Simultaneous Impact of Rhizobacteria Inoculation and Leaf-Chewing Insect Herbivory on Essential Oil Production and VOC Emissions in Ocimum basilicum

Abstract

Inoculation with rhizobacteria and feeding by herbivores, two types of abiotic stress, have been shown to increase the production of secondary metabolites in plants as part of the defense response. This study explored the simultaneous effects of inoculation with Bacillus amyloliquefaciens GB03 (a PGPR species) and herbivory by third-instar Spodoptera frugiperda larvae on essential oil (EO) yield and volatile organic compound (VOC) emissions in Ocimum basilicum plants. The density of glandular trichomes was also examined, given that they are linked to EO production and VOC emission. Herbivory increased EO content, but inoculation on its own did not. When combined, however, the two treatments led to a 10-fold rise in EO content with respect to non-inoculated plants. VOC emissions did not significantly differ between inoculated and non-inoculated plants, but they doubled in plants chewed by the larvae with respect to their undamaged counterparts. Interestingly, no changes were observed in VOC emissions when the treatments were tested together. In short, the two biotic stressors elicited differing plant defense responses, mainly when EO was concerned. PGPR did not stimulate EO production, while herbivory significantly enhanced it and increased VOC emissions. The combined treatment acted synergistically, and in this case, PGPR inoculation may have had a priming effect that amplified plant response to herbivory. Peltate trichome density was higher in inoculated plants, those damaged by larvae, and those subjected to the combination of both treatments. The findings highlight the intricate nature of plant defense mechanisms against various stressors and hint at a potential strategy to produce essential oil through the combined application of the two stressors tested here.

Keywords: Spodoptera frugiperda; essential oil; herbivory; plant growth-promoting bacteria; rhizobacteria; secondary metabolites; sweet basil; trichome density; volatile organic compounds.

Figure 1

Essential oil yield in Ocimum basilicum plants exposed to herbivory by Spodoptera frugiperda larvae herbivory and/or inoculated with Bacillus amyloliquefaciens GB03 (mean ± SE). Letters above bars indicate significant differences according to Fisher’s LSD test (p < 0.05).

Figure 2

Concentrations of major EO components (μg/g fw) in O. basilicum inoculated with PGPR and/or infested by S. frugiperda. Letters above bars indicate significant differences according to Fisher’s LSD test (p < 0.05).

Figure 3

Emission of volatile organic compounds (VOCs) by O. basilicum plants inoculated with B. amyloliquefaciens (GB03) and/or exposed to S. frugiperda. Different letters indicate statistically significant differences according to Fisher’s LSD test (p < 0.05).

Figure 4

Concentrations of major VOC compounds emitted by O. basilicum plants inoculated with PGPR and/or infested by S. frugiperda. Different letters indicate statistically significant differences according to Fisher’s LSD test (p < 0.05).

Figure 5

Clustered heat map of the effects of Spodoptera frugiperda herbivory, inoculation with B. amiloliquefaciens GB03, and their combination on: (A) EO composition, and (B) emitted VOCs.

Figure 6

Effects of inoculation with B. amiloliquefaciens GB03 and/or exposure to SF on the density of peltate and capitate glandular trichomes in O. basilicum plants. (A,E) Control; (B,F) plants inoculated with GB03; (C,G) plants exposed to SF; and (D,H) plants inoculated with GB03 and exposed to SF. Values are mean ± standard error (SE). Means followed by the same letter in a given column are not significantly different according to Fisher’s LSD test (p < 0.05).

Figure 7

Principal component analysis for the physiological response of O. basilicum to insect chewing herbivory and PGPR inoculation. EO: essential oil yield, VOC: volatile organic compound emissions, PTadax: density of peltate trichomes on the adaxial side, and PTabax: density of peltate trichomes on the abaxial side.