Nutrition vs association: plant defenses are altered by arbuscular mycorrhizal fungi association not by nutritional provisioning alone

Abstract

Background While it is known that arbuscular mycorrhizal fungi (AMF) can improve nutrient acquisition and herbivore resistance in crops, the mechanisms by which AMF influence plant defense remain unknown. Plants respond to herbivory with a cascade of gene expression and phytochemical biosynthesis. Given that the production of defensive phytochemicals requires nutrients, a commonly invoked hypothesis is that the improvement to plant defense when grown with AMF is simply due to an increased availability of nutrients. An alternative hypothesis is that the AMF effect on herbivory is due to changes in plant defense gene expression that are not simply due to nutrient availability. In this study, we tested whether changes in plant defenses are regulated by nutritional provisioning alone or the response of plant to AMF associations. Maize plants grown with or without AMF and with one of three fertilizer treatments (standard, 2 × nitrogen, or 2 × phosphorous) were infested with fall armyworm (Spodoptera frugiperda; FAW) for 72 h. We measured general plant characteristics (e.g. height, number of leaves), relative gene expression (rtPCR) of three defensive genes (lox3, mpi, and pr5), total plant N and P nutrient content, and change in FAW mass per plant. Results We found that AMF drove the defense response of maize by increasing the expression of mpi and pr5. Furthermore, while AMF increased the total phosphorous content of maize it had no impact on maize nitrogen. Fertilization alone did not alter upregulation of any of the 3 induced defense genes tested, suggesting the mechanism through which AMF upregulate defenses is not solely via increased N or P plant nutrition. Conclusion This work supports that maize defense may be optimized by AMF associations alone, reducing the need for artificial inputs when managing FAW.

ARBUSCULAR MYCORRHIZAL FUNGI INFLUENCE ON GROWTH OF CREOLE MAIZE AND LARVAL DEVELOPMENT OF Spodoptera frugiperda (Lepidoptera:Noctuidae)

“…These data agree with Harley and who set out that AMF promoted a defense against generalist herbivores, as well as reduction in larval weight (Roger et al, 2013). It has been shown that maize defense compounds such as protease inhibitors and benzoxazinoids are altered by AMF colonization of plant roots that affect weight gain of Lepidoptera larvae (Ramírez-Serrano et al 2022;Stratton et al 2022). Low weight in immature insect stages could affect development time, energy availability for the adult stage, survival, and consequently, greater predation probability by natural enemies.…”

Cascading Effects of Cover Crops on the Subsequent Cash Crop Defense against the Polyphagous Herbivore Fall Armyworm (Spodoptera frugiperda)

“…However, we did not estimate the AMF colonization success in cash crop, although this has been previously documented [ 3 ]. The direct and indirect effect of AMF on the alteration of plant defense against herbivores through the production of secondary metabolites, volatile organic compounds, and phytohormones has been enumerated in previous studies [ 3 , 11 , 34 ]. For example, [ 3 ] showed that plants grown with AMF received lower levels of herbivory compared to plants without AMF.…”

“…For example, [ 3 ] showed that plants grown with AMF received lower levels of herbivory compared to plants without AMF. Also, [ 34 ] showed that AMF upregulated the defense in maize plants against herbivores by enhancing the expression of defense-related genes ( mpi and pr5 ).…”

The link between changing in host carbon allocation and resistance to Magnaporthe oryzae : a possible tactic for mitigating the rice blast fungus

“… 84 , 85 As a result of these actions, the soil microbiome will become healthier, which will help strengthen the plant’s defense mechanisms. 86 Furthermore, carbon availability is impacted by adjusting nutrient management techniques, such as fertilizer and organic amendment application. A plant’s capacity to distribute carbon for defense toward rice blast may be impacted by balanced nutrient levels, which may contribute to optimal plant health.…”