Lateral root stimulation in the early interaction between Arabidopsis thaliana and the ectomycorrhizal fungus Laccaria bicolor: is fungal auxin the trigger?

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.