Lateral root stimulation in the early interaction betweenArabidopsis thalianaand the ectomycorrhizal fungusLaccaria bicolor

Abstract

Lateral root (LR) stimulation during early signal exchange between plant roots and ectomycorrhizal (ECM) fungi has recently been shown to be achieved by modulation of auxin gradients. We suggested that this modulation could occur through altered polar auxin transport (PAT) and through activation of auxin signalling pathways in the root. However, it remains unclear, which fungal molecules alter auxin pathways inside the plant partner. It has been suggested in previous studies that auxin released by the fungus could trigger observed plant responses during early signal exchange and later on during root colonization. Here we focus on the early interaction and we provide evidence for an alternative mechanism. Indeed, LR stimulation by the fungus in A. thaliana followed a totally different timing than with exogenously applied auxin. Furthermore, experimental conditions that excluded the exchange of soluble molecules while allowing exchange of volatile(s) between the plant and the fungus were sufficient for LR induction, therefore questioning the role of secreted fungal auxin. These data suggest that volatiles released by the fungus and sensed by the plant may act upstream of altered auxin signalling in the plant.

Identification of quantitative trait loci affecting ectomycorrhizal symbiosis in an interspecific F1 poplar cross and differential expression of genes in ectomycorrhizas of the two parents: Populus deltoides and Populus trichocarpa

“…Indeed, the activation of defense mechanisms in P. deltoides may inhibit L. bicolor colonization, while in P. trichocarpa this inhibition may be repressed by the overexpression of EREBP-4 ethylenesensitive transcription factor. Our results support the conclusions by Felten et al (2010), Rupp et al (1989), and Splivallo et al (2009) that ethylene, released by Laccaria, plays a role in primary response of the host plant. Thus, the initial steps in successful colonization may involve Laccaria manipulating the host's metabolic machinery and subsequently, decreasing their defenses.…”

Molecular signals required for the establishment and maintenance of ectomycorrhizal symbioses

“…Various roles have been proposed for other phytohormones, such as auxins, that are known to stimulate hyphal branching of ECM fungi (Debaud & Gay, 1987). Auxin is produced by both plant and fungal partners and seems to play an early role in the control of root development before fungal colonization (Felten et al, 2010). Cytokinins were described as stimulators of hyphal development in Suillus variegatus (Gogala, 1991).…”

The effect of elevated carbon dioxide on the interaction betweenEucalyptus grandisand diverse isolates ofPisolithussp. is associated with a complex shift in the root transcriptome

“…These differences include a reduction in the number of auxin-and ethylene-related genes in the roots of E. grandis undergoing colonization, while GA-regulated genes feature more prominently. The reduced transcriptomic representation of auxin-inducible genes in this core response to P. microcarpus is intriguing, as most other mycorrhizal and endophytic symbioses have reported up-regulation of auxin biosynthesis, transport, and signaling in host roots undergoing colonization (Felten et al, 2009(Felten et al, , 2010Splivallo et al, 2009;Hanlon & Coenen, 2011;Yin-Chen et al, 2011). The reasoning behind the reduced importance of auxin-related gene expression in mycorrhizal roots between E. grandis and P. microcarpus likely lies in the fact that Pisolithus produces hypaphorine during colonization (Beguiristain & Lapeyrie, 1997).…”