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Unravelling Citrus Huanglongbing Disease

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Front. Plant Sci. | doi: 10.3389/fpls.2018.01694

Construction of high-density genetic maps and detection of QTLs associated with Huanglongbing tolerance in citrus

 Ming Huang1,  Mikeal L. Roose2,  Qibin Yu1, Dongliang Du1,  Yuan Yu1, Yi Zhang1,  Zhanao Deng3, Ed Stover4 and  Frederick G. Gmitter1*
  • 1Citrus Research and Education Center, University of Florida, United States
  • 2Department of Botany and Plant Sciences, University of California, Riverside, United States
  • 3Gulf Coast Research and Education Center, University of Florida, United States
  • 4Agricultural Research Service, United States Department of Agriculture, U.S. Horticultural Research Laboratory, United States

Huanglongbing (HLB), or citrus greening, is the most devastating disease in citrus worldwide. Commercial citrus varieties including sweet orange (Citrus sinensis) are highly susceptible to HLB, and trifoliate orange (Poncirus trifoliata, a close Citrus relative) is widely considered resistant or highly tolerant to HLB. In this study, an intergeneric F1 population of sweet orange and trifoliate orange was genotyped by Genotyping-by-Sequencing, and high-density SNP-based genetic maps were constructed separately for trifoliate orange and sweet orange. The two genetic maps exhibited high synteny and high coverage of the citrus genome. Progenies of the F1 population and their parents were planted in a replicated field trial, exposed to intense HLB pressure for three years, and then evaluated for susceptibility to HLB over two years. The F1 population exhibited a wide range in severity of HLB foliar symptom and canopy damage. Genome-wide QTL analysis based on the phenotypic data of foliar symptom and canopy damage in two years identified three clusters of repeatable QTLs in trifoliate orange linkage groups LG-t6, LG-t8 and LG-t9. Co-localization of QTLs for two traits was observed within all three regions. Additionally, one cluster of QTLs in sweet orange (linkage group LG-s7) was also detected. The majority of the identified QTLs each explained 18% to 30% of the phenotypic variation, indicating their major role in determining HLB responses. These results show, for the first time, a quantitative genetic nature yet the presence of major loci for the HLB tolerance in trifoliate orange. The results suggest that sweet orange also contains useful genetic factor(s) for improving HLB tolerance in commercial citrus varieties. Findings from this study should be very valuable and timely to researchers worldwide as they are hastily searching for genetic solutions to the devastating HLB crisis through breeding, genetic engineering, or genome editing.

Keywords: Candidatus Liberibacter, Genetic map, QTL mapping, F1 population, genotyping-by-sequencing, Sweet orange (C. sinensis), Poncirus trifoliata, SNP

Received: 31 Jul 2018; Accepted: 31 Oct 2018.

Edited by:

Christopher I. Vincent, University of Florida, United States

Reviewed by:

Antonio Figueira, University of São Paulo, Brazil
Luis Felipe V. Ferrão, University of Florida, United States  

Copyright: © 2018 Huang, Roose, Yu, Du, Yu, Zhang, Deng, Stover and Gmitter. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Frederick G. Gmitter, Citrus Research and Education Center, University of Florida, Lake Alfred, 33850, Florida, United States, fgmitter@ufl.edu