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Type: Theses
Title: The responses of maize roots to nitrogen supply
Author: Sabermanesh, Kasra
Issue Date: 2014
School/Discipline: School of Agriculture, Food and Wine
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.
Advisor: Heuer, Sigrid
Garnett, Trevor Paul
Plett, Darren Craig
Tester, Mark Alfred
Dissertation Note: Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Agriculture, Food and Wine, 2014.
Keywords: maize
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
DOI: 10.4225/55/583e5c5c93e82
Appears in Collections:Research Theses

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