Complex interactions in the rhizosphere between plant growth-promoting rhizobacteria (PGPR) and plant hosts can lead to biochemical cascades within the plant, altering above-ground components, such as biomass or even the volatile organic compounds (VOC). These alterations in VOCs can confer beneficial traits for the plant, like resistance to herbivory. Numerous studies have identified changes in VOC patterns due to plant species and microbial communities, yet little information exists about whether PGPR alterations to VOC composition varies as a function of plant genotype and/or microbial community. Crop improvement in maize has led to new genotypes, and germplasm representing different phases in the directed evolution of maize are available. Therefore, we investigated whether PGPR-herbivory protection varies as a function of maize genotype and rhizosphere microbiome. Six maize genotypes were selected, spanning three decades of crop improvement and two major heterotic groups. Plants were grown in greenhouse conditions and inoculated with a PRGR, Bacillus altitudinus, at planting, and then weekly thereafter. Four microbial treatments were used to assess the interactions between genotype and microbial community: Bacillus+sterile soil (BS), Bacillus+live soil (BL), live soil (L), and sterile soil (S). Herbivory-induced volatiles were collected post-exposure to corn earworms and analyzed using gas chromatography.
Results/Conclusions
Breeding manipulations of plant genotype (PERMANOVA - breeding decade: R2 = 0.17410, P = 0.001; genotype: R2 = 0.10942, P = 0.001; and heterotic group: R2 = 0.23652, P = 0.001) significantly impacted herbivory-induced VOC composition. Microbial treatment had no influence on VOC composition (PERMANOVA: R2 = 0.01207, P = 0.589). Of the 35 individual VOCs assessed, only one, 2-ethylcyclopentanone, was significantly impacted by microbial treatment (ANOVA, F (3,93) = 4.3949, P = 0.006145), with the lowest abundance within the BS treatment, and highest within the S treatment. Abundance of 2-ethylcyclopentanone was significantly influenced by breeding decade (ANOVA: F (2, 93) = 19.5577, P < 0.0001), with the lowest abundance in the late (1980s) breeding decade. 2-ethylcyclopentanone is a fatty acid derivative, similar to the cyclopentanones within the jasmonic acid biosynthesis pathway. Assessment of these PGPR interactions on the rhizosphere microbiome is underway. The results of this research indicate that breeding-driven changes in plant genotype significantly impact herbivory-induced VOC phenotype, overwhelming any beneficial trait of a PGPR. To harness the effects of microbial inoculants, breeding maize which responds to PGPR inoculants is a necessity.