Overexpression of the Arabidopsis NPR1 protein in citrus confers tolerance to Huanglongbing

Cecile J Robertson,William O Dawson,Siddarame Gowda,Xudong Zhang,Zhonglin Mou,Vladimir Orbović

doi:10.5070/c451038911

Abstract

Author(s): Robertson, Cecile J; Zhang, Xudong; Gowda, Siddarame; Orbovic, Vladimir; Dawson, William O; Mou, Zhonglin | Abstract: Huanglongbing (HLB), also known as citrus greening, is one of the most destructive diseases of citrus worldwide. It is caused by unculturable phloem-limited bacteria that belong to the Candidatus Liberibacter genus including Ca. L. asiaticus (CLas), Ca. L. africanus, and Ca. L. americanus. Currently, there is no effective control strategy for HLB and no known cure for the disease. We have previously generated transgenic ‘Duncan’ grapefruit and ‘Hamlin’ sweet orange expressing the Arabidopsis NPR1 (AtNPR1) gene, which encodes a master regulator of systemic acquired resistance. Characterization of the transgenic lines indicated that overexpression of AtNPR1 confers resistance to citrus canker, another serious bacterial disease in citrus. In this study, we intensively screened these transgenic lines for resistance or tolerance to HLB under greenhouse conditions. Three independent transgenic lines (one line in the ‘Duncan’ grapefruit background and two lines in the ‘Hamlin’ sweet orange background) only occasionally displayed mild HLB symptoms in the presence of C Las and have kept growing normally over a period of nine years. Significantly, all vegetatively propagated progeny plants of these lines have retained the same levels of HLB tolerance. Furthermore, immunoblot analysis revealed that the three transgenic lines accumulate high levels of AtNPR1 protein. These results, together with the previous finding that the same three transgenic lines are resistant to citrus canker, demonstrate that overexpression of the AtNPR1 protein in citrus is able to provide robust tolerance to HLB.

Highlights

Huanglongbing (HLB), known as citrus greening, is one of the world’s most devastating citrus diseases
We have previously shown that transgenic citrus plants overexpressing the Arabidopsis thaliana NPR1 (AtNPR1) gene exhibited increased resistance to citrus canker (Zhang et al 2010)
To test if overexpression of AtNPR1 could provide resistance or tolerance to HLB, we evaluated HLB responses of the transgenic lines by psyllid inoculation of HLB under high levels of inoculum pressure

Summary

Introduction

Huanglongbing (HLB), known as citrus greening, is one of the world’s most devastating citrus diseases. In areas affected by HLB, the average productive lifespan of citrus trees drops dramatically from 50 or more years to 15 or less. HLB is causing significant damage in citrus production areas. Psyllids move quickly when disturbed and can fly at least 1.25 miles (Moffis et al 2016). They can be carried over long distances by wind, which significantly accelerates the spread of HLB. There is currently no effective control measure for HLB and no known cure for the disease

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Results

Conclusion