The response of the maize nitrate transport system to nitrogen demand and supply across the lifecycle.

An understanding of nitrate (NO3-) uptake throughout the lifecycle of plants, and how this process responds to nitrogen (N) availability, is an important step towards the development of plants with improved nitrogen use efficiency (NUE). NO3- uptake capacity and transcript levels of putative high- and low-affinity NO3- transporters (NRTs) were profiled across the lifecycle of dwarf maize (Zea mays) plants grown at reduced and adequate NO3-. Plants showed major changes in high-affinity NO3- uptake capacity across the lifecycle, which varied with changing relative growth rates of roots and shoots. Transcript abundances of putative high-affinity NRTs (predominantly ZmNRT2.1 and ZmNRT2.2) were correlated with two distinct peaks in high-affinity root NO3- uptake capacity and also N availability. The reduction in NO3- supply during the lifecycle led to a dramatic increase in NO3- uptake capacity, which preceded changes in transcript levels of NRTs, suggesting a model with short-term post-translational regulation and longer term transcriptional regulation of NO3- uptake capacity. These observations offer new insight into the control of NO3- uptake by both plant developmental processes and N availability, and identify key control points that may be targeted by future plant improvement programmes to enhance N uptake relative to availability and/or demand.

Nitrate transport capacity of the Arabidopsis thaliana NRT2 family members and their interactions with AtNAR2.1.

• Interactions between the Arabidopsis NitRate Transporter (AtNRT2.1) and Nitrate Assimilation Related protein (AtNAR2.1, also known as AtNRT3.1) have been well documented, and confirmed by the demonstration that AtNRT2.1 and AtNAR2.1 form a 150-kDa plasma membrane complex, thought to constitute the high-affinity nitrate transporter of Arabidopsis thaliana roots. Here, we have investigated interactions between the remaining AtNRT2 family members (AtNRT2.2 to AtNRT2.7) and AtNAR2.1, and their capacity for nitrate transport. • Three different systems were used to examine possible interactions with AtNAR2.1: membrane yeast split-ubiquitin, bimolecular fluorescence complementation in A. thaliana protoplasts and nitrate uptake in Xenopus oocytes. • All NRT2s, except for AtNRT2.7, restored growth and ß-galactosidase activity in the yeast split-ubiquitin system, and split-YFP fluorescence in A. thaliana protoplasts only when co-expressed with AtNAR2.1. Thus, except for AtNRT2.7, all other NRT2 transporters interact strongly with AtNAR2.1. • Co-injection into Xenopus oocytes of cRNA of all NRT2 genes together with cRNA of AtNAR2.1 resulted in statistically significant increases of uptake over and above that resulting from single cRNA injections.

Improving Nitrogen Use Efficiency Through Overexpression of Alanine Aminotransferase in Rice, Wheat, and Barley.

Nitrogen is an essential nutrient for plants, but crop plants are inefficient in the acquisition and utilization of applied nitrogen. This often results in producers over applying nitrogen fertilizers, which can negatively impact the environment. The development of crop plants with more efficient nitrogen usage is, therefore, an important research goal in achieving greater agricultural sustainability. We utilized genetically modified rice lines over-expressing a barley alanine aminotransferase (HvAlaAT) to help characterize pathways which lead to more efficient use of nitrogen. Under the control of a stress-inducible promoter OsAnt1, OsAnt1:HvAlaAT lines have increased above-ground biomass with little change to both nitrate and ammonium uptake rates. Based on metabolic profiles, carbon metabolites, particularly those involved in glycolysis and the tricarboxylic acid (TCA) cycle, were significantly altered in roots of OsAnt1:HvAlaAT lines, suggesting higher metabolic turnover. Moreover, transcriptomic data revealed that genes involved in glycolysis and TCA cycle were upregulated. These observations suggest that higher activity of these two processes could result in higher energy production, driving higher nitrogen assimilation, consequently increasing biomass production. Other potential mechanisms contributing to a nitrogen-use efficient phenotype include involvements of phytohormonal responses and an alteration in secondary metabolism. We also conducted basic growth studies to evaluate the effect of the OsAnt1:HvAlaAT transgene in barley and wheat, which the transgenic crop plants increased seed production under controlled environmental conditions. This study provides comprehensive profiling of genetic and metabolic responses to the over-expression of AlaAT and unravels several components and pathways which contribute to its nitrogen-use efficient phenotype.