Spatiotemporal variation of nitrate uptake kinetics within the maize (Zea mays L.) root system is associated with greater nitrate uptake and interactions with architectural phenes.

Larry M. York,Moshe Silberbush,Jonathan P. Lynch

doi:10.1093/jxb/erw133

Larry M. York, Moshe Silberbush + Show 1 more

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https://doi.org/10.1093/jxb/erw133

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Abstract

Increasing maize nitrogen acquisition efficiency is a major goal for the 21st century. Nitrate uptake kinetics (NUK) are defined by I max and K m, which denote the maximum uptake rate and the affinity of transporters, respectively. Because NUK have been studied predominantly at the molecular and whole-root system levels, little is known about the functional importance of NUK variation within root systems. A novel method was created to measure NUK of root segments that demonstrated variation in NUK among root classes (seminal, lateral, crown, and brace). I max varied among root class, plant age, and nitrate deprivation combinations, but was most affected by plant age, which increased I max, and nitrate deprivation time, which decreased I max K m was greatest for crown roots. The functional-structural simulation SimRoot was used for sensitivity analysis of plant growth to root segment I max and K m, as well as to test interactions of I max with root system architectural phenes. Simulated plant growth was more sensitive to I max than K m, and reached an asymptote near the maximum I max observed in the empirical studies. Increasing the I max of lateral roots had the largest effect on shoot growth. Additive effects of I max and architectural phenes on nitrate uptake were observed. Empirically, only lateral root tips aged 20 d operated at the maximum I max, and simulations demonstrated that increasing all seminal and lateral classes to this maximum rate could increase plant growth by as much as 26%. Therefore, optimizing I max for all maize root classes merits attention as a promising breeding goal.

Highlights

An increase of 100% in food production is necessary to meet the requirements of the global population of 9.7 billion predicted by 2050 (World Bank, 2014) to address global food insecurity, a defining challenge of this century (Funk and Brown, 2009)
Imax varied among root classes, root position, plant ages, and number of days of nitrate deprivation (Table 1; Fig. 3), with the slowest Imax being 14.02 pmol cm−2 s−1 observed in the 0–4 cm region of crown roots at 15 d of age after 2 d of nitrate deprivation, and the greatest Imax being 46.52 pmol cm−2 s−1 observed for crowns in the 4–8 cm region at 20 d of age after 2 d of nitrate deprivation
Phenotyping of root segment Nitrate uptake kinetics (NUK) coupled to genome-wide association studies could prove to be a very powerful approach to discover quickly genomic regions associated with optimal kinetics and to use those in breeding programs

Summary

Introduction

Nitrate is generally the most abundant form of available nitrogen in agricultural systems and acquired by crops in the greatest amounts (Miller and Cramer, 2004). The rate of nitrate absorption by a localized root segment is largely determined by nitrate uptake kinetics (NUK), which determine influx of nitrate as a function of external nitrate concentration. Uptake kinetics were modeled as an uptake rate that saturates as the nitrate concentration increases in solution surrounding the roots with first-order unidirectional kinetics. Given these assumptions, the relationship between uptake rate and external nitrate concentration is summarized with the Michaelis–Menten parameters Imax, Km, and Cmin (see Equation 5). The affinity of a transporter for its substrate is represented by Km and determines how well the transporter operates at low substrate concentrations

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