Studies On Mechanisms Of Low Nitrogen Tolerance In Tibetan Wild Barley

Nitrogen(N)is the most common limiting factor for crop productivity worldwide.The increasingly severe environmental problems caused by N fertilizer application urge alleviation of N fertilizer dependence in crop production.An effective approach to solve N deficiency is to develop low N tolerant crop cultivars.Thus,it's imperative to have a comprehensive undstanding of physiological and molecular mechanism of the low nitrogen tolerance in crops.Tibetan annual wild barley is well-known for its wide genetic diversity and high tolerance to poor soil fertility.In the present study,the physiological and molecular characteristics of Tibetan wild barley were investigated to make sight into the low N tolerance mechanisms.The main findings were summarized as follows:1.Genotypic differences of physiological changes between two contrasting Tibetan wild barley genotypes in response to low nitrogenPhysiological differences involving in absorbtion and utilization of N were studied on two wild barley genotypes differing in LN tolerance(XZ149,tolerant and XZ56,sensitive)in response to LN stress,with a main cutivar ZD9 as comparison.Results have shown that LN tolerance of XZ149 was obviously higher than that of the other two genotypes.Firstly,XZ149 have stronger N uptake ability under LN stress,displaying that XZ149 had the smallest decline of shoot dry matter,total dry matter and N accumulation among the three barley genotypes.This study also found that there was no significant difference in the N content(concentration)among the three genotypes under different N conditions,suggesting that LN tolerance is mainly associated with the untilization of N.Secondly,XZ149 also showed a stronger ability of nitrogen assimilation or reassimilation.Decline of soluble protein content and NR activity in XZ149 was less than that of XZ56,and increase of leaf GS activity in XZ149 was higher than that of XZ56,maintaining its more nitrogen compounds synthesized under LN stress.Therefore,it is worthy for further study on wild barley to reveal the physiological and molecular mechanism about the tolerance to LN and explore genetic network or genes associated with LN tolerance.2.RNA-Seq based comparative transcriptome profiling analysis of different N-tolerant barley genotypes in responses to low nitrogenIn this study,we employed Illumina RNA-Sequencing to determine the genotypic difference in transcriptome profile using two Tibetan wild barley genotypes differing in LN tolerance(XZ149,tolerant and XZ56,sensitive).A total of 1469 differentially expressed genes(DEGs)were identified in the two genotypes at 6 h and 48 h after LN treatment.Genetic difference existed in DEGs between XZ149 and XZ56,including transporters,transcription factors(TFs),kinases,antioxidant stress and hormone signaling related genes.Meanwhile,695 LN tolerance-associated DEGs were mainly mapped to amino acid metabolism,starch and sucrose metabolism and secondary metabolism,and involved in transporter activity,antioxidant activities,and other gene ontology(GO).XZ149 had a higher capability of N absorption and use efficiency under LN stress than XZ56.The higher expression of nitrate transporters and energy-saving assimilation pattern could be attributed to its more N uptake and higher LN tolerance.In addition,auxin(IAA)and ethylene(ETH)response pathways may be also related to the genotypic difference in LN tolerance.The identified candidate genes related to LN tolerance may provide new insights into comprehensive understanding of the genotypic difference in N utilization and LN tolerance.3.Metabolomic analysis of two contrasting wild barley genotypes reveals adaptive strategies in response to low nitrogen stressMetabolomic analysis was done on two wild genotypes(XZ149,tolerant and XZ56,sensitive)to understand the mechanism of LN tolerance.XZ149 was much less affected by LN stress than XZ56 in plant biomass.A total of 51 differentially accumulated metabolites were identified between LN and normal N treated plants.LN stress induced tissue-specific changes in carbon and nitrogen partitioning,and XZ149 had a pattern of energy-saving ammino acids accumulation and carbon allocation in favor of plant growth that contribute to its higher LN tolerance.Moreover,XZ149 is highly capable of producing ernergy and maitaining redox homeostasis under LN stress.The current results revealed the mechanisms underlying the wild barley in high LN tolerance and provided the valuable references for developing barley cultivars with LN tolerance.4.Genotypic differences in ionomic responses to low nitrogen between two contrasting Tibetan wild barley genotypesIn order to understand adaptive nutrient homeostasis in barley under LN stress,XZ149,XZ56 and one main barley cultivar ZD9 were used to investigate ionomic changes in response to LN employing ICP-OES.Principal component analysis(PCA)showed that LN had a significantly effect on barley ionome,and there were genotiypic differences under LN stress.The study showed that P and K accumulation increased in the shoots and decreased in roots under LN stress.This response varies among genotypes in which P content of XZ149 exhibited smallest changes among the three genotypes,and K content of XZ149 showed the largest increase in shoots and the smallest decrease in roots under LN stress.In addition,LN stress significantly reduced Fe content of cultivated barley ZD9 both in shoots and roots,but had little impact on that of two wild barley genotypes both in shoots and roots,suggesting that wild barley has relatively stable capacity of Fe uptake and transport under the LN stress.LN stress promoted Zn accumulation in roots of XZ149 and ZD9 and inhibited its translocation to shoots,in the meantime,reduced the accumulation of Mn in roots of XZ56 and ZD9.Generally,the LN tolerant genotype XZ149 was less affected than the other two barley genotypes under LN stress,so maintaining stable absorption of elements may contribute to its tolerance to LN stress.