Stress can alter important plant life-history traits. Here, we report the long-term effects of the stress hormone jasmonic acid (JA) on the defence phenotype, transcriptome and DNAmethylome of Arabidopsis. Three weeks after transient JA signalling activity, 5-week-old plants retained induced resistance (IR) against herbivory but showed enhanced susceptibility to necrotrophic and biotrophic pathogens. Transcriptome analysis of these plants revealed priming and/or up-regulation of JA-dependent defence genes but repression of ethyleneand salicylic acid-dependent genes. Long-term JA-IR against herbivory was associated with shifts in glucosinolate composition and required MYC2/3/4 transcription factors, DNA (de)methylation pathways and the small RNA (sRNA)-binding protein ARGONOUTE1 (AGO1). Although methylome analysis did not reveal consistent changes in DNA methylation near MYC2/3/4-controlled genes, JA-treated plants were specifically enriched with hypomethylated ATREP2 transposable elements (TEs), while ATREP2-derived sRNAs showed increased association with AGO1. Our results indicate that AGO1-associated sRNAs from hypomethylated ATREP2 TEs trans-regulate long-lasting memory of JAdependent immunity.

Competitive and facilitative interactions influence bacterial community composition, diversity and functioning. However, the role of genetic diversity for determining interactions between coexisting strains of the same, or closely related, species remains poorly understood. Here, we investigated the type (facilitative/inhibitory) and potential underlying mechanisms of pairwise interactions between 24 genetically diverse bacterial strains belonging to three genospecies (gsA,C,E) of the Rhizobium leguminosarum species complex. Interactions were determined indirectly, based on secreted compounds in cell-free supernatants, and directly, as growth inhibition in cocultures. We found supernatants mediated both facilitative and inhibitory interactions that varied greatly between strains and genospecies. Overall, gsE strains indirectly suppressed growth of gsA strains, while their own growth was facilitated by other genospecies' supernatants. Similar genospecies-level patterns were observed in direct competition, where gsA showed the highest susceptibility and gsE the highest inhibition capacity. At the genetic level, increased gsA susceptibility was associated with a non-random distribution of quorum sensing and secondary metabolite genes across genospecies. Together, our results suggest that genetic variation is associated with facilitative and competitive interactions, which could be important ecological mechanisms explaining R. leguminosarum diversity.

Rhizobia - nitrogen-fixing, root-nodulating bacteria - play a critical role in both plant ecosystems and sustainable agriculture. Rhizobia form intracellular infections within legumes roots where they produce plant accessible nitrogen from atmospheric nitrogen and thus reduce the reliance on industrial inputs. The rhizobia-legume symbiosis is often treated as a pairwise relationship between single genotypes, both in research and in the production of rhizobial inoculants. However in nature individual plants are infected by a high diversity of rhizobia symbionts. How this diversity affects productivity within the symbiosis is unclear. Here, we use a powerful statistical approach to assess the impact of diversity within the Rhizobium leguminosarum - clover symbiosis using a biodiversity-ecosystem function framework. Statistically, we found no significant impact of rhizobium diversity. However this relationship was weakly positive - rather than negative - indicating that there is no significant cost to increasing inoculant diversity. Productivity was influenced by the identity of the strains within an inoculant; strains with the highest individual performance showed a significant positive contribution within mixed inoculants. Overall, inoculant effectiveness was best predicted by the individual performance of the best inoculant member, and only weakly predicted by the worst performing member. Collectively, our data suggest that the Rhizobium leguminosarum - clover symbiosis displays a weak diversity-function relationship, but that inoculant performance can be improved through the inclusion of high performing strains. Given the wide environmental dependence of rhizobial inoculant quality, multi-strain inoculants could be highly successful as they increase the likelihood of including a strain well adapted to local conditions across different environments.

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