Combined Allosteric Responses Explain the Bifurcation in Non-Linear Dynamics of 15N Root Fluxes Under Nutritional Steady-State Conditions for Nitrate.

Erwan Le Deunff,Philippe Malagoli,Julien Lecourt,Carole Deleu,Patrick Beauclair

doi:10.3389/fpls.2020.01253

Abstract

With regard to thermodynamics out of equilibrium, seedlings are open systems that dissipate energy towards their environment. Accordingly, under nutritional steady-state conditions, changes in external concentrations of one single ion provokes instability and reorganization in the metabolic and structure/architecture of the seedling that is more favorable to the fluxes of energy and matter. This reorganization is called a bifurcation and is described in mathematics as a non-linear dynamic system. In this study, we investigate the non-linear dynamics of 15N fluxes among cellular compartments of B. napus seedlings in response to a wide range of external concentrations (from 0.05 to 20 mM): this allows to determine whether any stationary states and bifurcations could be found. The biphasic behavior of the root uptake rate (vin) was explained by the combined cooperative properties between the vapp (N uptake, storage and assimilation rate) and vout (N translocation rate) 15N fluxes that revealed a unique and stable stationary state around 0.28 mM nitrate. The disappearance of this stationary state around 0.5 mM external nitrate concentrations provokes a dramatic bifurcation in 15N flux pattern. This bifurcation in the vin and vout 15N fluxes fits better with the increase of BnNPF6.3/NRT1.1 expression than BnNRT2.1 nitrate transporter genes, confirming the allosteric property of the BnNPF6/NRT1.1 transporter, as reported in the literature between low and high nitrate concentrations. Moreover, several statistically significant power-law equations were found between variations in the shoots tryptophan concentrations (i.e., IAA precursor) with changes in the vapp and vout 15N fluxes as well as a synthetic parameter of plant N status estimated from the root/shoot ratio of total free amino acids concentrations. These relationships designate IAA as one of the major biological parameters related to metabolic and structural-morphological reorganization coupled with the N and water fluxes induced by nitrate. The results seriously challenge the scientific grounds of the concept of high- and low-affinity of nitrate transporters and are therefore discussed in terms of the ecological significance and physiological implications on the basis of recent agronomic, physiological and molecular data of the literature.

Highlights

The integrated understanding of the nitrogen nutritional function at the whole plant level continues to remain elusive
This study presents a re-examination of the 15N fluxes in seedlings of B. napus growing under nutritional steady-state conditions in agar plates over a wide range of external nitrate concentrations in order to determine whether stationary states and bifurcations in the dynamic fluxes of 15N may be found
The results showed that changes in the 15N fluxes were significantly correlated with the root transcriptional induction of the two nitrate transporter genes BnNPF6.3/BnNRT1.1 and BnNRT2.1 (Figure 7) indicating a transcriptional coordination of the two transporters along the symplastic pathway with the increase in 15N flow rates throughout changes in the external nitrate concentrations (Figures 7A, B)

Summary

Introduction

The integrated understanding of the nitrogen nutritional function at the whole plant level continues to remain elusive. They revealed a biphasic K+ absorption pattern mainly explained by two distinct absorption mechanisms involving two different potassium transport systems (kinetic components of ion flux across the root): one acting at low external concentrations (< 0.5– 1 mM) and the other operating for high external concentrations (> 1 mM) Such as NO−3 , K+, Cl-, and Ca2+ the underlying process responsible for the abrupt increase in ion influx rate between mechanism I and mechanism II beyond a threshold value around 0.5–1 mM has remained unexplained (Epstein et al, 1963; Thellier, 1973; Siddiqi et al, 1990). From the seminal works of Epstein and co-workers and their Enzyme-substrate interpretation of mechanisms I (renamed HATS for Low-Affinity Transport System) and II (renamed LATS for High-Affinity Transport System) of ion isotherms, a dynamic explanation about the abrupt transition between these two mechanisms has not been found yet (Le Deunff and Malagoli, 2014a)

Methods

Results

Discussion

Conclusion