ARS | Publication request: The destructive citrus pathogen, ‘Candidatus Liberibacter asiaticus’ encodes a functional flagellin characteristic of a pathogen-associated molecular pattern

Submitted to: American Phytopathological Society Annual Meeting
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 12, 2012
Publication Date: N/A

Interpretive Summary: The citrus disease Huanglongbing (HLB) has spread worldwide and is economically devastating the citrus industry. The bacterium associated with the major form of the disease is ‘Candidatus Liberibacter asiaticus’ (Las). Even though it has a very small genome, this intracellular plant pathogen and insect symbiont has retained the entire flagellum-encoding gene cluster. Las encodes a flagellin and hook-associated protein (Fla) that contains a conserved 22 amino acid domain (flg22). This domain is characterized as a plant bacterial pathogen-associated molecular pattern (PAMP) with is recognized by pathogen recognition receptors as part of the plants basal defense against pathogen attack. A Sinorhizobium meliloti mutant lacking the fla genes had its motility partially restored by constitutive expression of FlaLas. In planta transient expression revealed that FlaLas induced cell death and callose deposition in Nicotiana benthamiana, and that some transcription factors associated with plant innate immunity were upregulated. A synthetic flg22Las peptide could not induce plant cell death but retained the ability to induce callose deposition at high concentrations. These results indicate that, while the activity of flg22 in FlaLas was weaker than in other well-studied plant pathogenic bacteria, it still acts as a PAMP and may play an important role in triggering host plant resistance to the HLB bacteria.

Technical Abstract: 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 FlaLas. Agrobacterium-mediated transient expression in planta revealed that FlaLas 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 FlaLas were essential for callose induction. The synthetic flg22Las peptide could not induce plant cell death but retained the ability to induce callose deposition at a concentration of 20 µM or above. This indicated that the pathogen-associated molecular pattern (PAMP) activity of flg22 in Las was weaker than in other well-studied plant pathogenic bacteria. These results indicate that FlaLas acts as a PAMP and may play an important role in triggering host plant resistance to the HLB bacteria.