Phytohormone Profile of Medicago in Response to Mycorrhizal Fungi, Aphids, and Gibberellic Acid.

Drew Olson,Jamie D Riggs,Hannah M Berry,Cristiana T Argueso,Susana Karen Gomez

doi:10.3390/plants11060720

Abstract

Although gibberellic acid (GA) is widely used in agriculture, it is unclear whether exogenous GA makes aphid-infested, mycorrhizal plants more susceptible to herbivory. This study investigates the role of GA in modulating defenses in barrel medic plants (Medicago truncatula) that are infested with pea aphids (Acyrthosiphon pisum) and colonized by the beneficial symbiont Rhizophagus intraradices. Mock- and R. intraradices-inoculated potted plants were grown in a topsoil: sand mix for 42 days and were treated with GA or solvent. Subsequently, plants were exposed to herbivory or no aphid herbivory for 36 h and 7 days. Afterwards, plant growth parameters, aphid fitness, and foliar phytohormone concentrations were measured. The results revealed that GA regulates plant defenses during arbuscular mycorrhizal (AM) fungus–plant–aphid interactions as aphids that fed for 7 days on mycorrhizal, GA-untreated plants weighed more than those that fed on mycorrhizal, GA-treated plants. No major differences were detected in phytohormone levels at 36 h. Overall, mycorrhizal plants showed more shoot biomass compared to non-mycorrhizal controls. The arbuscule density and fungal biomass of R. intraradices were not altered by exogenous GA and aphid herbivory based on molecular markers. This study indicates that exogenous GA may help reduce aphid fitness when feeding on mycorrhizal plants.

Highlights

Soil microorganisms such as arbuscular mycorrhizal (AM) fungi form relationships with approximately 80% of plant roots, providing essential nutrients in exchange for carbohydrates and lipids from host plants [1–6]
We investigated the role of exogenous gibberellic acid (GA) application to roots in modulating M. truncatula susceptibility during interactions with pea aphids and the AM fungus R. intraradices
The 36 h time-point was mainly used to examine the early changes in phytohormone levels that could lead to an impact on aphid fitness (7 days)

Summary

Introduction

Soil microorganisms such as arbuscular mycorrhizal (AM) fungi form relationships with approximately 80% of plant roots, providing essential nutrients in exchange for carbohydrates and lipids from host plants [1–6]. AM fungi indirectly interact with aboveand below-ground organisms by ‘priming’ plants for future engagement with pathogenic microbes and insect herbivores. This priming response appears to be regulated by jasmonic acid (JA) and can spread through extraradical mycorrhizal networks [7–14]. AM fungi can prime plants for future insect attacks, certain insects such as aphids can benefit from feeding on mycorrhizal plants [15–21]. Aphid feeding on mycorrhizal plants results in above-ground changes in volatile compounds and foliar chemistry as well as below-ground changes in AM fungal root colonization and composition, 4.0/)

Methods

Results

Conclusion