Over-use of N fertilizer in crop production has resulted in a series of environmental problems in the North China Plain (NCP).  Thus, improvement of nitrogen use efficiency (NUE) in summer maize has become an effective strategy for promoting sustainable agriculture in this region.  Using twenty maize cultivars, plant dry matter production, N absorption and accumulation, yield formation, and NUE in summer maize were investigated under three N levels in two growing seasons.  Based on their yield and yield components, these maize cultivars were categorized into four groups including efficient-efficient (EE) cultivars, high-nitrogen efficient (HNE) cultivars, low-nitrogen efficient (LNE) cultivars and nonefficient-nonefficient (NN) cultivars.  In both two seasons, the EE cultivars improved grain yield together with increased plant biomass, and enhanced accumulative amounts as well as higher average grain yields than the other cultivar groups under deficient-N conditions.  Significant correlations were observed between yield and kernel numbers (KN), dry matter (DM) amount and N accumulation at both post-silking and maturity stages.  DM and N accumulation at late growth stage (i.e., from silking to maturity) contributed largely to the enhanced yield capacity and improved NUE under N-deficient conditions.  Compared with the NN cultivars, the EE cultivars also showed increased N assimilation amount (NAA) and N remobilization content (NRC), and elevated N remobilization efficiency (NRE), NUE and nitrogen partial factor productivity (PFPN).  Our investigation has revealed N-associated physiological processes and may provide guidance for cultivation and breeding of high yield and NUE summer maize under limited N conditions in the NCP.

Plant N starvation response is closely associated with the N signaling components that involve transduction of the low-N cues.  In this study, we functionally characterized TaARR1, a cytokinin (CK) response regulator gene in Triticum aestivum, in mediating the N starvation adaptation in plants.  TaARR1 harbors two conserved domains specified by plant ARR family members; subcellular localization analysis indicated its target onto nucleus after endoplasmic reticulum assortment.  TaARR1 displayed modified expression upon the N starvation stressor, showing upregulated expression in roots and leaves over a 27-h N starvation treatment and whose induced transcripts were gradually recovered along with progression of the N recovery treatment.  The tobacco lines overexpressing TaARR1 displayed improved low-N stress tolerance, displaying enlarged phenotype, increased biomass and N accumulation, and enhanced glutamine synthetase (GS) activities compared with wild type (WT) following the N starvation treatment.  Expression analysis on genes encoding the nitrate transporter (NRT) and GS proteins in Nicotiana tabacum revealed that NtNRT2.2 and NtGS3 are upregulated in expression in the N-deprived transgenic lines, whose expression patterns were contrasted to other above family genes that were unaltered on transcripts between the transgenic lines and WT.  Transgene analysis validated the function of NtNRT2.2 and NtGS3 in regulating N accumulation, GS activity, growth traits, and N use efficiency in plants.  These results suggested the internal connection between the TaARR1-mediated N starvation tolerance and the modified transcription of distinct N acquisition- and assimilation-associated genes.  Our investigation together indicates that TaARR1 is essential in plant N starvation adaptation due to the gene function in transcriptionally regulating distinct NRT and GS genes that affect plant N uptake and assimilation under the N starvation condition.

Through regulating target genes via the mechanisms of posttranscriptional cleavage or translational repression, plant miRNAs involve diverse biological processes associating with plant growth, development, and abiotic stress responses.  In this study, we functionally characterized TaMIR1119, a miRNA family member of wheat (Triticum aestivum), in regulating the drought adaptive response of plants.  TaMIR1119 putatively targets six genes categorized into the functional classes of transcriptional regulation, RNA and biochemical metabolism, trafficking, and oxidative stress defense.  Upon simulated drought stress, the TaMIR1119 transcripts abundance in roots was drastically altered, showing to be upregulated gradually within a 48-h drought regime and that the drought-induced transcripts were gradually restored along with a 48-h recovery treatment.  In contrast, most miRNA target genes displayed reverse expression patterns to TaMIR1119, exhibiting a downregulated expression pattern upon drought and whose reduced transcripts were re-elevated along with a normal recovery treatment. These expression analysis results indicated that TaMIR1119 responds to drought and regulates the target genes mainly through a cleavage mechanism.  Under drought stress, the tobacco lines with TaMIR1119 overexpression behaved improved phenotypes, showing increased plant biomass, photosynthetic parameters, osmolyte accumulation, and enhanced antioxidant enzyme (AE) activities relative to wild type.  Three AE genes, NtFeSOD, NtCAT1;3, and NtSOD2;1, encoding superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) proteins, respectively, showed upregulated expression in TaMIR1119 overexpression lines, suggesting that they are involved in the regulation of AE activities and contribution to the improved cellular reactive oxygen species (ROS) homeostasis in drought-challenged transgenic lines.  Our results indicate that TaMIR1119 plays critical roles in regulating plant drought tolerance through transcriptionally regulating the target genes that modulate osmolyte accumulation, photosynthetic function, and improve cellular ROS homeostasis of plants.