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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 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 FlaLas acts as a PAMP and may play an important role in triggering host plant resistance to the HLB bacteria.
Highlights
Huanglongbing (HLB, known as citrus greening) is a devastating disease of citrus
We demonstrated that Liberibacter asiaticus’ (Las) encodes a functional flagellin that acts as a pathogen-associated molecular pattern (PAMP), by expressing the Las fla gene in S. meliloti and through Agrobacterium-mediated transient expression
The structure of the bacterial flagellum consists of three distinct parts: a basal body serving as a rotary motor, a rigid filament, and a flexible hook that couples the motor to the filament [23]
Summary
Huanglongbing (HLB, known as citrus greening) is a devastating disease of citrus. The prevalent species of HLB bacteria, ‘Candidatus Liberibacter asiaticus’ (Las) is a Gramnegative bacterium with a significantly reduced genome (1.23 Mb) [6]. As an insect-transmitted and obligate plant pathogen, Las attacks all citrus species and citrus hybrids in the genus of Citrus, causing a systemic disease by residing in the phloem of the plant hosts [7,8,9]. There is no highly resistant cultivar, some resistance or field tolerance to HLB within citrus and citrus relatives has been described [7,8,9]. Different isolates of Las cause different levels of disease in citrus cultivars [11]
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