Transcriptomics of Actinorhizal Symbioses Reveals Homologs of the Whole Common Symbiotic Signaling Cascade  

Abstract

Comparative transcriptomics of two actinorhizal symbiotic plants, Casuarina glauca and Alnus glutinosa, was used to gain insight into their symbiotic programs triggered following contact with the nitrogen-fixing actinobacterium Frankia. Approximately 14,000 unigenes were recovered in roots and 3-week-old nodules of each of the two species. A transcriptomic array was designed to monitor changes in expression levels between roots and nodules, enabling the identification of up-and downregulated genes as well as root-and nodule-specific genes. The expression levels of several genes emblematic of symbiosis were confirmed by quantitative polymerase chain reaction. As expected, several genes related to carbon and nitrogen exchange, defense against pathogens, or stress resistance were strongly regulated. Furthermore, homolog genes of the common and nodule-specific signaling pathways known in legumes were identified in the two actinorhizal symbiotic plants. The conservation of the host plant signaling pathway is all the more surprising in light of the lack of canonical nod genes in the genomes of its bacterial symbiont, Frankia. The evolutionary pattern emerging from these studies reinforces the hypothesis of a common genetic ancestor of the Fabid (Eurosid I) nodulating clade with a genetic predisposition for nodulation.

Alnus peptides modify membrane porosity and induce the release of nitrogen-rich metabolites from nitrogen-fixing Frankia

“…More recently, similar results were found for another actinorhizal plant, Datisca (Demina et al, 2013). Remarkably, all three actinorhizal plants induce a small family of defensin genes in their nodules (Hocher et al, 2011;Demina et al, 2013). In the present study, we sought to confirm the noduleenhanced expression of defensin peptides produced by actinorhizal plants and to understand their role in the symbiotic process.…”

“…), inoculated roots (7 d.p.i.) and nodules (4 weeks post inoculation) were collected and immediately frozen in liquid nitrogen as done by Hocher et al (2011). Five biological replicates were used for each condition.…”

“…Plant growth conditions and mRNA extraction A. glutinosa seeds were harvested from an A. glutinosa specimen growing on the Rhô ne River banks in Lyon, France, as described previously (Alloisio et al, 2010;Hocher et al, 2011). Seeds were grown and inoculated with the compatible F. alni strain ACN14a (Normand and Lalonde, 1982) as described previously (Alloisio et al, 2010).…”

“…In actinorhizal symbiosis, a collection of Alnus glutinosa and Casuarina glauca ESTs has been sequenced from roots and 3-week-old nodules (Hocher et al, 2011). These analyses showed that several genes related to carbon and nitrogen metabolism, defense against pathogens or stress resistance were strongly upregulated in nodules.…”

“…However, key steps in the symbiotic process, including how fixed nitrogen is transferred from symbionts to host plants, are still only partially understood despite efforts made in this area (Benson and Silvester, 1993;Benson et al, 2011;Oldroyd et al, 2011;Pawlowski et al, 2011). High-throughput sequencing of expressed sequence tags (ESTs) and DNA arrays are powerful tools to understand biological processes and have been widely used recently to study nitrogen-fixing symbiotic interactions (Ampe et al, 2003;Alloisio et al, 2010;Hocher et al, 2011;Heath et al, 2012). Nevertheless, there is presently no obvious candidate gene identified in either rhizobia or Frankia to account for release of nitrogen compounds to plants.…”