Nitro Systems · Reaching the roots of systemic nitrogen (N) signaling in plants

Reaching the roots of systemic nitrogen (N) signaling in plants

The goal of this project was to develop a molecular understanding of how a multi-organ organism responds to environmental perturbation as an integrated system. Plants especially need to mount a system-wide response to fluctuating signals and are an ideal model to study systemic responses to perturbations. An example of such response is illustrated by a long-distance “root-shoot-root” relay that enables roots to specifically forage for nitrogen (N)-rich patches in a heterogeneous soil environment. Indeed, the level of N is a limiting factor for plant growth and varies by several orders of magnitude in soil. Plants can adapt to this challenge thanks to the ability of roots to sense uneven N concentrations in the soil and to forage via root growth within the N-rich patches. To uncover the underlying regulatory mechanisms, a “split-root” system has been previously studied into the genomic era. Roots of a single plant are split into two halves and exposed to distinct N environments (N-replete vs. N-deplete) (Fig. 1A). It mimics the heterogeneous soil N conditions and allows to distinguish systemic from local N-responses (Fig. 1B). The N-foraging response of roots was shown to be mediated by two distinct systemic N-signals (N-demand and N-supply), dependent on a root-shoot-root signal relay (Fig. 1A). Therefore, the split-root system is a great model to study systemic inter-organ responses to external stimulus in a multi-organ organism. The goal of Nitro Systems is to decipher the identity of the long-distance signals mediating the systemic signals in Arabidopsis. Our first aim was to generate time-series RNA-seq data from shoots and roots of plants grown on heterogeneous N-environments (Fig.2B, left panel and Fig.2C). The following aims were to physically capture inter-organ travelling RNAs by sequencing RNA in phloem cells (Fig.2B, right panel and Fig.2C), to identify candidate genes involved in inter-organ signaling by using predictive time-based modeling of RNA-seq data and to validate the highest-rank candidate genes using a genetic approach. The system used is a two compartment Vertical Heterogeneous N-environment (VHN) system (Fig.2A). The growth media on each plate is divided into 2 sections (top/bottom) of medium with one section containing KNO3 (N-replete) and the other section an equivalent concentration of KCl (N-depleted) (Fig.2A). Controls are (i) homogeneous N-depleted (KCl) on each, and (ii) homogeneous N-replete (KNO3) media on each. After having performed an integrative study of N-foraging response in this VHN system, we concluded that systemic N-demand but not systemic N-supply signaling operates in this system to control root architecture and N-accumulation. Specifically in the older part of the root system, we show for the first time that this systemic N-demand signaling required the activity of the transceptor NRT1.1. Using this system, our last and most interesting conclusion is that in addition to previously known systemic signaling, a « N at tip » signal regulates a part of the molecular responses of the shoots to the nitrate provision allowing the plant to know at a systems level if nitrate is available at the tip of the primary root.

Data: CORDIS, © European Union