High-affinity iron uptake is required for optimal Epichloë festucae colonization of Lolium perenne and seed transmission

Is the endophyte-based plant protection against aphids mediated by changes in the insect microbiome?

Aphids are important herbivores in natural and managed environments. We studied the response of aphids and their associated microbiota to the presence of the fungal endophyte Epichloë sp. LpTG-3 strain AR37, and the AR37-derived alkaloids in plants. We hypothesized that AR37 and/or AR37-derived alkaloids would reduce the aphid performance, and that this reduction would be associated with endophyte-mediated changes in the abundance, composition, and diversity of beneficial bacterial endosymbionts of aphids (e.g., Buchnera). Plants of Lolium perenne associated with AR37 variants able (wild type and ∆idtA) and unable (∆idtM) to produce indole diterpene alkaloids were challenged with Rhopalosiphum padi aphids. We measured aphid population size, plant biomass, and the abundance, composition and diversity of the aphid's bacterial microbiota. The presence of AR37 increased the resistance of plants against R. padi aphids via the production of indole diterpene alkaloids, and this effect was independent of the plant biomass. The endophyte-mediated reduction in aphid performance was not associated with changes in the abundance, composition and diversity of the insect's bacterial microbiota. However, we cannot rule out that the reduction in aphid performance could be associated with a putative endophyte effect on the bacterial provision of benefits to aphids. Our study highlighted the protective role of endophyte-derived indole diterpene alkaloids against aphids. Further investigations will be needed to determine if there is a link between the endophyte-mediated aphid resistance and the integrity of the insect's bacterial microbiota.

The Role of FpfetC from Fusarium proliferatum in Iron Acquisition, Fumonisin B1 Production, and Virulence

Iron is an essential micronutrient required for the fungal growth and propagation. Fusarium proliferatum is the causal agent of rice spikelet rot disease. In this study, we characterized the role of F. proliferatum multicopper ferroxidase (FpfetC), which mediated the oxidization of ferrous to ferric iron in the reductive system of iron assimilation. Deletion of FpfetC led to impaired growth under iron-deprived conditions, and the growth defect could be restored by exogenous iron. Compared to wild-type Fp9 strain, ΔFpfetC showed increased conidiation, resistance to copper stress, and sensitivity to zinc stress. FpfetC deficiency rendered a transcription remodeling of genes involved in high-affinity iron assimilation, iron homeostasis and iron storage. Moreover, production of fumonisin B1 (FB1) and transcript levels of fumonisin biosynthesis (Fpfums) genes were elevated in ΔFpfetC. ΔFpfetC exhibited hypervirulence to rice, accompanied with aggravation of invasive hyphae and activation of siderophore synthesis at the sites of inoculation. Additionally, disruption of FpfetC attenuated penetration ability to cellophane membrane under iron starvation. Taken together, these results demonstrated that FpfetC played important roles in iron uptake, conidiation, response to metal stress, fumonisin biosynthesis, and virulence in F. proliferatum.

The growth promotion in endophyte symbiotic plants does not penalise the resistance to herbivores and bacterial microbiota

A trade-off between growth and defence against biotic stresses is common in plants. Fungal endophytes of the genus Epichloë may relieve this trade-off in their host grasses since they can simultaneously induce plant growth and produce antiherbivore alkaloids that circumvent the need for host defence. The Epichloë ability to decouple the growth-defence trade-off was evaluated by subjecting ryegrass with and without Epichloë endophytes to an exogenous treatment with gibberellin (GA) followed by a challenge with Rhopalosiphum padi aphids. In agreement with the endophyte-mediated trade-off decoupling hypothesis, the GA-derived promotion of plant growth increased the susceptibility to aphids in endophyte-free plants but did not affect the insect resistance in endophyte-symbiotic plants. In line with the unaltered insect resistance, the GA treatment did not reduce the concentration of Epichloë-derived alkaloids. The Epichloë mycelial biomass was transiently increased by the GA treatment but at the expense of hyphal integrity. The response of the phyllosphere bacterial microbiota to both GA treatment and Epichloë was also evaluated. Only Epichloë, and not the GA treatment, altered the composition of the phyllosphere microbiota and the abundance of certain bacterial taxa. Our findings clearly demonstrate that Epichloë does indeed relieve the plant growth-defence trade-off.