Study On The Mechanisms Of Nitrogen And Cytokinins In The Regulation Of Rice Seminal Root Growth

Rice(Oryza sativa L.)is one of the most important food crops in the world and about half of the world’s population feeds on rice.In the past 50 years,rice yield has steadily increased worldwide,partly owing to an increase in nitrogen(N)application,but the efficiency of nitrogen absorption and utilization is low.High N input and low N use efficiency not only increase crop production costs but also cause severe environmental pollution.Therefore,decreasing N application is an important goal of sustainable agriculture.However,decreasing N application may lead to N deficiency and affect rice root growth,and the underlying mechanism by which N deficiency affects rice root growth is still poorly understood.Root is the plant organ that is most important for acquiring soil nitrogen.In general,N deficiency promotes root growth,while supraoptimal N supply inhibits root growth.The lack or excessing of cytokinin content in root can also promote or inhibit root growth,which indicates that there may be some relationship between nitrogen and cytokinin in regulating root growth,but the interaction mechanism is still unclear.Root growth is mainly determined by cell proliferation in meristem zone and cell elongation in elongation zone.The effects of nitrogen deficiency on cytokinin metabolism,root meristem cell proliferation and the maturation cell length was compared,which clarifyed the mechanism of N deficiency-induced decrease in CK content promotes rice seminal root growth.This result provides a theoretical basis for nitrogen-reducing and root-promoting cultivation.The main results are as follows:1.A certain threshold content of CK is required for rapid growth of rice seminal root,supraoptimal or too low CK content inhibits rice seminal root growth.This study analyzed the effect of exogenous lovastatin(cytokinin inhibitor,LOV),kinetin(synthetic cytokinin,KT)and zeatin(endogenous cytokinin,Z)on the root growth of 9311 and Nipponbare to investigate the regulation of CK on rice seminal root growth.The results show that low concentrations of lovastatin had no apparent effect on rice seminal root growth,but high concentrations of lovastatin obviously inhibited rice seminal root growth,KT and Z treatments also inhibited the seminal root growth of rice.In addition,LOV treatment reduced the CKs content of rice seminal root,while KT treatment increased the CKs content.These results indicate that a certain threshold content of CK is required for rapid growth of rice seminal root and whether LOV treatment reduce CK content or KT treatment increase CK content will inhibit for rapid growth of rice seminal root.Further studies showed that,on the one hand,supraoptimal cytokinin promoted the early differentiation of meristem cells and reduced the meristem size by up-regulating OsIAA3.On the other hand,supraoptimal cytokinin inhibited cell elongation by down-regulating cell elongation-related genes expression,and then inhibited rice seminal root growth.2.N deficiency-induced decrease in CK content promotes growth of rice seminal root.To investigated the effects of nitrogen deficiency on the seminal root growth,9311 and Nip seminal root growth under N deficiency(1 mmol N,1/4 mmol N,1/16 mmol N,0 mmol N)was compared.The results showed that N deficiency promoted the growth of 9311 and Nip rice seminal roots,and the greater the degree of N deficiency,the longer the length of the seminal roots.Even in long-term(30 d,60 d)N deficient cultivation,the root length still increased compared with control.In addition,application of Z(a type of CK)at different concentrations significantly inhibited the growth of rice seminal roots,and 4 n M Z completely blocked the N deficiency-induced growth.Further studies showed that nitrogen deficiency inhibited CK biosynthesis by down-regulating the cytokinin synthesis gene OsIPTs,and up-regulating the cytokinin-degrading gene OsCKXs to promote CK degradation,among which led to a decreases of CK content in rice seminal roots,and then promotes rice seminal root growth.3.N deficiency-induced decrease in CK content increases root meristem cell number and enhances root meristem cell proliferation rate.To investigated the regulation of N deficiency on the root meristem growth,the effect of root meristem size,meristem cell number,cell division activity,cell proliferation rate under control(1 N),N deficiency and N deficiency + Z were compared.The results show that decreased CK content up-regulating OsPLTs and down-regulation of OsIAA3 increases meristem cells number.The increases of the meristem cell number increased the cell proliferation rate of root meristem and promoted the rice seminal root growth.However,N deficiency had no effect on cell division activity of meristem in rice seminal roots.4.N deficiency-induced decrease in CK content promotes rice seminal root cell elongation in elongation zone.To investigated the regulation of N deficiency on the root cell elongation in elongation zone,the effect of cell length and transcription levels of cell elongation-related genes under control(1 N),N deficiency and N deficiency + Z were compared.The result showed that N deficiency-induced decrease in CK content increased root mature zone cell length.Except OsXTH25 and OsEXP11,all root-specific OsXTHs and OsEXPs were up-regulated by N deficiency,and this up-regulation was inhibited by exogenous zeatin.These evidences indicate that N deficiency promotes root cell elongation by increasing expression of root-specific OsXTHs and OsEXPs,which in turn promotes rice seminal root growth.In summary,N deficiency reduces CK content by inhibiting CK biosynthesis and promoting CK degradation.N deficiency-induced decrease in CK content not only upregulating OsPLTs and down-regulation of OsIAA3 enhanced the root meristem cell proliferation rate by increasing the number of root meristem cells number,but also promoted root cell elongation by up-regulating root-specific OsXTHs and OsEXPs.