The mutualistic association of roots with ectomycorrhizal fungi promotes plant health and is a hallmark of boreal and temperate forests worldwide. In the pre-colonization phase, before direct contact, lateral root (LR) production is massively stimulated, yet little is known about the signals exchanged during this step. Here, we identify sesquiterpenes (SQTs) as biologically active agents emitted by Laccaria bicolor while interacting with Populus or Arabidopsis. We show that inhibition of fungal SQT production by lovastatin strongly reduces LR proliferation and that (–)-thujopsene, a low-abundance SQT, is sufficient to stimulate LR formation in the absence of the fungus. Further, we show that the ectomycorrhizal ascomycote, Cenococcum geophilum, which cannot synthesize SQTs, does not promote LRs. We propose that the LR-promoting SQT signal creates a win-win situation by enhancing the root surface area for plant nutrient uptake and by improving fungal access to plant-derived carbon via root exudates.

Infection of Arabidopsis thaliana with avirulent Pseudomonas syringae induces camphene and α- and β-pinene emissions contributing to systemic resistance inducti

Summary: The physiological role of isoprene emission in plants is a matter of much debate. One of the most widely propagated hypotheses suggests a function of isoprene in the protection of leaf physiological processes against thermal and oxidative stress. To test this hypothesis, we developed transgenic Grey poplar (Populus×canescens) plants in which gene expression of isoprene synthase (ISPS) was either silenced by RNA interference (RNAi) or upregulated by over‐expression of the ISPS gene. Despite increased ISPS mRNA levels, we did not observe consistent increases in isoprene emission in the over‐expressing lines, indicating post‐transcriptional control of ISPS by co‐suppression. In the RNAi lines, levels of isoprene emission were effectively suppressed to virtually zero. Transgenic plants were subjected to temperature stress with three transient heat phases of 38–40°C, each followed by phases of recovery at 30°C. Parallel measurements of gas exchange, chlorophyll fluorescence and isoprene emission provided new insights into the physiological link between isoprene and enhanced temperature tolerance. Transgenic non‐isoprene‐emitting poplars showed reduced rates of net assimilation and photosynthetic electron transport during heat stress, but not in the absence of stress. The decrease in the efficiency of photochemistry was inversely correlated with the increase in heat dissipation of absorbed light energy, measured as NPQ (non‐photochemical quenching). Isoprene‐repressed poplars also displayed an increased formation of the xanthophyll cycle pigment zeaxanthin in the absence of stress, which can cause increased NPQ or may indicate an increased requirement for antioxidants. In conclusion, using a molecular genetic approach, we show that down‐regulation of isoprene emission affects thermotolerance of photosynthesis and induces increased energy dissipation by NPQ pathways.