Shoot nitrate underlies a perception of nitrogen satiety to trigger local and systemic signaling cascades in Arabidopsis thaliana

Abstract

Nitrate is an important ion for plant growth and development. It serves not only as a building block for amino acid synthesis but also as a signaling molecule. Changes in the exogenous nitrate concentrations affect the expression of nitrate-responsive genes within minutes. Following these rapid transcriptional events, nitrate and its downstream organic nitrogen (N) compounds accumulate in the plant body, inducing secondary responses to the internal N level. Nevertheless, the respective roles of nitrate and organic N in triggering plant responses to internal N remain to be clarified. Several studies have implied that internal nitrate levels regulate root N uptake independent of the levels of N assimilation products. However, little is known about the specific effects of internal nitrate levels on plant growth and gene expression. To manipulate the internal nitrate levels independently of internal organic N, we grew wildtype Arabidopsis and a nitrate reductase (NR)-null mutant under a series of modified N conditions. Using their shoots and roots, we performed analyses of plant growth and RNA sequencing. The results showed that elevated shoot nitrate accumulation in the NR-null mutant was accompanied by increased expression of nitrate assimilatory genes in the shoots, decreased gene expression of high-affinity nitrate and ammonium uptake transporters in the roots, and decreased lateral root growth. Furthermore, the genes normally induced by N deficiency were significantly downregulated both in the shoots and roots of the NR-null mutant, compared with the wild-type. Our transcriptional profiling suggests that the transcription factors NLP7 and NIGT mediate a wide range of these transcriptional responses. Taken together, we conclude that shoot nitrate acts as a N satiety signal to trigger local and systemic signaling cascades in A. thaliana. The present study illustrates an adaptive strategy of plants to survive in N-limited environments, depending on the residual nitrate storage. ARTICLE HISTORY

Excessive ammonium assimilation by plastidic glutamine synthetase causes ammonium toxicity in Arabidopsis thaliana

“…The seeds were placed in the dark at 4 °C for 3 d. Plants were grown in a horizontal position under a photosynthetic photon flux density of 100–130 μmol m −2 s −1 (16 h light/8 h dark cycle) at 23 °C. For the transfer experiments, surface-sterilized seeds were sown in plastic Petri dishes (length, 140 mm; width, 100 mm; depth, 20 mm; Eiken Chemical Co. Ltd., Taito-ku, Tokyo, Japan) containing 50-mL half-strength N-modified Murashige and Skoog medium containing 2.5 mM ammonium as the sole N source, supplemented with 4.7 mM MES, 1% (w/v) sucrose, and 0.4% (w/v) gellan gum (pH 6.7) 40 . This culture condition allowed almost uniform growth of different lines, alleviating ammonium toxicity.…”

“…Immunoblot analysis of GLN proteins was performed based on a previously described method 40 . Shoots were harvested, frozen with liquid N 2 , and stored at −80 °C until use.…”

A reevaluation of the contribution of NRT1.1 to nitrate uptake in Arabidopsis under low‐nitrate supply

“…Keywords: Arabidopsis thaliana; dual-affinity nitrate transporter; nitrate uptake; NRT1.1; NRT2.1; NRT2.2 Nitrate (NO À 3 ) is the major form of nitrogen absorbed by the roots of most terrestrial plants [1]. It not only acts an N source for growth and development but has also been proposed to act as a signal regulating several physiological processes, such as seed germination [2], root development [3], and flowering time [4,5], among others [6][7][8][9]. To better adapt to fluctuating nitrate availability, plants have evolved two uptake systems: one operating at low external nitrate concentrations, where nitrate is absorbed by high-affinity transport systems (HATS), the other acting at high nitrate concentrations through low-affinity transport systems (LATS) [10][11][12].…”

Modeling of Root Nitrate Responses Suggests Preferential Foraging Arises From the Integration of Demand, Supply and Local Presence Signals

“…First, while split root experiments are generally taken to indicate the presence of growth stimulating demand and growth suppressing supply signals, thus far no supply signal specific for heterogeneous nitrate conditions has been proposed. Additionally, while the role for systemic nitrate levels in modulating root growth dynamics was recently further substantiated (Okamoto et al, 2019), how systemic survival, FIGURE 1 | Response of root system architecture to environmental and internal nitrate status. (A) Range of growth responses occurring for increasing homogeneous external nitrate levels.…”