| A better understanding of the biochemical and genetic responses associated with N metabolism in Maize plants;with potential to adapt to low nitrogen nutrients availability can help to select better germplasm for future breeding program.A hydroponic study was designed to evaluate the biochemical and morphological responses associated with N metabolism in two elite Chinese maize inbred lines.The study revealed significant phenotypic responses under low nitrogen condition including a relative increase in plant biomass,total N content,root length and GS activity for N tolerant plants.Inbred line,Qi319 was found more tolerant to N stress under stress conditions;suggesting that Qi319 could be germplasm resource for improving abiotic stress resistance comparing Ye478.Therefore,it can be assumed that genetically improved abiotic stress resistant lines can be obtained by hybridization of Qi319 and Ye478.To further investigate the best performing germplasm as well as explore the genes associated to the given trait,a diverse maize germplasm of 779 maize accession lines constituting of 475 inbred and 304 RIL accession lines,were evaluated for low and normal nitrogen stress under hydroponic conditions.Genome wide association studies(GWAS)was performed using40,757 high-quality SNP markers in temperate and tropical maize groups;whereas,the RIL population was characterized using genotyping by sequencing(GBS)technology with high density linkage map using 86,257SNPs;phenotype in controlled high lux plant growth room for hydroponic culture.The maize germplasm showed significant differential responses.A total of 31 genes for plant biomass under normal nitrogen conditions including 11 genes with well-known function were identified.These include proteins responsible for mineral translocation,biotic stress and abiotic stress responses and regulation of biological pathways,Proteins kinase and cell signaling,nutrients translocation and pre-m RNA-processing.For obtaining an overview of the molecular events,genes regulation and regulatory elements involved in nitrogen metabolism pathways for early and late low nitrogen stress,we selected the most tolerant and most sensitive maize RIL accession lines out of 304 screened lines from thorough screening under hydroponics conditions in controlled environment supplied with normal(control)and low nitrogen for 1 day and 5 days,respectively.Based on biochemical and morphological screening(GS activity test;root and shoot fresh weight biomass),we evaluated the genetic responses and identified the genes related to early and late under low nitrogen conditions for the most tolerant ZYQ151 and the most sensitive ZYQ129 line based on their Transcriptome analysis.Differentially expressed genes(DEGs)ontology analysis for low nitrogen tolerant lines Qi319 and ZYQ151 revealed up regulation of biological processes for photosynthesis and responses to light stimulus but down regulated in Ye478 and ZYQ129,concluding that in low nitrogen tolerant plants,the GS activity enhances photosynthesis and hence reduce sharp loss of the plant biomass.The common up-regulated DEGs for both Ye478 and ZYQ129 by low nitrogen stress were related to fatty acid metabolism,fatty acid biosynthesis and biosynthesis of monocarboxylic acid suggesting that low nitrogen stress directly affects fatty acid metabolism.The relative upregulation of DEGs for the shoots of Qi319 and ZYQ151 under N stress show an increase in cell cycle,cell cycle process,DNA replication,regulation of mitotic cell cycle,nuclear division and mitotic cell cycle represents an increase in the crop biomass due to relative increase in GS activity for the mentioned germplasm.Field experiment for determining genes related to low and normal nitrogen stress affecting maize flowering time revealed a total of 13 QTL including 5 QTL for normal nitrogen while 8 QTL for low nitrogen treatments on chromosome 1,2,8 and 10 respectively.For normal nitrogen treatment,QTL were detected on chromosome 2,8 and 10 whereas for low nitrogen treatment,QTL were found on chromosome 1,8 and 10.The highest LOD score was recorded for QTL on Chromosome 10 and chromosome8 respectively.Furthermore,QTLs,with both negative and positive additive effect were recorded,indicating that both parents contributed favorable alleles.These detected QTL were confirmed during 2019 experiment with the same field conditions at the same location.We found 3 major QTL for both low and normal nitrogen treatment.The QTL on chromosome 8 with a LOD score of 3.99 confirms our candidate gene.Therefore based on gene ontology,it was found that ZCN8;member of Zea CENTRORADIALIS(ZCN)genes family that codes for phosphatidylethanolamine-binding protein8,plays and important role in maize flowering initiation(SD)under low nitrogen stress conditions.As ZCN8 has been demonstrated as major candidate gene for flowering time but no study has been carried out regarding its association with low nitrogen stress conditions.Therefore it suggests that it might have an important role in controlling flowering time(SD)in maize under low nitrogen. |
