Destructive Citrus Pathogen, ‘Candidatus Liberibacter asiaticus’ Encodes a Functional Flagellin Characteristic of a Pathogen-Associated Molecular Pattern

Huanglongbing (HLB) is presently the most devastating citrus disease worldwide. As an intracellular plant pathogen and insect symbiont, the HLB bacterium, ‘ Candidatus Liberibacter asiaticus’ (Las), retains the entire flagellum-encoding gene cluster in its significantly reduced genome. Las encodes a...

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Published inPloS one Vol. 7; no. 9; p. e46447
Main Authors Zou, Huasong, Gowda, Siddarame, Zhou, Lijuan, Hajeri, Subhas, Chen, Gongyou, Duan, Yongping, Yang, Ching-Hong
Format Journal Article
LanguageEnglish
Published United States Public Library of Science 28.09.2012
Public Library of Science (PLoS)
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Summary:Huanglongbing (HLB) is presently the most devastating citrus disease worldwide. As an intracellular plant pathogen and insect symbiont, the HLB bacterium, ‘ Candidatus Liberibacter asiaticus’ (Las), retains the entire flagellum-encoding gene cluster in its significantly reduced genome. Las encodes a flagellin and hook-associated protein (Fla) of 452 amino acids that contains a conserved 22 amino acid domain (flg22) at positions 29 to 50 in the N-terminus. The phenotypic alteration in motility of a Sinorhizobium meliloti mutant lacking the fla genes was partially restored by constitutive expression of Fla Las . Agrobacterium -mediated transient expression in planta revealed that Fla Las induced cell death and callose deposition in Nicotiana benthamiana , and that the transcription of BAK1 and SGT1 , which are associated with plant innate immunity, was upregulated. Amino acid substitution experiments revealed that residues 38 (serine) and 39 (aspartate) of Fla Las were essential for callose induction. The synthetic flg22 Las peptide could not induce plant cell death but retained the ability to induce callose deposition at a concentration of 20 µM or above. This demonstrated that the pathogen-associated molecular pattern (PAMP) activity of flg22 in Las was weaker than those in other well-studied plant pathogenic bacteria. These results indicate that Fla Las acts as a PAMP and may play an important role in triggering host plant resistance to the HLB bacteria.
Bibliography:http://dx.doi.org/10.1371/journal.pone.0046447
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Competing Interests: The authors have declared that no competing interests exist.
Conceived and designed the experiments: YPD HSZ. Performed the experiments: HSZ SG LJZ SH GYC. Analyzed the data: HSZ YPD. Contributed reagents/materials/analysis tools: HSZ LJZ. Wrote the paper: HSZ YPD.
ISSN:1932-6203
1932-6203
DOI:10.1371/journal.pone.0046447