Nitrogen-Controlled Host Gatekeeping: Regulatory Chokepoints Across Four Windows for Diazotroph Access.

Although diazotrophy is compatible with low-carbon agriculture and aligned with sustainability goals, its benefits could be expanded by better leveraging associative plant-diazotroph partnerships. Host control, however, remains underemphasized despite increasing resolution of microbial determinants of colonization. Understanding how plants tune permissiveness across fluctuating mineral N landscapes is therefore central to explaining when microbial presence translates into measurable diazotrophic function and plant N gain. Here, we propose an N-mediated host gatekeeping framework that organizes existing evidence into four licensing windows: (i) spatial positioning of permissive sites, (ii) N-sensitive transcriptional thresholding, (iii) local immune tuning at the contact interface, and (iv) carbon energy arbitration sustaining fixation and N transfer. Our model predicts that moderate, spatially heterogeneous mineral N biases the root interface toward permissive states in which microdomain colonization can translate into measurable biological nitrogen fixation, whereas at either extreme one or more windows tend to close. In crops, soil heterogeneity and genotype-linked root functional traits act as filters shaping when functional engagement becomes possible. By reframing N as both a resource and a signal acting through host arbitration, this model clarifies how permissiveness can be tuned to better realize diazotrophic potential and support plant N gain under rational mineral N management.