Screening Of Low-nitrogen Tolerant Varieties Of Chinese Cabbage

Nitrogen plays a decisive role in the health and yield of crops.If nitrogen element is insufficient,the quality of crops will decline and the growth rate will slow down.However,increasing the application of nitrogen fertilizer does not improve the utilization efficiency of nitrogen fertilizer.Low nitrogen utilization rate not only wastes resources,reduces economic benefits,but also brings a series of environmental problems.In order to solve this problem,starting from the crops themselves,improve the nitrogen utilization efficiency of crops,which is the key to reduce the amount of nitrogen fertilizer and achieve sustainable development.Therefore,it has always been an important task to explore the mechanism of low nitrogen tolerance of crops.Chinese cabbage is one of the most important vegetable crops in China.Reseach on its low-nitrogen responsive mechanisms and mining the low-nitrogen responsive genes would help the breeder for developing molecular markers for screening low-nitrogen tolerant Chinese cabbage germplasm resources.In this study,1)the tolerance to low N-fertilized level for 25 Chinese cabbage cultivars were preliminarily evaluated with principal component analysis and cluster analysis;2)we systematically analyzed the NPF and NRT2 encoding genes of Chinese cabbage which play vital roles in the nitrate uptake and transportation;3)the expression patterns of BrNPFs and BrNRT2 s were analyzed to identify the low nitrogen responsive genes in roots of Chinese cabbage by RT-PCR and qRT-PCR.The main research results are as follows:1.Pot-culture experiment was conducted to evaluate and analyze the nitrogen tolerance of 25 Chinese cabbage varieties.The results showed that plant height,root length,fresh weight,root-crown ratio,leaf area and chlorophyll were affected by nitrogen level and variety.The response of different cultivars to nitrogen level is different.Principal component analysis showed that the original data of 7 physical indexes of 25 Chinese cabbage varieties were summed up into two principal components: plant height and root length,representing more than 77% of the information.According to the calculation of principal component value of each variety and clustering analysis,all 25 varieties divided into nitrogen tolerant type(3 varieties),the medium-nitrogen tolerant type(9 varieties)and the low-nitrogen sensitive type(13 varieties)respectively.2.Eighty-two BrNPFs genes were isolated from the genome of Chinese cabbage by Blast analysis using Arabidopsis thaliana NPF protein sequence as reference.Two contrast Chinese cabbage cultivars,Shengqiu80(low nitrogen tolerance type)and Fengkang50(low nitrogen sensitivity type)were used as experimental materials to analyze the expression patterns of BrNPFs in the root of Chinese cabbabge under different N levels by semi-RT-PCR and Qrt-PCR.Among the 82 BrNPFs,four genes responded to N treatments,BrNPF2.6,BrNPF3.1c,BrNPF4.2,and BrNPF5.7b.Among them,the expression of BrNPF5.7b was induced by the low-nitrogen treatment,and highly expressed in Fengkang50 which suggested BrNPF5.7b might related to the differences between Shengqiu80 and Fengkang50.The expression levels of BrNPF3.1c,BrNPF2.6 and BrNPF4.2 were inhibited under low nitrogen stress,and highly expressed in Fengkang50 under normal condition,suggesting their might play a negative role during the uptake processese of Chinese cabbage.3.14 BrNRT2 genes were isolated from the genome of Chinese cabbage based on Blast analysis.Among them,four BrNRT2 genes responded to low N treatment,BrNRT2.1a,BrNRT2.1c,BrNRT2.4,and BrNRT2.5.BrNRT2.1a showed highly expressd in Fengkang50 under low N treatment which indicated it might also play roles between the differences of Fengkang50 and Shengqiu80.Comapring with Shengqiu80,the expression levels of BrNRT2.1a and BrNRT2.1c were induced in Fengkang50 under low N treatment,which indicated that they may also be related to the difference of Fengkang50 and Shengqiu80.Based on the expression patterns of BrNRT2.4 and BrNRT2.5,they were considered as the common responsive genes of low N in Chinese cabbage.