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

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.

A snapshot of the transcriptome ofMedicago truncatula(Fabales: Fabaceae) shoots and roots in response to an arbuscular mycorrhizal fungus and the pea aphid (Acyrthosiphon pisum) (Hemiptera: Aphididae)

“…Gibberellin 20-oxidases are mainly involved in synthesizing bioactive gibberellins (GA 1 , GA 3 , GA 4 , and GA 7 ) (Schwechheimer 2012). Previously, we found that pea aphids weighed more when fed for 7 days on gibberellic acid-untreated, mycorrhizal plants than those fed on gibberellic acid-treated, mycorrhizal plants (Olson et al 2022). Regarding nutrient transport, we observed upregulation of nitrate and zinc transporter genes (Medtr8g469310; Medtr2g064310) in shoots of mycorrhizal plants with aphids, whereas in nonmycorrhizal plants with aphids, there was upregulation of copper and lysine histidine transporter genes (Medtr4g064963; Medtr2g101920).…”