Nitrogen-modulated effects of the diazotrophic bacterium Cupriavidus taiwanensis on the non-nodulating plant Arabidopsis thaliana

Nitrogen (N) is essential for plant growth, yet the role of diazotrophic bacteria in non-nodulating plants, particularly from the β-class Pseudomonadota, remains unclear. We explored the mechanisms underlying the interaction of Cupriavidus taiwanensis LMG19424, belonging to this group and known for nodulation induction in Mimosa sp., with the non-nodulating plant Arabidopsis thaliana. In vitro experiments with LMG19424 and Arabidopsis, varying N levels (KNO3), and utilizing plant and bacterial mutant lines were conducted. Then, we analyzed the physiological, nutritional, and molecular effects in the inoculated plants. We also evaluated the rhizospheric and endophytic colonization of the strain in this plant species. Inoculation with LMG19424 consistently increased rosette and root growth across a KNO3 gradient, except in roots in specific plant mutant lines (auxin and ethylene pathways). A nifH-deletion mutant of LMG19424 induced root development but lost the ability to stimulate rosette growth. C. taiwanensis colonized plants endophytically, increasing N and P content while decreasing Fe in plant tissues under varying KNO3 conditions. Furthermore, strain LMG19424 influenced plant gene expression in hormonal pathways and N transport/metabolism, with N conditions strongly affecting this regulation. In Arabidopsis, LMG19424 acts as a Plant Growth Promoting Bacterium, with N availability critically influencing its effect on plant physiology and gene regulation. Microbial-N fixation modulates host plant growth, with contrasting shoot and root responses. This emphasizes the pivotal role of the abiotic environment, particularly N availability, in shaping microbial-plant interactions and offers insights into interactions among diazotrophic β-class Pseudomonadota and non-nodulating plants.