Bacterial Volatiles Known to Inhibit Phytophthora infestans Are Emitted on Potato Leaves by Pseudomonas Strains

Abstract

Bacterial volatiles play important roles in mediating beneficial interactions between plants and their associated microbiota. Despite their relevance, bacterial volatiles are mostly studied under laboratory conditions, although these strongly differ from the natural environment bacteria encounter when colonizing plant roots or shoots. In this work, we ask the question whether plant-associated bacteria also emit bioactive volatiles when growing on plant leaves rather than on artificial media. Using four potato-associated Pseudomonas, we demonstrate that potato leaves offer sufficient nutrients for the four strains to grow and emit volatiles, among which 1-undecene and Sulfur compounds have previously demonstrated the ability to inhibit the development of the oomycete Phytophthora infestans, the causative agent of potato late blight. Our results bring the proof of concept that bacterial volatiles with known plant health-promoting properties can be emitted on the surface of leaves and warrant further studies to test the bacterial emission of bioactive volatiles in greenhouse and field-grown plants.

Keywords: 1-undecene; Solanum tuberosum; bacterial volatile emission; dimethyl disulfide; dimethyl trisulfide; phyllosphere.

Figure 1

Principal component analysis of the volatiles emitted by leaves inoculated with Pseudomonas strains. (A) Score Plot of PC1 and PC2 of potato leaves non-inoculated (N) or inoculated with different Pseudomonas strains (R32, R47, S04, S35) following PCA. Superimposed on the plot are mean scores on the components for qualitative variables that are included in the PCA command and the colored ellipses illustrating the 95% confidence interval with each score. (B) Results of principal components analysis (PCA): projection on two first factors (PC1 and PC2) of the VOCs with a cosinus square superior to 0.6. VOC.1: 2-methyl-3-methylthiofuran; VOC.3: dimethyl disulfide, VOC.6: dimethyl trisulfide, VOC.7: methanethiol, VOC.8: thiocyanic methylester, VOC.11: 3-pentanone, VOC.12: Unknown 5C, VOC.15: hexanal, VOC.16: 1-undecene, VOC.18: 2-heptanone, VOC.21: (Z)-2-heptenal, VOC.22: 6-octene-2-one, VOC.25: (E)-2-octenal, VOC.26: putative, longiverbenone or isomer, VOC.28: Cyclobutane, tetrakis (1-methylethylidene)-, VOC.29: (2,6,6-trimethylcyclohexen-1-yl) methylsulfonylbenzene.

Figure 2

Comparative emission of VOCs by different Pseudomonas inoculated on potato leaves. Boxplot of the 16 VOCs with a cosinus square superior to 0.6 according to the two first dimensions of the principal component analysis (A–P). Tukey HSD post-hoc tests with Holm correction were performed on each VOC between potato leaves non-inoculated (N) or inoculated with different Pseudomonas strains (R32, R47, S04, S35; N = 6; Quantitative Ion Peak Area/g plant (QIPA/g plant)). Different letters indicate statistically significant differences at a significance level of p < 0.05).