Periodic Reporting for period 2 - Nitro Systems (Reaching the roots of systemic nitrogen (N) signaling in plants)
Reporting period: 2017-09-01 to 2018-08-31
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.
The analysis of the gene expression patterns at 8h in the different tissues converge to indicate that various systemic signals co-exist. We identified 3 root-to-shoot signals (Fig.9 and 11): a N-supply signal, a N-demand signal and a “N at tip” signal coming from the root apex and indicating to the shoot whether N is present of not (Fig. 10). The latter signals is independent of any nutritional consideration and represent a so far unknown and very specific systemic signaling pathway. We showed that “N at tip signal” is dependent on root apex (3mm) and on the nitrate sensor previously mentioned. Besides, we discovered that shoots seem to “sense” how much N has been accumulated in aerial parts and to regulate genes accordingly (Fig. 9 and 11 “N-dose” pattern). In addition, shoots would integrate the “N-status” information with the 3 ascending signals and would send an integrative signal back to the roots (N-repletion, N-depletion, heterogeneous N, etc.) (Fig.7). In roots, those systemic signals would be integrated with local one to adjust gene expression.
The dissemination of these results will be done through the publication of a scientific article in one of the best journal. This article is currently under writing. After the publication, the question of a root tip signal directly transduced to the shoots will be further explored. Notably, we will address the respective role of the root and shoot apical meristems by exploiting the shoot transcriptional responses highlighted in this project.