Candidatus Liberibacter

Event Date: 
Wednesday, March 25, 2015 - 18:15 - 18:30
Institution: 
University of Florida / UWS
Title: 

Candidatus Liberibacter asiaticus encodes a functional salicylic acid hydroxylase which degrades SA and contributes to the suppression of plant defence

Abstract: 

Salicylate (SA) is a plant hormone and plays important roles in plant defence. SA is synthesized in the chloroplast and transmitted in the phloem. SA hydroxylase is a flavoprotein monooxygenase with the enzyme activity of degradation of SA and is a proximal component of the naphthalene degradation pathway in many bacteria. Candidatus Liberibacter asiaticus, the causal agent of the most devastating citrus disease, is phloem limited and encodes a SA hydroxylase. In this study, we have shown that the SA hydroxylase is functional in degrading SA and its analogs. Ca. L. asiaticus infected plants have reduced PR gene (PR1, PR2, and PR5) expression and SA accumulation in Duncan grapefruit and Valencia sweet orange in response to subsequent inoculation with Xanthomonas citri subsp. citri (Xac) Aw, which is nonpathogenic on both citrus varieties. Ca. L. asiaticus also increased citrus susceptibility to subsequent infection by X. citri. The bacterial populations of XacA and XacAw in grapefruit were significantly higher in Ca. L. asiaticus infected plants compared to healthy control. Our data suggest that Ca. L. asiaticus encodes a functional salicylic acid hydroxylase which degrades SA and contributes to the suppression of plant defence. To counteract this virulence mechanism of Ca. L. asiaticus, foliar spray of SA analogs 2, 6-Dichloroisonicotinic acid (INA) and 2,1,3-Benzothiadiazole (BTH) and SA producing bacterial isolates was conducted to control HLB in large scale field trials. Both INA and BTH in combination with selected bacterial strains slowed down the increase of Ca. L. asiaticus titers in planta and HLB disease severity compared to negative control. SA hydroxylase seems to be an ideal target to develop small molecule inhibitors since no human homolog is present and it is not essential for bacterial growth, hence, the possibility of resistance development is minimized.      Salicylate (SA) is a plant hormone and plays important roles in plant defence. SA is synthesized in the chloroplast and transmitted in the phloem. SA hydroxylase is a flavoprotein monooxygenase with the enzyme activity of degradation of SA and is a proximal component of the naphthalene degradation pathway in many bacteria. Candidatus Liberibacter asiaticus, the causal agent of the most devastating citrus disease, is phloem limited and encodes a SA hydroxylase. In this study, we have shown that the SA hydroxylase is functional in degrading SA and its analogs. Ca. L. asiaticus infected plants have reduced PR gene (PR1, PR2, and PR5) expression and SA accumulation in Duncan grapefruit and Valencia sweet orange in response to subsequent inoculation with Xanthomonas citri subsp. citri (Xac) Aw, which is nonpathogenic on both citrus varieties. Ca. L. asiaticus also increased citrus susceptibility to subsequent infection by X. citri. The bacterial populations of XacA and XacAw in grapefruit were significantly higher in Ca. L. asiaticus infected plants compared to healthy control. Our data suggest that Ca. L. asiaticus encodes a functional salicylic acid hydroxylase which degrades SA and contributes to the suppression of plant defence. To counteract this virulence mechanism of Ca. L. asiaticus, foliar spray of SA analogs 2, 6-Dichloroisonicotinic acid (INA) and 2,1,3-Benzothiadiazole (BTH) and SA producing bacterial isolates was conducted to control HLB in large scale field trials. Both INA and BTH in combination with selected bacterial strains slowed down the increase of Ca. L. asiaticus titers in planta and HLB disease severity compared to negative control. SA hydroxylase seems to be an ideal target to develop small molecule inhibitors since no human homolog is present and it is not essential for bacterial growth, hence, the possibility of resistance development is minimized.      

Event Date: 
Wednesday, March 27, 2013 - 18:15 - 18:30
Institution: 
Hawkesbury Institute for the Environment, University of Western Sydney
Title: 

Effect of Huanglongbing on the structure and functional diversity of microbial communities associated with citrus.

Abstract: 

Plant-microbe interactions lie at the heart of plant performance and ecology. It has been postulated that disruption of multi-trophic interactions in a stable ecosystem under the influence of invading phytopathogens will cause community reorganization and changes in the local feedback interactions. However, there is a paucity of knowledge on the extent to which such community shifts may occur, on the dynamics of changes and on the putative effects regarding the functioning of ecosystems. We have used Citrus-‘Candidatus Liberibacter asiaticus’ [Las, causal agent of devastating Huanglongbing (HLB) disease] as a host pathogen model to characterize the structure, function and interactions of plant-associated microbial communities. We applied a suit of metagenomic techniques to provide detailed census of citrus associated microbiomes. Our results confirmed that Las is the sole causal agent of HLB in Florida and revealed that HLB significantly re-structures the composition of native microbial community present either in leaf, roots and rhizosphere of citrus. Functional microarray (Geochip) and shotgun metagenomic sequencing showed that HLB has severe effects on various functional guilds of bacteria involved in key ecological processes including nitrogen cycling and carbon fixation. Overall, the metagenomic studies provided evidence that change in plant physiology mediated by Las infection could elicit shifts in the composition and functional potential of plant associated microbial communities. In the long term, these fluctuations might have important implications for the productivity and sustainability of citrus producing agro-ecosystems.

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