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Title: Identification of huanglongbing tolerance-associated genes using Candidatus Liberibacter asiaticus flagellin 22 as a proxy to challenge citrus

Author
item SHI, QINGCHUN
item FEBRES, VICENTE - UNIVERSITY OF FLORIDA
item ZHANG, SHUJIAN
item YU, FAHONG - UNIVERSITY OF FLORIDA
item MCCOLLUM, THOMAS
item HALL, DAVID
item MOORE, GLORIA - UNIVERSITY OF FLORIDA
item STOVER, EDDIE

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2017
Publication Date: 2/1/2018
Citation: Shi, Q., Febres, V.J., Zhang, S., Yu, F., McCollum, T.G., Hall, D.G., Moore, G.A., Stover, E. 2018. Identification of huanglongbing tolerance-associated genes using Candidatus Liberibacter asiaticus flagellin 22 as a proxy to challenge citrus. Molecular Plant-Microbe Interactions. 31:200-211.

Interpretive Summary: Huanglongbing (HLB) disease tolerance is a highly desirable trait for breeding better citrus types. However, the identification of tolerance is a lengthy processes because the development of disease symptoms is slow. In this study, we used a recent technology called RNA sequencing to analyze all of the citrus genes and compare between HLB tolerant and sensitive citrus. This allowed us to find a group of genes possibly associated with HLB tolerance. The identified genes may work as markers to accelerate the screening process of HLB tolerance.

Technical Abstract: Plant defense elicited by pathogen-associated molecular patterns (PAMPs) is an important component of disease resistance. Previous research indicated the canker resistance in citrus correlates with responsiveness to Xcc-flg22, the 22 amino acid PAMP from the flagellin of Xanthomonas citri ssp. citri (Xcc), the causal agent of citrus canker (Shi, Febres et al. 2015). In this study, the flg22 of Candidatus Liberibacter asiaticus (CLas), the putative pathogen of Huanglongbing (HLB), was analyzed for defense-eliciting activities in citrus. CLas-flg22 induced defense-associated genes including EDR1, EDS1, NDR1, PBS1, RAR1, SGT1, PAL1, AZI1, NPR2, NPR3 and RdRp1 in the HLB tolerant ‘Sun Chu Sha’ mandarin but not in the HLB susceptible ‘Duncan’ grapefruit. In comparison to Xcc-flg22, PAMP-triggered immunity (PTI) marker genes (WRKY22 and GST1) and flagellin receptor genes (FLS2-1 and FLS2-2) were less induced by CLas-flg22 and no reactive oxygen species (ROS) production was observed following treatment with CLas-flg22. Transcriptomics (RNA-seq) was used to compare the effect of CLas-flg22 and Xcc-flg22 between ‘Sun Chu Sha’ mandarin and ‘Duncan’ grapefruit, and identified 86 genes with differential expression with CLas-flg22 that was not present with Xcc-flg22 in ‘Sun Chu Sha’ mandarin. Validation of 16 selected genes by RT-qPCR indicated consistency with the RNA-seq data, and the expression of these genes was subsequently evaluated in citrus during CLas infection. Differential expression of a number of genes were observed between the two citrus genotypes under CLas inoculation, suggesting their involvement in HLB tolerance. In addition, several genes were similarly regulated by the CLas-flg22 and CLas treatments, while others were oppositely regulated in the tolerant mandarin genotype, providing evidence on the similarity between CLas-flg22 and CLas triggered defense, as well as the interplay between host defense activation and suppression by the pathogen. These genes identified are valuable in furthering the study of HLB tolerance mechanisms and may have potential for screening for HLB-tolerant citrus materials using CLas-flg22 as a pathogen proxy.