. By contrast, D. citri induced the degradation of most carotenoids and accelerated chlorophyll biosynthesis, leading to chlorophyllide a accumulation. Chlorophyllide a might have an antiherbivory role. Accordingly, we suggest that citrus plants try to defend themselves against ‘Ca. L. asiaticus’ or D. citri using multifaceted defense systems, based on the stressor type. These findings will help in better understanding the tritrophic interactions among plant, pathogen, and vector.

Host acquisition of a new microbial species can readily perturb the dynamics of preexisting microbial associations. Molecular cross talk between microbial associates may be necessary for efficient resource allocation and enhanced survival. Classic examples involve quorum sensing (QS), which detects population densities and is both used and coopted to control expression of bacterial genes, including host adaptation factors. We report that a 56-amino-acid repressor protein made by the resident psyllid endosymbiont Wolbachia can enter cells of Liberibacter crescens , a cultured proxy for the uncultured psyllid endosymbiont “ Ca . Liberibacter asiaticus” and repress “ Ca . Liberibacter asiaticus” phage lytic cycle genes. Such repression in “ Ca . Liberibacter asiaticus” may be critical to survival of both endosymbionts, since phage-mediated lysis would likely breach the immunogenic threshold of the psyllid, invoking a systemic and nonspecific innate immune reaction.