Abstract

Asymptomatic infections are by their nature challenging to study and even more difficult to monitor across broad geographical ranges, particularly as methods are reliant on expensive molecular techniques. The plant pathogen that causes Witches’ Broom disease of lime (Candidatus Phytoplasma aurantifolia) is a major limiting factor in lime production across the Middle East and was recently detected in Brazil, but without the typical symptoms from the Middle East. Here, we discuss the difficulty of monitoring asymptomatic infections and highlight the threat posed by highlight future outbreaks. Asymptomatic infections have important implications for understanding the evolution of pathogens within perennial hosts. We use three model systems of asymptomatic infections: (i) a Phytoplasma and (ii) a bacterial infection of lime (Citrus aurantifolia) and (iii) an “out-group” Phytoplasma of Cassava (Manihot esculenta) to demonstrate consistency across divergent hosts. We found that although all plants in the study were intentionally infected, assays typically did not confirm this diagnosis. Emergent technologies monitoring gene expression could be used to both study novel biology associated with asymptomatic infections and develop monitoring technologies. We highlight the difficulty of monitoring asymptomatic infections in possible future outbreaks and have important implications for understanding the evolution of pathogens within perennial hosts.

Highlights

  • Vector-borne plant pathogens of perennial crop species provide an opportunity to study the impacts of long-term infections, in terms of epidemiology and vector ecology

  • As the host plant remains in situ after harvest, perennial cropping systems theoretically allow for evolution of slower pathologies, which may be cryptic in nature

  • This study has addressed two key questions regarding the nature of asymptomatic infections: [1] that being invisible or “silent” infections makes them inherently challenging to monitor; and [2] that this organism interacts with its plant host in a distinct manner that we have observed in the present study

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Summary

Introduction

Vector-borne plant pathogens of perennial crop species provide an opportunity to study the impacts of long-term infections, in terms of epidemiology and vector ecology. This silent infection was observed through molecular testing of plant material, yet the host plants themselves show no obvious visible symptoms These infected trees do collapse within the 5 year post infection period [13], making this asymptomatic variant potentially even more of a threat to global lime production. Detailed research into this system has been limited, some suggest that the silent infection may be due to ultra-low pathogen titre levels within the host plant [12, 14] or due to different interactions with plant defences [15] or insect vectors [16, 17]. These findings provide an important and novel examination of the nature of asymptomatic infections, a poorly understood, emerging area of plant pathology

Pathogen detection in the absence of visible symptoms: study system
Sample locations
Plant material
Molecular detection of Phytoplasma
Pathogen detection in the absence of visible symptoms: results and discussion
Novel asymptomatic infection biology: study system
RNA extraction
Gene expression
Statistical analysis Analyses of differential gene expression in asymptomatic
Novel asymptomatic infection biology: results and discussion
Findings
Conclusions

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