Rice Ferredoxin-Dependent Glutamate Synthase Regulates Nitrogen-Carbon Metabolomes and Is Genetically Differentiated between japonica and indica Subspecies

Plants assimilate inorganic nitrogen absorbed from soil into organic forms as Gin and Glu through the glutamine synthetase/glutamine：2-oxoglutarate amidotransferase （GS/GOGAT） cycle. Whereas GS cata- lyzes the formation of Gin from Glu and ammonia, GOGAT catalyzes the transfer of an amide group from Gin to 2-oxoglutarate to produce two molecules of Glu. However, the regulatory role of the GS/GOGAT cycle in the carbon-nitrogen balance is not well understood. Here, we report the functional characterization of rice ABNORMAL CYTOKININ RESPONSE 1 （ABC1） gene that encodes a ferredoxin-dependent （Fd）- GOGAT. The weak mutant allele abcl-1 mutant shows a typical nitrogen-deficient syndrome, whereas the T-DNA insertional mutant abcl-2 is seedling lethal. Metabolomics analysis revealed the accumulation of an excessive amount of amino acids with high N/C ratio （Gin and Asn） and several intermediates in the tricarboxylic acid cycle in abcl-1, suggesting that ABC1 plays a critical role in nitrogen assimilation and carbon-nitrogen balance. Five non-synonymous single-nucleotide polymorphisms were identified in the ABC1 coding region and characterized as three distinct haplotypes, which have been highly and specifically differentiated between japonica and indica subspecies. Collectively, these results suggest that ABC1/ OsFd-GOGAT is essential for plant growth and development by modulating nitrogen assimilation and the carbon-nitrogen balance.

The Ghd7 transcription factor represses ARE1 expression to enhance nitrogen utilization and grain yield in rice

The genetic improvement of nitrogen use efficiency(NUE)of crops is vital for grain productivity and sustainable agriculture.However,the regulatory mechanism of NUE remains largely elusive.Here,we report that the rice Grain number,plant height,and heading date7(Ghd7)gene genetically acts upstream of ABC1 REPRESSOR1(ARE1),a negative regulator of NUE,to positively regulate nitrogen utilization.As a transcriptional repressor,Ghd7 directly binds to two Evening Element-like motifs in the promoter and intron 1 of ARE1,likely in a cooperative manner,to repress its expression.Ghd7 and ARE1 display diurnal expression patterns in an inverse oscillation manner,mirroring a regulatory scheme based on these two loci.Analysis of a panel of 2656 rice varieties suggests that the elite alleles of Ghd7 and ARE1 have undergone diversifying selection during breeding.Moreover,the allelic distribution of Ghd7 and ARE1 is associated with the soil nitrogen deposition rate in East Asia and South Asia.Remarkably,the combination of the Ghd7 and ARE1 elite alleles substantially improves NUE and yield performance under nitrogen-limiting conditions.Collectively,these results define a Ghd7–ARE1-based regulatory mechanism of nitrogen utilization,providing useful targets for genetic improvement of rice NUE.

Genetic manipulations of TaARE1 boost nitrogen utilization and grain yield in wheat

Nitrogen is a key element essential for plant growth and crop production,and the improvement of the nitrogen use efficiency(NUE)of crops largely contributes to yield production.The improvement of NUE is a major challenge in agriculture,not only for reducing the planting cost of crops.

Regulation of nitrogen starvation responses by the alarmone(p)ppGpp in rice

Nitrogen is an essential macronutrient for all living organisms and is critical for crop productivity and quality.In higher plants,inorganic nitrogen is absorbed through roots and then assimilated into amino acids by the highly conserved glutamine synthetase/glutamine:2-oxoglutarate aminotransferase(GS/GOGAT)cycle.How nitrogen metabolism and nitrogen starvation responses of plants are regulated remains largely unknown.Previous studies revealed that mutations in the rice ABNORMAL CYTOKININ RESPONSE1(ABC1)gene encoding Fd-GOGAT cause a typical nitrogen deficiency syndrome.Here,we show that ARE2(for ABC1 REPRESSOR2)is a key regulator of nitrogen starvation responses in rice.The are2 mutations partially rescue the nitrogen-deficient phenotype of abc1 and the are2 mutants show enhanced tolerance to nitrogen deficiency,suggesting that ARE2 genetically interacts with ABC1/Fd-GOGAT.ARE2 encodes a chloroplast-localized Rel A/Spo T homolog protein that catalyzes the hydrolysis of guanosine pentaphosphate or tetraphosphate(p)pp Gpp,an alarmone regulating the stringent response in bacteria under nutritional stress conditions.The are2 mutants accumulate excessive amounts of(p)pp Gpp,which correlate with lower levels of photosynthetic proteins and higher amino acid levels.Collectively,these observations suggest that the alarmone(p)pp Gpp mediates nitrogen stress responses and may constitute a highly conserved mechanism from bacteria to plants.