Summary1. Cool-season grasses can be simultaneously infected by foliar fungal endophytes and colonised by mycorrhizal fungi, the integrated functions of which are strong predictors of plant fitness within grassland ecosystems. Evidence has been presented previously that infection of grass species with foliar endophytes can negatively affect mycorrhizal colonisation. Here, we tested the hypothesis that mycorrhizal colonisation in turn adversely affects Neotyphodium endophyte concentrations and that the competitive interaction between the two endosymbionts is affected by resource supply. 2. Specifically, we report how competition between Glomus (G. mosseae -GM, G. intraradices -GI) mycorrhizal fungi and N. lolii (common strain (CS) and AR1) foliar endophytic strains is affected by P supply and water-soluble carbohydrate (WSC) content in two Lolium perenne (perennial ryegrass) cultivars: a high sugar grass, AberDart, and a conventional (control) grass, Fennema. 3. The presence of Glomus mycorrhizae reduced the concentrations of endophytes and alkaloids in leaf blades and pseudostems. The reduction depended on P supply, ryegrass cultivar (notably WSC content) and endophyte strain. Conversely, foliar endophyte infection reduced mycorrhizal colonisation rates and concentrations in the roots of the control cultivar Fennema, although not in the high sugar cultivar, AberDart. 4. Neither GM nor N. lolii infection had an effect per se on the yield of root or blade compared with mycorrhiza-free (M-) and endophyte-free (E-) plants, respectively; though, yield of roots and blades was reduced by GI infection and at low P. 5. Competitive interactions between ecologically widespread foliar endophytes (valuable for plant protection) and mycorrhizal endosymbionts (valuable for P acquisition) as seen in this study are of critical importance especially in areas of high pest prevalence and low P availability. Our work stresses the need for elucidating the physiological ⁄ metabolic basis for such interactions between endosymbionts to understand how these processes contribute to plant performance and fitness in grassland ecosystems.

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The promoting effects of both high nitrogen (N) and exogenous gibberellin (GA) supply on regrowth of Lolium 7 perenne have been widely reported. The mobilisation of carbohydrate reserves in response to N is a critical 8 mechanism for promoting plant regrowth. However, our knowledge about GA regulation of carbohydrate metabolism remains limited. Here, we analysed the effects of both N and exogenous GA on the molecular 10 mechanisms controlling perennial ryegrass regrowth and investigated the similarities and differences. Our 11 analyses show that both high N and exogenous GA supply lead to a decline in the accumulation of carbohydrate 12 reserves, but the regulatory mechanisms responsible for this decline varied between N and GA supply. The effects 13 of elevated N were mainly through declining fructan biosynthesis which results in improving photosynthate use 14 efficiency to promote plant regrowth, whereas the application of exogenous GA resulted in an increase in the 15 hydrolytic activities of fructan exohydrolase and invertases capable of cleaving reserved carbohydrates to release 16 energy sources for plant regrowth.