Université de Lyon, Lyon F-69003, France.
Environ Microbiol. 2010 Dec;12(12):3290-301. doi: 10.1111/j.1462-2920.2010.02305.x.
The plant pathogenic bacteria Dickeya dadantii is also a pathogen of the pea aphid Acyrthosiphon pisum. The genome of the bacteria contains four cyt genes, encoding homologues of Bacillus thuringiensis Cyt toxins, which are involved in its pathogenicity to insects. We show here that these genes are transcribed as an operon, and we determined the conditions necessary for their expression. Their expression is induced at high temperature and at an osmolarity equivalent to that found in the plant phloem sap. The regulators of cyt genes have also been identified: their expression is repressed by H-NS and VfmE and activated by PecS. These genes are already known to regulate plant virulence factors, but in an opposite way. When tested in a virulence assay by ingestion, the pecS mutant was almost non-pathogenic while hns and vfmE mutants behaved in the same way as the wild-type strain. Mutants of other regulators of plant virulence, GacA, OmpR and PhoP, that do not control Cyt toxin production, also showed reduced pathogenicity. In an assay by injection of bacteria, the gacA strain was less pathogenic but, surprisingly, the pecS mutant was slightly more virulent. These results show that Cyt toxins are not the only virulence factors required to kill aphids, and that these factors act at different stages of the infection. Moreover, their production is controlled by general virulence regulators known for their role in plant virulence. This integration could indicate that virulence towards insects is a normal mode of life for D. dadantii.
植物病原细菌迪凯氏菌也是豌豆蚜 Acyrthosiphon pisum 的病原体。该细菌的基因组包含四个 cyt 基因，编码苏云金芽孢杆菌 Cyt 毒素的同源物，这些基因参与了其对昆虫的致病性。我们在这里表明，这些基因作为一个操纵子转录，并且我们确定了它们表达的必要条件。它们的表达在高温和与植物韧皮部汁液等渗的渗透压条件下被诱导。Cyt 基因的调控因子也已被确定：它们的表达受到 H-NS 和 VfmE 的抑制，受到 PecS 的激活。这些基因已经被证实可以调节植物毒力因子，但方式相反。在通过摄入进行的毒力测定中，pecS 突变体几乎没有致病性，而 hns 和 vfmE 突变体的行为与野生型菌株相同。其他植物毒力调控因子 GacA、OmpR 和 PhoP 的突变体，它们不控制 Cyt 毒素的产生，也表现出降低的致病性。在细菌注射测定中，gacA 菌株的致病性较低，但令人惊讶的是，pecS 突变体的毒性略有增加。这些结果表明，Cyt 毒素不是杀死蚜虫所需的唯一毒力因子，这些因子在感染的不同阶段发挥作用。此外，它们的产生受到一般毒力调控因子的控制，这些因子因其在植物毒力中的作用而被熟知。这种整合可能表明，对昆虫的毒力是 D. dadantii 的一种正常生活方式。
