Ectopic expression of the GRAS-type transcriptional regulator NSP2 in Parasponia triggers contrasting effects on symbioses

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