Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Tian, Lining

2nd Supervisor

Norman Hüner

Joint Supervisor

Abstract

Plant agricultural productivity relies heavily on nitrogenous fertilizers. Excess N fertilizer application can often lead to lower nitrogen use efficiency (NUE) along with energy waste and environmental problems. Therefore, understanding and improving NUE in plants are of key importance. The small monocot plant Brachypodium distachyon (Brachypodium) is rapidly emerging as a powerful model system to study questions unique to monocot crops (wheat, maize, rice, etc.).

Here, through an intensive BLAST search, six putative orthologues of the Arabidopsis NRT2 gene family were identified in the fully sequenced Brachypodium genome (Phytozome v11.0), among which I isolated a T-DNA mutant (bdnrt2.1) lacking BdNRT2.1. Analysis of individual BdNRT2 gene expression, plant nitrogen uptake, assimilation, remobilization, metabolic change under different nitrogen sources, concentrations, and developmental stage in wild-type and the bdnrt2.1 was performed.

Results demonstrate that BdNRT2 gene expressions are governed by internal nitrogen status rather than external nitrate concentrations. Genes in the BdNRT2 family have diverse roles differing from AtNRT2 in response to different nitrogen conditions. The BdNRT2.1 knock-out mutant showed an impaired inducible high-affinity transport system (iHATS), reduced nitrogen utilization efficiency (NUtE) and overall NUE (37% on average) under a N non-limited condition, whereas the constitutive high-affinity transport system (cHATS), low-affinity transport system (LATS) and nitrogen uptake efficiency (NUpE) were not affected. The mutant’s reduced NUE and iHATS phenotype could be rescued by complementation. Furthermore, BdNRT2.1 had a contrasting impact on nitrogen metabolism at different developmental stages, suggesting it serves a more important role (signal transducer) after anthesis. Finally, BdNRT2.1 overexpressing transgenic Brachypodium lines had significantly higher grain yield. This demonstrates that BdNRT2.1, serving as a key member of the family, is involved in nitrogen remobilization, and it has potential application for more efficient use of nitrogen fertilizer in monocot crops. These results provide the possibility for future experiments to elucidate the specific roles of each NRT2 transporter in monocot plants.

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