Modulation (i.e., priming) of plant defense responses is a known benefit of arbuscular mycorrhizal symbiosis, but it is not well known how common this modulation is, what is its typical magnitude and mechanism, or how it is affected by biotic and abiotic context. Priming often manifests as a mild but effective boost of plant immune responses to biotic or abiotic stress but may also occur in its absence. Differential production of plant antioxidant enzymes is thought to be a common mechanism of priming, but it is not clear which enzymes are most important. We used meta-analysis across 1129 observations from 88 studies to estimate the magnitude of priming of host plant defense in response to arbuscular mycorrhiza, and to test how priming depends on the presence and type of stress (water, heavy metal, organic compounds, salt, temperature, transplant conditions, and pathogen), the diversity of mycorrhizal fungal species, the type of antioxidant enzyme (peroxidase, polyphenol oxidase, superoxide dismutase, catalase, and reductase) and which plant parts were assayed.
Results/Conclusions
Overall, priming was observed in mycorrhizal plants regardless of stress conditions. Total anti-oxidant enzyme production in both stress and non-stress conditions was approximately 17% higher in mycorrhizal plants than non-mycorrhizal plants. Under stress, the magnitude of priming was not affected by the type of stress or other contextual factors. Both with and without stress, priming was lower when measured only in leaves compared to the whole plant. Under non-stress conditions, polyphenol oxidase showed differential production compared to the other enzymes tested. Higher production of polyphenol oxidase in response to mycorrhizal fungi in non-stress conditions corroborates earlier results indicating mycorrhiza induced priming irrespective of stress. Priming response did not depend on number of mycorrhizal species inoculated under stress or non-stress conditions. These results lend general insight into the overall magnitude and typical mechanisms of plant defense priming, suggesting that it is lower in leaf tissue, involving polyphenol oxidase, and does not substantially depend on the presence or type of stress.