Metabolomic Analysis Provides New Insight Into Tolerance of Huanglongbing in Citrus

There have been efforts to develop citrus cultivars that are tolerant of Huanglongbing (HLB), a catastrophic phloem-limited disease. Previous studies demonstrated that continuous plant growth with phloem regeneration is one of the major characteristics of HLB tolerance. In this study, the metabolic mechanisms of HLB tolerance in citrus were elucidated using a multiple pathway-targeted metabolomic approach. Comparative analysis of healthy and infected HLB-tolerant and HLB-sensitive mandarin cultivars (Citrus reticulata) revealed differentially expressed metabolic responses among different groups. Pathway enrichment analysis indicated aspartate and glutamate metabolism, purine metabolism, and biosynthesis of plant hormones were upregulated in the tolerant group, except salicylic acid signaling. Catabolic pathways linked to energy-yielding metabolism were also upregulated in the tolerant group. These metabolisms and pathways were interconnected with each other, unveiling a pivotal metabolic network associated with HLB tolerance. In the network, auxins and cytokinins, the plant hormones responsible for plant growth and phloem regeneration, were accumulated. In addition, purine metabolites serving as energy carriers and nitrogen sources of plants were increased. Only salicylic acid-related metabolites for plant defense responses were decreased in the tolerant group. Our findings may evidence the strategy of HLB-tolerant cultivars that sustain plant growth and phloem formation rather than displaying direct plant defense to overcome the disease.

Dual transcriptional profiling of carrot and ‘Candidatus Liberibacter solanacearum’ at different stages of infection suggests complex host-pathogen interaction

The interactions between the phloem-limited pathogen ‘Candidatus Liberibacter solanacearum’ (CLso) haplotype C and carrot (Daucus carota subsp. sativus) were studied at 4, 5 and 9 weeks post inoculation (wpi), by combining dual RNA-Seq results with data on bacterial colonization and observations of the plant phenotype. In the infected plants, genes involved in jasmonate biosynthesis, salicylate signaling, PAMP and effector-triggered immunity and production of pathogenesis-related proteins were upregulated. At 4 wpi, terpenoid synthesis-related genes were upregulated, presumably as a response to the psyllid feeding, whereas at 5 and 9 wpi, genes involved in both the terpenoid and flavonoid production were downregulated and phenylpropanoid genes were upregulated. Chloroplast-related gene expression was downregulated, in concordance with the observed yellowing of the infected plant leaves. Both the RNA-Seq data and electron microscopy suggested callose accumulation in the infected phloem vessels, likely to impair the transport of photosynthates, while phloem regeneration was suggested by the formation of new sieve cells and the upregulation of cell wall-related gene expression. The CLso genes involved in replication, transcription and translation were expressed at high levels at 4 and 5 wpi, whereas at 9 wpi the Flp pilus genes were highly expressed, suggesting adherence and reduced mobility of the bacteria. The CLso genes encoding ATP and C4-dicarboxylate uptake were differentially expressed between the early and late infection stages, suggesting a change in the dependence on different host-derived energy sources. HPE1 effector and salicylate hydroxylase were expressed, presumably to suppress host cell death and salicylic acid-dependent defenses during the infection.