Minerdi Daniela, Moretti Marino, Gilardi Giovanna, Barberio Claudia, Gullino Maria Lodovica, Garibaldi Angelo
Centre of Competence for the Innovation in the Agro-Environmental field, Agroinnova, University of Torino, via Leonardo da Vinci, 44, 10095 Grugliasco, Italy.
Environ Microbiol. 2008 Jul;10(7):1725-41. doi: 10.1111/j.1462-2920.2008.01594.x. Epub 2007 Apr 4.
In the present article we have ascertained the presence of a consortium of ectosymbiotic bacteria belonging to Serratia, Achromobacter, Bacillus and Stenotrophomonas genera associated to the mycelium of the antagonistic Fusarium oxysporum MSA 35 [wild-type (WT) strain]. Morphological characterization carried out on the WT strain, on the F. oxysporum MSA 35 without ectosymbionts [cured (CU) strain] and on the pathogenic F. oxysporum f.sp. lactucae (Fuslat 10) showed that the ectosymbionts, present only in the WT strain, caused a depleted production of micro conidia and aerial hyphae, and a change in shape and dimension of the latter. Virulence tests showed that the cured Fusarium was a pathogenic strain and, as shown by polymerase chain reaction and microscope analysis, pathogenicity was correlated with the capability of the cured hyphae of penetrating lettuce roots. Accordingly, the hyphae of the WT strain were impaired in entering the plant roots. Typing experiments provided evidence that both CU and WT strains belong to F. oxysporum f.sp. lactucae. This implies that the antagonistic effect of WT Fusarium is not a fungal trait, but it is due to the interaction with the ectosymbiotic bacteria. Expression analysis showed that fmk1, chsV and pl1 genes involved in F. oxysporum pathogenicity are not expressed in the WT strain whereas they are expressed in the cured fungus. These results, together with the hyphal characteristics, suggest that the inability of WT strain to penetrate the plant roots could be due to alterations in the expression profile of cell wall-degrading enzymes. In conclusion, we demonstrated a modulation of F. oxysporum gene expression in response to the interaction with the ectosymbiotic bacteria. Preliminary researches indicated that the presence of bacteria attached to the hyphae of antagonistic F. oxysporum is not an isolated phenomenon. Further investigations are necessary to better understand the rule and the diffusion of ectosymbiotic bacteria among antagonistic Fusarium.
在本文中，我们已确定存在一个外共生细菌群落，它们属于沙雷氏菌属、无色杆菌属、芽孢杆菌属和嗜麦芽窄食单胞菌属，与拮抗性尖孢镰刀菌MSA 35[野生型（WT）菌株]的菌丝体相关。对WT菌株、不含外共生体的尖孢镰刀菌MSA 35[除菌（CU）菌株]和致病性尖孢镰刀菌莴苣专化型（Fuslat 10）进行的形态学特征分析表明，仅存在于WT菌株中的外共生体导致微分生孢子和气生菌丝的产生减少，以及气生菌丝的形状和尺寸发生变化。毒力测试表明，除菌后的镰刀菌是一种致病菌株，并且如聚合酶链反应和显微镜分析所示，致病性与除菌后的菌丝穿透莴苣根的能力相关。因此，WT菌株的菌丝进入植物根的能力受损。分型实验提供的证据表明，CU菌株和WT菌株均属于尖孢镰刀菌莴苣专化型。这意味着WT镰刀菌的拮抗作用不是一种真菌特性，而是由于与外共生细菌的相互作用。表达分析表明，参与尖孢镰刀菌致病性的fmk1、chsV和pl1基因在WT菌株中不表达，而在除菌后的真菌中表达。这些结果与菌丝特征一起表明，WT菌株无法穿透植物根可能是由于细胞壁降解酶表达谱的改变。总之，我们证明了尖孢镰刀菌基因表达响应于与外共生细菌的相互作用而发生调节。初步研究表明，附着在拮抗性尖孢镰刀菌菌丝上的细菌的存在不是一个孤立的现象。需要进一步研究以更好地了解外共生细菌在拮抗性镰刀菌中的规律和传播情况。
