The responses of maize roots to nitrogen supply

Abstract

Substantial quantities of costly nitrogen (N) fertilisers are applied to cereal crops each year to maximise yields, but only approximately half of the N is captured by cereals, providing scope to increase root N uptake. However, our understanding of how the nitrate (NO₃⁻) uptake system is regulated and how it could be improved is limited. Furthermore, the changes to root morphology in response to NO₃⁻ supply are not well understood, in this case due to the difficulties associated with phenotyping roots in soil. To investigate how the NO₃⁻ uptake system is up-regulated, maize (Zea mays var. B73 and Mo17) was grown hydroponically with low or sufficient NO₃⁻ supply, and a range of physiological parameters associated with NO₃⁻ uptake were measured across the transition from seed N use, to external N capture. This transition provides an ideal system to clarify how the NO₃⁻ uptake system up-regulates as this is when plants first rely on increasing root N capture to meet demand. Across both lines and treatments, concentrations of shoot N and free amino acids in roots and shoots rapidly decrease as seed N reserves exhaust. Once free amino acid concentrations decrease to a critical level, root NO₃⁻ uptake capacity rapidly increased, corresponding with a rise in transcript levels of putative NO₃⁻ transporter genes ZmNRT2.1 and ZmNRT2.2. As NO₃⁻ uptake capacity reached maximum levels, shoot N concentrations stabilised. Despite shoot N concentrations stabilising, B73 was unable to maintain net N uptake and shoot growth in low NO₃⁻, relative to sufficient NO₃⁻. Conversely, Mo17 maintained shoot growth and net N uptake, and increased root mass in low NO₃⁻ relative to sufficient NO₃⁻. The effects of NO₃⁻ limitation on growth were reflected by an increased root:shoot, which emerged just prior to shoot N concentrations stabilising. In order to understand how root morphology may reflect the NO₃⁻ treatments differences observed in growth and net N uptake, morphological root traits were quantified across seedling development. Analysis showed that although B73 achieved greater absorption area per unit root mass than Mo17, its morphology was unresponsive to NO₃⁻ supply. Conversely, Mo17 responded to NO₃⁻ limitation by increasing lateral and axial root length before increasing root mass or volume. Subsequently, 11 putative quantitative trait loci (QTL) associated with morphological root traits corresponding with shoot growth or N uptake were detected across low and sufficient NO₃⁻, with one major QTL for lateral root length and surface area being identified in low NO₃⁻ on chromosome 5. These results provide insight into the processes involved in up-regulating root NO₃⁻ uptake capacity and how root morphology can adapt to NO₃⁻ supply. These findings identify potential control points in the regulation of NO₃⁻ uptake capacity and root morphology, which may be investigated further via global transcriptional analysis or fine-mapping of identified QTL respectively. Ultimately, this work may lead to identification of candidate regulatory genes that could be either manipulated to generate new lines with enhanced N uptake efficiencies, or allow the identification of germplasm with this trait.