Molecular mechanisms underlying heat or tetracycline treatments for citrus HLB control

Fang Ding,Weiqi Luo,Shouan Zhang,Yongping Duan,Victoria Allen

doi:10.1038/s41438-018-0038-x

Abstract

Huanglongbing (HLB), a destructive plant bacterial disease, severely impedes worldwide citrus production. In our previous reports, we revealed the molecular mechanisms of host plant responses that underlie thermotherapy against HLB. In this study, we investigated the molecular mechanism underlying heat or tetracycline treatments on the HLB bacterium, ‘Candidatus Liberibacter asiaticus’ (Las) by focusing on Las prophage/phage conversion under stress conditions. By comparing the prophage FP1 and FP2 copy number to the copy number of 16S rDNA in HLB-affected plants, we found that the relative copy number of both FP1 and FP2 increased significantly, ranging from 3.4- to 6.7-fold change when Las-infected samples underwent a temperature shift from 23 to 37, 42 or 45 °C. When treated with tetracycline at 50–150 and 200–250 µg/ml, respectively, the relative copy number of both FP1 and FP2 increased by 3.4- to 6.0-fold. In addition, analyses of Las prophage structural gene and antirepressor gene copy numbers showed similar trends for all treatments. Furthermore, transmission electron microscopy provided direct evidence of lysogenic to lytic conversion upon temperature increase. These results not only provide new insight into the molecular mechanisms underlying heat or tetracycline treatment but also suggest a novel HLB control strategy by enhancing the endogenous conversion from Las prophages to phages.

Highlights

Citrus Huanglongbing (HLB), known as citrus greening, is one of the most destructive diseases that affects the citrus industry throughout the world
FP1 and FP2 copy number In order to investigate the mechanism of thermotherapy against Liberibacter asiaticus’ (Las), we first confirmed that the heat-treatment protocols used in this study resulted in the same outcome as the heat treatments from our previous study, despite slight temporal and temperature differences
We hypothesized that environmental stresses may trigger the induction of prophage in Las, reducing the viability and the study of Las is currently hindered by the inability to grow the bacterium in a pure culture, Las is believed to be a relatively heat-sensitive bacterium[43,44]

Summary

Introduction

Citrus Huanglongbing (HLB), known as citrus greening, is one of the most destructive diseases that affects the citrus industry throughout the world. The causal agents are insect-vectored, phloem-limited, and fastidious α-proteobacteria, including ‘Candidatus Liberibacter asiaticus’ (Las), ‘Ca. L. africanus’ (Laf), and ‘Ca. L. americanus’ (Lam)[1,2,3]. Diaphorina citri Kuwayama, Cacopsylla (Psylla) citrisuga (Hemiptera: Psyllidae)[4], and African citrus psyllid, Trioza erytreae (Del Guercio), are the main transmission vectors of HLB pathogen[2]. All known citrus species and cultivars regardless of rootstock or scion are susceptible to HLB5–8, Florida citrus varieties showed different sensitivities to HLB bacteria[9]. Because of the systemic distribution of Las in infected plants, HLB is extremely difficult to cure

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Conclusion