Potential soil transmission of a novel Candidatus Liberibacter strain detected in citrus seedlings grown in soil from a huanglongbing infested citrus grove

Background

The bacteria Candidatus Liberibacter spp. reportedly causes Huanglongbing (HLB) that is a disease of citrus plants. The symptoms of HLB are yellow blotches on the leaves, droopy leaves & branches and ultimately death of the whole plant.

Ca. Liberibacter is transmitted by psyllids, which are phloem feeding insects that potentially transmit the bacteria while feeding on the plant tissues. Despite the destruction of the infected plants, subsequent replanting of disease-free plants still caused HLB associated symptoms in the new plants. It is not however entirely clear whether HLB associated  Ca Liberibacter is soil-borne.

 

Summary picture of the preprint

The data in this preprint suggest that bacteria are transmitted to the healthy plants upon replanting via the soil particles associated with previously infected plants. The researchers used soil samples from two sites which were reportedly infected with HLB back in 2009-11. The soil samples were broadly divided into 2 parts: sterile (via autoclaving) and non-sterile. Citrus seedlings were then grown and their growth monitored for 1 year in green-house in the absence of any kind of insects. After around 8.5 months, the researchers observed prominent HLB related symptoms in the plants growing on soil from one of the sites. Using the plant parts that showed HLB symptoms, the authors confirmed the presence of the Ca Liberibacter spp. using molecular methods. Moreover, according to the authors, phylogenetic analysis has revealed a novel strain of Ca Liberibacter.

 

Possible implication of this preprint

This article tries to ask a basic question on the transmission mode of an uncultured bacterium. It also attempts to find how a plant pathogen infects a plant in absence of its insect vector.  The idea in this preprint opens up avenues for studies dealing with replant diseases.

 

Open questions

This article further conveys the message towards understanding the disease transmission mode.

• During collection of the soil samples from the disease infested sites, did the authors also check for the presence of HLB associated Ca Liberibacter in the original location (i.e. grove B)?
• Finally, when the HLB associated symptoms were observed on the citrus seedlings grown on the non autoclaved soil, did the authors attempt to check for the Ca Liberibacter presence (again!)?
• Would it be sufficient to conclude that the putative Ca Liberibacter is indeed a novel strain based on the amplicon size (difference), cloning, phylogenetic analysis and quantitative PCR?

 

Reference

• da Rocha, U. N., Shin, K., Timilsina, S., Jones, J. B., Singer, B. H., & Van Bruggen, A. H. (2019). Potential soil transmission of a novel Candidatus Liberibacter strain detected in citrus seedlings grown in soil from a huanglongbing infested citrus grove. bioRxiv, 821553.
• Picture source: Unsplash

Tags: bacteria, insect vector, liberibacter, microbes, plant pathogen, preprint

Posted on: 9 November 2019

doi: https://doi.org/10.1242/prelights.15101

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