Transcriptome of Dickeya dadantii Infecting Acyrthosiphon pisum Reveals a Strong Defense against Antimicrobial Peptides

The plant pathogenic bacterium Dickeya dadantii has recently been shown to be able to kill the aphid Acyrthosiphon pisum. While the factors required to cause plant disease are now well characterized, those required for insect pathogeny remain mostly unknown. To identify these factors, we analyzed the transcriptome of the bacteria isolated from infected aphids. More than 150 genes were upregulated and 300 downregulated more than 5-fold at 3 days post infection. No homologue to known toxin genes could be identified in the upregulated genes. The upregulated genes reflect the response of the bacteria to the conditions encountered inside aphids. While only a few genes involved in the response to oxidative stress were induced, a strong defense against antimicrobial peptides (AMP) was induced. Expression of a great number of efflux proteins and transporters was increased. Besides the genes involved in LPS modification by addition of 4-aminoarabinose (the arnBCADTEF operon) and phosphoethanolamine (pmrC, eptB) usually induced in Gram negative bacteria in response to AMPs, dltBAC and pbpG genes, which confer Gram positive bacteria resistance to AMPs by adding alanine to teichoic acids, were also induced. Both types of modification confer D. dadantii resistance to the AMP polymyxin. A. pisum harbors symbiotic bacteria and it is thought that it has a very limited immune system to maintain these populations and do not synthesize AMPs. The arnB mutant was less pathogenic to A. pisum, which suggests that, in contrast to what has been supposed, aphids do synthesize AMP.

近期研究证实，植物病原细菌狄氏迪基氏菌（Dickeya dadantii）能够致死豌豆蚜（Acyrthosiphon pisum）。尽管目前学界已充分阐明其引发植物病害所需的致病因子，但其作为昆虫病原所需的致病因子仍大多未被探明。为鉴定此类致病因子，本研究对从受感染豌豆蚜体内分离得到的该细菌进行了转录组分析。感染后3天时，共有超过150个基因呈上调表达、300个基因呈下调表达，且表达差异倍数均超过5倍。上调表达的基因中未发现与已知毒素基因同源的序列。上调基因反映了该细菌对豌豆蚜体内生存环境的应激响应。尽管仅少量参与氧化应激响应的基因被诱导表达，但针对抗菌肽（antimicrobial peptides, AMP）的防御通路被显著激活。大量外排蛋白与转运蛋白的表达水平显著升高。除了革兰氏阴性菌通常在应对抗菌肽时会被诱导表达的、参与脂多糖（lipopolysaccharide, LPS）修饰的相关基因——包括通过添加4-阿拉伯糖胺修饰脂多糖的arnBCADTEF操纵子，以及通过添加磷酸乙醇胺修饰脂多糖的pmrC、eptB基因之外，dltBAC与pbpG基因同样被诱导表达；这类基因可通过为磷壁酸添加丙氨酸，赋予革兰氏阳性菌抗菌肽抗性。上述两类修饰均可使狄氏迪基氏菌获得对抗抗菌肽多粘菌素（polymyxin）的抗性。豌豆蚜体内定植有共生细菌，此前学界认为其免疫系统极为有限，仅能维持共生菌群的存活，且无法合成抗菌肽。而arnB基因缺失突变体对豌豆蚜的致病力显著下降，这表明与此前的推测相反，豌豆蚜实则能够合成抗菌肽。