BACKGROUND: Nitrogen-fixing nodules occur in ten related taxonomic lineages interspersed with lineages of non-nodulating plant species. Nodules result from an endosymbiosis between plants and diazotrophic bacteria; rhizobia in the case of legumes and Parasponia and Frankia in the case of actinorhizal species. Nodulating plants share a conserved set of symbiosis genes, whereas related non-nodulating sister species show pseudogenization of several key nodulation-specific genes. Signalling and cellular mechanisms critical for nodulation have been co-opted from the more ancient plant-fungal arbuscular endomycorrhizal symbiosis. Studies in legumes and actinorhizal plants uncovered a key component in symbiotic signalling, the LRR-type SYMBIOSIS RECEPTOR KINASE (SYMRK). SYMRK is essential for nodulation and arbuscular endomycorrhizal symbiosis. To our surprise, however, despite its arbuscular endomycorrhizal symbiosis capacities, we observed a seemingly critical mutation in a donor splice site in the SYMRK gene of Trema orientalis, the non-nodulating sister species of Parasponia. This led us to investigate the symbiotic functioning of SYMRK in the Trema-Parasponia lineage and to address the question of to what extent a single nucleotide polymorphism in a donor splice site affects the symbiotic functioning of SYMRK.RESULTS: We show that SYMRK is essential for nodulation and endomycorrhization in Parasponia andersonii. Subsequently, it is revealed that the 5'-intron donor splice site of SYMRK intron 12 is variable and, in most dicotyledon species, doesn't contain the canonical dinucleotide 'GT' signature but the much less common motif 'GC'. Strikingly, in T. orientalis, this motif is converted into a rare non-canonical 5'-intron donor splice site 'GA'. This SYMRK allele, however, is fully functional and spreads in the T. orientalis population of Malaysian Borneo. A further investigation into the occurrence of the non-canonical GA-AG splice sites confirmed that these are extremely rare.
CONCLUSION: SYMRK functioning is highly conserved in legumes, actinorhizal plants, and Parasponia. The gene possesses a non-common 5'-intron GC donor splice site in intron 12, which is converted into a GA in T. orientalis accessions of Malaysian Borneo. The discovery of this functional GA-AG splice site in SYMRK highlights a gap in our understanding of splice donor sites.