Abstract
Citrus greening, also known as Huanglongbing (HLB), is a devastating citrus plant disease caused predominantly by Liberibacter asiaticus. While nearly all Liberibacter species remain uncultured, here we used the culturable L. crescens BT-1 as a model to examine physiological changes in response to the variable osmotic conditions and nutrient availability encountered within the citrus host. Similarly, physiological responses to changes in growth temperature and dimethyl sulfoxide concentrations were also examined, due to their use in many of the currently employed therapies to control the spread of HLB. Sublethal heat stress was found to induce the expression of genes related to tryptophan biosynthesis, while repressing the expression of ribosomal proteins. Osmotic stress induces expression of transcriptional regulators involved in expression of extracellular structures, while repressing the biosynthesis of fatty acids and aromatic amino acids. The effects of osmotic stress were further evaluated by quantifying biofilm formation of L. crescens in presence of increasing sucrose concentrations at different stages of biofilm formation, where sucrose-induced osmotic stress delayed initial cell attachment while enhancing long-term biofilm viability. Our findings revealed that exposure to osmotic stress is a significant contributing factor to the long term survival of L. crescens and, possibly, to the pathogenicity of other Liberibacter species.
Highlights
Citrus greening has become one of the most devastating citrus diseases in the world
While the mechanism which enables tolerance to these extreme changes in environmental conditions is yet to be fully elucidated, we show that long-term osmotic stress is a significant driving factor behind changes in gene expression that facilitates cell adhesion, biofilm formation, and increased longterm viability in L. crescens
While L. asiaticus does not maintain a canonical system for osmoregulation, the changes seen in L. crescens provide a steppingstone in the understanding of such complex system
Summary
Citrus greening has become one of the most devastating citrus diseases in the world. Known at Huanglongbing (HLB), once infected with this pathogen, there currently is no cure. Considered the causative agent, Liberibacter asiaticus is transferred from the psyllid vector to the citrus host during feeding on the phloem sap. Due to the inability to sustain an axenic culture, there is limited understanding of this phloem-restricted Alphaproteobacteria’s physiology [1,2,3]. Most Liberibacter species remain uncultured, L. crescens BT-1 can be maintained in culture and shares 77.4% nucleotide identity to L. asiaticus, with 832 homologous genes. L. crescens BT-1 has been thoroughly used as a surrogate strain to provide insights on L. asiaticus physiology [4, 5]
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