Identification of Gene Candidates Associated with Huanglongbing Tolerance, Using 'Candidatus Liberibacter asiaticus' Flagellin 22 as a Proxy to Challenge Citrus.

The 22-amino acid (flg22) pathogen-associated molecular pattern from the flagellin of Xanthomonas citri subsp. citri has been shown to induce defense responses correlated with citrus canker resistance. Here, flg22 of 'Candidatus Liberibacter asiaticus', the putative causal agent of Huanglongbing (HLB), elicited differential defense responses that were weaker than those from Xcc-flg22, between those of the HLB-tolerant mandarin cultivar Sun Chu Sha and susceptible grapefruit cultivar Duncan. Transcriptomics was used to compare the effect of CLas-flg22 and Xcc-flg22 between the citrus genotypes and identified 86 genes induced only by CLas-flg22 in the tolerant mandarin. Expression of 16 selected genes was validated, by reverse transcription-quantitative polymerase chain reaction, and was evaluated in citrus during 'Ca. L. asiaticus' infection. Differential expression of a number of genes occurred between tolerant and susceptible citrus infected with 'Ca. L. asiaticus', suggesting their involvement in HLB tolerance. In addition, several genes were similarly regulated by CLas-flg22 and 'Ca. L. asiaticus' treatments, while others were oppositely regulated in the tolerant mandarin, suggesting similarity and interplay between CLas-flg22 and 'Ca. L. asiaticus'-triggered defenses. Genes identified are valuable in furthering the study of HLB tolerance mechanisms and, potentially, for screening for HLB-tolerant citrus using CLas-flg22 as a pathogen proxy.