Ectopic expression of the GRAS-type transcriptional regulator in triggers contrasting effects on symbioses.

Introduction: Plants strictly control root endosymbioses with nutrient-scavenging arbuscular endomycorrhizal fungi or nodule inducing diazotrophic bacteria. The GRAS-type transcriptional regulator NODULATION SIGNALING PATHWAY 2 () is a conserved hub in this process. The -regulated transcriptional network is instrumental in balancing nutrient homeostasis with symbiotic interactions. activity is modulated post-transcriptionally by a specific microRNA. Overriding this control mechanism by ectopic expression of a miRNA-resistant transgene enhances the symbiotic permissiveness to arbuscular endomycorrhizal fungi. Such engineered plants may possess enhanced capacities for nutrient uptake. However, the trade-off of this strategy on plant development or other symbiotic interactions, like nodulation, is yet to be fully understood.

Method: We used the nodulating species as an experimental system to study the effect of ectopic expression. Parasponia and legumes (Fabaceae) diverged 100 million years ago, providing a unique comparative system to dissect the nodulation trait.

Results: Six independent transgenic lines were generated that differed in the level of expression in the root from 6 to 95-fold higher when compared to the empty vector control plants. Analysis of these plants revealed a positive correlation between mycorrhization and the expression level, as well as with the expression of the symbiosis transcription factor and the rate-limiting enzyme in the carotenoid biosynthetic pathway (). Yet ectopic expression of affected plant architecture and root nodule organogenesis.

Discussion: This indicates a significant trade-off when leveraging over-expression to enhance endomycorrhization.