A Novel Interaction between Plant-Beneficial Rhizobacteria and Roots: Colonization Induces Corn Resistance against the Root Herbivore <i>Diabrotica speciosa</i>

<div><p>A number of soil-borne microorganisms, such as mycorrhizal fungi and rhizobacteria, establish mutualistic interactions with plants, which can indirectly affect other organisms. Knowledge of the plant-mediated effects of mutualistic microorganisms is limited to aboveground insects, whereas there is little understanding of what role beneficial soil bacteria may play in plant defense against root herbivory. Here, we establish that colonization by the beneficial rhizobacterium <i>Azospirillum brasilense</i> affects the host selection and performance of the insect <i>Diabrotica speciosa</i>. Root larvae preferentially orient toward the roots of non-inoculated plants versus inoculated roots and gain less weight when feeding on inoculated plants. As inoculation by <i>A. brasilense</i> induces higher emissions of (<i>E</i>)-β-caryophyllene compared with non-inoculated plants, it is plausible that the non-preference of <i>D. speciosa</i> for inoculated plants is related to this sesquiterpene, which is well known to mediate belowground insect-plant interactions. To the best of our knowledge, this is the first study showing that a beneficial rhizobacterium inoculant indirectly alters belowground plant-insect interactions. The role of <i>A. brasilense</i> as part of an integrative pest management (IPM) program for the protection of corn against the South American corn rootworm, <i>D. speciosa</i>, is considered.</p></div

Identification of compounds in the root volatile profile.

<p>Retention time (Rt), Kovats Index (KI), peak area (%) and identification of compounds emitted by inoculated and non-inoculated corn roots through combined GC-MS analysis.</p><p>Identification of compounds in the root volatile profile.</p

Effect of <i>Azospirillum brasilense</i> on <i>Diabrotica speciosa</i> larval host choice.

<p><i>Diabrotica speciosa</i> larval choice between inoculated plants and the blank treatment (non-inoculated soil), inoculated and non-inoculated corn, and the plant-beneficial rhizobacterium (PBR) inoculant and the blank. Bars represent the mean number of larvae ± SE. Pie charts on the right represent non-responsive (no choice) and responsive (choice) larvae. Asterisks indicate a significant difference between treatments according to a quasi-Poisson glm (n = 10, <i>P</i><0.05).</p

Effect of <i>Azospirillum brasilense</i> on <i>Diabrotica speciosa</i> performance.

<p><i>Diabrotica speciosa</i> larval performance when fed on inoculated and non-inoculated corn plants. Bars represent the mean larval weight ± SE. Asterisks indicate a significant difference between treatments according to according to a glmm (n = 10, <i>P</i><0.05).</p

Scanning electron microscopy images of inoculated and non-inoculated corn roots.

<p>Scanning electron microscopy images showing the colonization of corn roots by the plant-beneficial rhizobacterium <i>Azospirillum brasilense</i>. (A) Inoculated corn roots and (B) non-inoculated corn roots.</p

Root volatile profile induced by <i>Azospirillum brasilense</i> colonization.

<p>Emissions of volatile compounds from inoculated and non-inoculated corn roots. Bars represent the mean ± SE. Different letters indicate a significant difference between treatments according to <i>One-Way</i> ANOVA followed by Tukey’s HSD test (n = 4, <i>P</i><0.05).</p