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

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

Two-Component Elements Mediate Interactions between Cytokinin and Salicylic Acid in Plant Immunity

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant–pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant–pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA–dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA–dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses agains

Differential expression of ABA up-regulated genes at high temperature.

<p>(a) Kernel density estimate of log₂ fold change for ABA up-regulated genes differentially regulated in mock-inoculated plants. (b) Kernel density estimates of log₂ fold change for ABA up-regulated genes differentially regulated in plants during susceptible and resistant interactions at 3, 12, and 24 hpi.</p

Differential gene expression analysis.

<p>(a) Genes differentially up and down-regulated at high relative to normal temperature in mock-inoculated plants, and during susceptible and resistant interactions. (b) Number of DEGs per time point, with DEGs from the susceptible and resistant interactions combined per each time point. (c–e) Number of DEGs up or down-regulated by high temperature in plants in the susceptible (S) plants or resistant (R) interaction at (c) 3 h, (d) 12 h, and (e) 24 hpi. The red-squared number represents DEGs which were oppositely regulated by susceptibility/resistance.</p

Differential expression of upstream and downstream hormone genes at high temperature.

<p>(a) Fold change for hormone biosynthesis genes at high temperature relative to normal temperature is represented for mock-inoculated plants, and plants during susceptible and resistant interactions. Hormone biosynthesis genes were selected for display only if they were differentially expressed in at least one column. (b) Downstream hormone-responsive genes represented as proportions of total DEGs for mock-inoculated plants, and plants during susceptible and resistant interactions.</p

GO term analysis.

<p>Left panel shows the log₂ odds ratio of genes regulated by high temperature with the GO annotation to genes not regulated by high temperature with the GO annotation. Positive value indicates the term is over-represented in regulated genes, while negative value indicates the term is under-represented in regulated genes. Enrichment analysis was conducted for each term, in each treatment condition (Fisher’s exact test, FDR-corrected p-value < 0.05). The arrows indicate the genes were either up-regulated or down-regulated by high temperature treatment. Terms not statistically enriched are shown as white/zero log₂ odds ratio. Right panel shows the median log₂ fold change per term. Positive value indicates more genes annotated with the term are up-regulated, while negative value indicates more genes annotated with the term are down-regulated.</p