To limit damage from insect herbivores, plants rely on a blend of defensive mechanisms that includes partnerships with beneficial microbes, particularly those inhabiting roots. While ample evidence exists for microbially mediated resistance responses that directly target insects through changing phytotoxin and volatile profiles, we know surprisingly little about the microbial underpinnings of plant tolerance. Tolerance defenses counteract insect damage via shifts in plant physiology that reallocate resources to fuel compensatory growth, improve photosynthetic efficiency, and reduce oxidative stress. Despite being a powerful mitigator of insect damage, tolerance remains an understudied realm of plant defenses. Here we propose a novel conceptual framework that can be broadly applied across study systems to characterize microbial impacts on expression of tolerance defenses. We conducted a systematic review of studies quantifying the impact of rhizosphere microbial inoculants on plant tolerance to herbivory based on several measures— biomass, oxidative stress mitigation, or photosynthesis. We identified 40 studies, most of which focused on chewing herbivores (n=31) and plant growth parameters (e.g. biomass). Next, we performed a meta-analysis investigating the impact of microbial inoculants on plant tolerance to herbivory, which was measured via differences in plant biomass, and compared across key microbe, insect, and plant traits. Thirty-five papers comprising 113 observations were included in this meta-analysis, with effect sizes (Hedges’ d) ranging from -4.67 (susceptible) to 18.38 (overcompensation). Overall, microbial inoculants significantly reduce the cost of herbivory via plant growth promotion, with overcompensation and compensation comprising 25% of observations of microbial-mediated tolerance. The grand mean effect size 0.99 [0.49-1.49] indicates that addition of a microbial inoculant increased plant biomass by ~1 standard deviation under herbivore stress, thus improving tolerance. This effect was influenced most by microbial attributes, including functional guild and total soil community diversity. Overall, results highlight the need for additional investigation of microbially mediated plant tolerance, particularly in sap-feeding insects and across a more comprehensive range of tolerance mechanisms. Such attention would round out our current understanding of anti-herbivore plant defenses, offer insight into the underlying mechanisms that promote resilience to insect stress, and inform application of microbial biotechnology to support sustainable agricultural practices.

A meta-analysis was performed following systematic review of the literature. We identified papers from our primary literature that fit the following criteria: (1) examined interactions between at least one insect herbivore, one root microbe, and one plant, (2) considered herbivore damage that was real rather than simulated (i.e., mechanical damage), or mechanical damage supplemented with insect regurgitant, (3) applied microbial inoculants to plant roots or soil (i.e., excluded cases of strictly shoot inoculation), (4) focused on the root microbial community (i.e., not phyllosphere/aerial plant tissues), (5) considered at least one measure of plant tolerance (e.g., yield, plant biomass, oxidative stress mitigation, or photosynthesis), (6) included no-microbe control treatments with which to compare treatments that added one or more microbes, and (7) considered wild-type varieties/cultivars, rather than mutant genotypes. The resultant 40 papers that fit these criteria are summarized in this dataset.

We first examined each paper included in our systematic review to assess how often measures of tolerance such as yield, biomass, oxidative stress, and photosynthetic components were evaluated to identify measures that could be used for a meta-analysis. While all 40 papers considered tolerance as proxied by biomass, relatively few considered whole plant fitness (e.g., yield) or measures reflective of tolerance mechanisms such as oxidative stress and/or photosynthesis. Because of the few data related to yield (n = 7 papers), oxidative stress and photosynthesis, we focused the meta-analysis on summarizing how the presence of one or more microbes influenced tolerance to herbivory as captured by plant shoot, root, reproductive, and/or total biomass.

We next extracted means and estimates of uncertainty (standard deviation or standard error) of biomass measures (shoot, root, reproductive, and/or total) from the infested host treatment group (i.e., “insect alone” or “+insect /-microbe”) and the infested holobiont treatment group (i.e., “microbe + insect” or “+insect/+microbe”) where a microbial inoculant was applied to the host plant and then exposed to herbivory. These parameters were taken from publicly available data or tables whenever possible, and estimated from figures with WebPlotDigitizer when necessary. We were unable to extract biomass measures from five of the 40 papers due to either (1) insufficient detail being provided about estimates of uncertainty, (2) figures not depicting the treatments of interest, or (3) biomass being presented as percent regrowth.

From each paper included in our meta-analysis (n = 35), data on key insect, plant, microbial, and environmental attributes were collected for each independent observation. Insect-focused traits included herbivore feeding guild and site of herbivore feeding. Microbial attributes included microbial functional guild, microbial inoculant source, microbial community species richness, and the effect of microbe treatment on insect fitness. Plant information included family, age at infestation, “regrowth period” (i.e., how long the plant was allowed to regrow after the herbivory treatment ended), and plant cultivation status (i.e., domesticated or wild). Finally, we collected information on the experimental environment, including whether experiments were conducted under controlled conditions or in the field. These data were later used as moderators in our meta-analysis models to test how the strength of microbe-mediated expression of tolerance responses changed according to key insect, microbe, and plant traits (e.g., different insect feeding guilds). Collectively, these attributes were chosen based on existing research demonstrating or predicting differences in the expression of tolerance or impacts on underlying tolerance mechanisms across identified levels/categories (e.g., chewers vs. phloem feeders, wild vs. crops, growth promoting root microbes).

We estimated effect sizes that capture microbial impacts on plant tolerance to insect herbivory with Hedges’ d using the metafor package (v. 4.2-0) (Viechtbauer 2010) in R (v. 4.2.3) (R Core Team 2021).