| Nitrogen is an important component of plant proteins and has important influence on plant chlorophyll synthesis.Nitric oxide(NO)is a by-product produced by Nitrate reductase(NR)during plant nitrogen assimilation and affects plant chlorophyll synthesis.The effects of nitrogen and NO on plant chlorophyll synthesis are still unclear.In this study,wild type Arabidopsis thaliana(Col-0)and wild type barley(Hordeum vulgare L.,Zaoshu No.3)were used to study the effects of different nitrogen levels(1/20,1/2,2 times),different nitrogen forms(nitrate,ammonium)and NO treatments on chlorophyll synthesis in barley and Arabidopsis seedlings.The results were as followeds:(1)Barley seedlings were more sensitive to high nitrate treatment than Arabidopsis.The main performance was as followeds: high nitrate nitrogen significantly reduced the chlorophyll biosynthesis rate of barley etiolated seedlings compared with that of the Arabidopsis thaliana.The anthocyanin content of barleyde-etiolated seedlings treated with high nitrate increased by 9.52 times compared with no light treatment,while the Arabidopsis seedlings only increased by about 2 times.The maximum photosynthetic efficiency(Fv/Fm)and actual photosynthetic efficiency(?PSII)of barley de-etiolated seedlings treated with high nitrate were also reduced by 11.16% and 42.33% compared with Arabidopsis thaliana;Futhermore,the non-photochemical quenching of barley seedlings non-photochemical quenching(NPQ)was 263% of the NPQ of Arabidopsis,indicating that its photoprotective ability is also significantly reduced.(2)With the increase of N level,the NR activity of barley and Arabidopsis seedlings also increased,while the NR activity of barley was about twice that of Arabidopsis.Among them,high nitrate-treated barley de-etiolated seedlings had the highest NR activity.Nitrate treatment could significantly induce endogenous NO production in barley seedlings but not Arabidopsis seedlings,especially in high nitrate treatment,barley seedlings had the highest endogenous NO content.(3)Compared with the control,the content of chlorophyll precursor,?-amino levulinic acid(ALA)in barley seedlings decreased under NO treatment,while the contents of protoporphyrin IX,Mg-protoporphyrin IX,especially pchlide(protochlorophyllide)accumulated significantly.Moreover,activity of chlorophyll synthesis related enzymes POR and ALA forming enzyme decreased.NO causes S-nitrosylation of POR cysteine residues and disassembly of POR aggregates in barley and Arabidopsis seedlings.However,this modification only reduced the POR activity of barley seedlings,hindering the transformation of pchlide into chlorophyllide(chlide),which slowed the chlorophyll synthesis rate and blocked chlorophyll synthesis during the greening process.(4)NO treatment resulted in significant photobleaching damage in barley leaf apex upon dark-to-high-light shift(and therefore inducing photobleaching lesions),compared with the Arabidopsis seedlings.Reactive oxygen species(ROS)staining and ROS-related gene expression detection confirmed that superoxide,hydrogen peroxide and singlet oxygen accumulated significantly in barley seedlings after the NO treatments,when exposed to continuous light,but not in Arabidopsis seedlings.Among them,the superoxide anion and singlet oxygen of barley seedlings increased by 15.7/7 times compared with the control.In summary,the results suggest that high nitrate induced a large amount of NO in barley seedlings.While NO causes S-nitrosylation of POR cysteine residues and disassembly of 5:1 POR aggregates in barley seedlings,which reduced POR activity in barley seedlings,but not in Arabidopsis.It further hinders the chlorophyll synthesis and can destroy its redox homeostasis during barley seedlings greening.The above results indicate that barley seedlings and Arabidopsis thaliana seedlings have different tolerance to nitrate,and barley seedlings were less tolerant to high nitrate,which affected the initial development of barley seedlings. |
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