Analysis Of Physiological And Transcriptome Characteristics Under Low Nitrogen Stress At Seedling Stage In Foxtail Millet

Nitrogen is an essential element for plant growth,and it is also one of the important factors to maintain crop metabolism,growth and development,yield and quality.In agriculture,application of nitrogen fertilizer is the main means to obtain yield and quantity for crops.However,excessive utilization of nitrogen fertilizer not only leads to low nitrogen utilization rate,but also brings a series of ecological problems.Therefore,how to improve the nitrogen use efficiency of crops has become one of the hot issues.Foxtail millet holds a great tolerance to nutrient starvation and is widely planted in area with poor soil condition.Based on the analysis of phenotype,physiological and biochemical characteristics and transcriptome profiling of Xiaomi under low nitrogen stress,and also the NAC transcription factor gene family was identified and analysed,we analysed the response of Xiaomi under low nitrogen stress and screened pivotal genes which participated or regulated nitrogen uptake of Xiaomi under nitrogen stress.The main research results are as follows:1.The total root length,lateral root number,root surface area,root volume and root-shoot ratio of Xiaomi increased,while the length,width and leaf area of flag leaf decreased under low nitrogen stress.The leaves performed premature senescence,also plant height,stem diameter,leaf thickness and total nitrogen content decreased,which indicated that Xiaomi might adjust low nitrogen environment by promoting root growth,and inhibited the growth of aerial parts to adapt to low nitrogen environment.2.The photosynthetic performance,chlorophyll content,nitrate reductase(NR),peroxidase(POD),catalase(CAT),superoxide dismutase(SOD)and malondialdehyde content(MDA)of Xiaomi leaves under low nitrogen stress were analysed.It were found that the chlorophyll content(2 h,72 h,15 d),stomatal conductance(7 d)and photosynthetic rate(7 d)decreased and intercellular CO2 concentration(7 d)increased of Xiaomi under low nitrogen stress;while the PI absolute photosynthetic rate(ABS)of PSII system,total photosynthetic rate(ETo/CSm)and total photosynthetic rate(Phi Eo)of Xiaomi significantly decreased after 15-day-low nitrogen stress,indicating that the electron transport efficiency of the PSII system was inhibited under nitrogen starvation.After 2-hour low nitrogen stress,POD,SOD activities,MDA content in leaves and NR,POD,CAT activities in roots of Xiaomi significantly increased,while MDA content in roots decreased.Under 72-hour low nitrogen stress,the activities of NR in leaves decreased and POD in roots increased.Under 15-day low nitrogen stress,POD activity,MDA content in leaves and CAT activity in roots increased,NR,CAT activities in leaves and SOD activity in roots showed a significant decreased,the activity of SOD in leaves,in roots it showed a decreased trend of NR,POD activities and MDA content.The results showed that Xiaomi could adapt to nitrogen starvation environment through physiological and biochemical metabolic pathways to maintains its life activity.3.Based on the transcriptome profiling of leaves and roots of Xiaomi under low nitrogen stress,we screened 1671(2 h)and 124(72 h)differentially expressed genes in leaves and 167(2 h)and 67(72 h)differentially expressed genes in roots.Only 1 coexpressed gene was found among different tissues(leaves and roots)and treat time(2 h and 72 h);32 coexpressed genes were found in leaves,while only 7 coexpressed genes were found in roots under 2-hour and 72-hour low nitrogen stress.GO and KEGG enrichment analysis showed that differentially expressed genes in leaves were mainly enriched in catalytic activity,oxidase,metabolic process,linolenic acid metabolism and so on under 2-hour low nitrogen stress,and enriched in transcription factor activity and response to oxidative stress under 72-hour low nitrogen stress.Differentially expressed genes in Xiaomi roots were mainly enriched in metabolic process,cation binding,redox process and photosynthetic protein pathway,and enriched in photosynthesis,photosystem II,thylakoid,defense reaction and nitrogen metabolic pathway under low nitrogen stress for 2 hours and 72 hours.4.NAC transcription factors plays an important role in plant for responses to abiotic stresses.In this study,a total of 20 differentially expressed NAC family genes were screened.Using bioinformatics methods,129 NAC family genes were identified at the whole genome level of Xiaomi.We found that the length of amino acids ranged from 152 to 705,the theoretical isoelectric points ranged from 4.46 to 11.03,and the molecular weight difference ranged from 17486.64 to77200.87 among 129 NAC family genes.The 129 NAC family genes were distributed on 9chromosomes of Xiaomi,of which 20 genes were distributed on chromosome 5,and the least genes were distributed on chromosomes 6 and 8.The 20 differentially expressed genes of NAC family were distributed on chromosome 7,among these genes,5 genes were distributed on chromosome 9and only one gene was distributed on chromosome 4.The conserved domains or motifs were located at the N-terminal of 129 NAC family genes,and all of them held the typical NAM domain.Based on Phylogenetic analysis,129 genes were divided into 18 subgroups,indicating that the genes in the same subgroup might hold similar functions.The cluster analysis of relative expression of 20 NAC family genes in Xiaomi showed that they were mainly clustered into 4 groups,and the number of each group genes was quite different.The 20 NAC family genes were expressed in different degrees in leaves and roots of Xiaomi,and the expression patterns of NAC family genes clustered into one group were the same,which laid the foundation for further gene cloning and functional verification.