Transformed Hairy Roots of the actinorhizal shrub Discaria trinervis: a valuable tool for studying actinorhizal symbiosis in the context of intercellular infection

Abstract

Background Nitrogen is a major limiting factor of plant growth in many ecosystems. Root nodule symbiosis (RNS) is one of the most efficient adaptations allowing plants to cope with nitrogen deficiency by establishing a symbiotic association with diazotrophic bacteria able to produce ammonium from atmospheric nitrogen. Nevertheless RNS is restricted to two groups of plants: legumes and Parasponia (Celtidaceae), that interact with a group of gram-negative proteobacteria collectively called rhizobia, and actinorhizal plants, a group of 220 species, mostly shrubs and trees distributed in the orders Fagales, Cucurbitales and Rosales, that interact with gram-positive actinomycetes of the genus Frankia[1]. All these plants belong to the Rosid I clade, suggesting a common origin for the ability to establish RNS [2]. In recent decades a strong research effort focused on model legumes lead to the identification of key molecular actors involved in nodulation, including the bacterial signalling molecules, the Nod factors and several genes involved in the symbiotic signalling pathways [3]. Much less is known in non model legumes and actinorhizal plants, particularly in species that are not infected like model legumes through root hairs but show more ancestral infection mechanisms like crack entry or intercellular infection. Yet important cues regarding the diversity and evolution of RNS are being found precisely in these more primitive non-model systems [4,5]. Among infection mechanisms leading to root nodule symbiosis, the intercellular infection pathway is probably the most ancestral but also one of the least characterized [6,7]. Intercellular infection has been described in Discaria trinervis, an actinorhizal shrub belonging to the Rosales order [8]. To decipher the molecular mechanisms underlying intercellular infection with Frankia, we set up an efficient genetic transformation protocol for D. trinervis based on A. rhizogenes.