United States Horticultural Research Laboratory, United States Department of Agriculture-Agricultural Research Service, Fort Pierce, Florida, United States of America ; School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai, China.
PLoS One. 2012;7(9):e46447. doi: 10.1371/journal.pone.0046447. Epub 2012 Sep 28.
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.
黄龙病（HLB）是目前世界上最具破坏性的柑橘病害。作为一种细胞内植物病原体和昆虫共生体，HLB 细菌“亚洲韧皮杆菌”（Las）在其显著缩小的基因组中保留了整个鞭毛编码基因簇。Las 编码 452 个氨基酸的鞭毛蛋白和钩相关蛋白（Fla），其 N 端 29 到 50 位含有一个保守的 22 个氨基酸结构域（flg22）。缺失 fla 基因的 Sinorhizobium meliloti 突变体的运动表型改变，通过组成型表达 Fla（Las）得到部分恢复。农杆菌介导的瞬时表达在体内表明，Fla（Las）在 Nicotiana benthamiana 中诱导细胞死亡和胼胝质沉积，并且与植物先天免疫相关的 BAK1 和 SGT1 的转录被上调。氨基酸取代实验表明，Fla（Las）的 38 位（丝氨酸）和 39 位（天冬氨酸）残基对于诱导胼胝质沉积是必需的。合成的 flg22（Las）肽不能诱导植物细胞死亡，但在 20µM 或更高浓度下仍保留诱导胼胝质沉积的能力。这表明 flg22 在 Las 中的病原体相关分子模式（PAMP）活性比其他研究充分的植物病原菌弱。这些结果表明，Fla（Las）作为一种 PAMP，可能在触发宿主植物对 HLB 细菌的抗性方面发挥重要作用。
