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One Target, Two Mechanisms: The Impact of 'Candidatus Liberibacter asiaticus' and Its Vector, Diaphorina citri, on Citrus Leaf Pigments.

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.