AUTHOR=Alhusayni Sultan , Kersten Nick , Huisman Rik , Geurts Rene , Klein Joël 

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

JOURNAL=Frontiers in Plant Science

VOLUME=15

YEAR=2024

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2024.1468812

DOI=10.3389/fpls.2024.1468812

ISSN=1664-462X

ABSTRACT=<sec><title>Introduction</title><p>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 (<italic>NSP2</italic>) is a conserved hub in this process. The <italic>NSP2</italic>-regulated transcriptional network is instrumental in balancing nutrient homeostasis with symbiotic interactions. <italic>NSP2</italic> activity is modulated post-transcriptionally by a specific microRNA. Overriding this control mechanism by ectopic expression of a miRNA-resistant <italic>NSP2</italic> 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.</p></sec><sec><title>Method</title><p>We used the nodulating <italic>Cannabaceae</italic> species <italic>Parasponia andersonii</italic> as an experimental system to study the effect of ectopic <italic>NSP2</italic> expression. Parasponia and legumes (Fabaceae) diverged 100 million years ago, providing a unique comparative system to dissect the nodulation trait.</p></sec><sec><title>Results</title><p>Six independent transgenic <italic>Parasponia</italic> lines were generated that differed in the level of <italic>NSP2</italic> 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 <italic>NSP2</italic> expression level, as well as with the expression of the symbiosis transcription factor <italic>CYCLOPS</italic> and the rate-limiting enzyme in the carotenoid biosynthetic pathway <italic>PHYTOENE SYNTHASE1</italic> (<italic>PSY1</italic>). Yet ectopic expression of <italic>NSP2</italic> affected plant architecture and root nodule organogenesis.</p></sec><sec><title>Discussion</title><p>This indicates a significant trade-off when leveraging <italic>NSP2</italic> over-expression to enhance endomycorrhization.</p></sec>