The Rice OsNAR2.1 Participates The Regulating Root Growth By Nitatre

Rice roots can excrete O2 to surrounding soil and this can convert ammonium (NH4+) to nitrate (NO3-) in the process of nitrification at root surface or in rhizosphere. And rice roots are actually always exposed to a mixture of NH4+ and NO3- although the predominant species of mineral nitrogen (N) in the soil collected from the lowland rice field is NH4+ Therefore NO3 nutrition on the surface of rice roots is an important part of N nutrition of rice plants. Low and medium NO3 can stimulate the growth of the root of rice, especially on lateral roots. Hydroponic experiments were conducted to study the effects of different NO3 concentration on N uptake and root morphology in rice nar2.1-RNAi and WT, which can illuminate NO3 regulating the root growth in rice.The results are listed as follows:1. Hydroponic experiments were conducted to study the effects of different ratios of NH4+to NO3- (NH4+:NO3- as 100:0,75:25 and 0:100) on N uptake and root morphology in rice (Oryza sativa L.) at the seedling stage. The results obtained were as follows. Higher biomass and N accumulation were found in the mixed solution of NH4+and NO3- while no difference was found between sole NH4+and NO3- solution. Amino acid content in the mixed solution of NH4 and NO3- was the highest and that in sole NO3- solution was the lowest. Nitrate content in sole NO3- solution was four times of that under two ratios of NH4+ to NO3-(100:0 and 75:25). And mixed NH4+ and NO3- solution increased NO3- content by 18% compared to sole NH4+solution. From 0 to 5d, total root length was similar to that under two ratios of NH4+to NO3-(75:25 and 0:100), which was higher than that in sole NH4+ supply. After 5d, total root length markedly increased under two ratios of NH4+to NO3-(100:0 and 75:25). Mixed NH4+ and NO3- solution increased total root length by 35% compared with sole NH4+ and NO3-supply while there no obvious difference between sole NH4+ and NO3-supply. The change in total root length mainly resulted from total lateral root length which is affected by lateral root initialing.2. Hydroponic experiments were conducted to compare the root growth between nar2.1-RNAi and WT under three NO3- concentrations (0.2,0.5 and 5mM) and three ratios of NH4+to NO3- (100/0,75/25 and 0/100) with total N concentration of 0.2mM. The root growth in MT and WT was significantly different under 0.2 and 0.5mM NO3-concentrations. The total root length of two MT lines was significantly lower than that of WT. Further analysis showed that the difference of total root length mainly resulted from the initiation of adventitious and lateral roots. Furthermore, the number of adventitious and lateral roots in the mutant was significantly lower than that in WT. No difference was recorded in the root growth for MT and WT under 5mM NO3- or sole NH4+ nutrition. Furthermore, difference in total root length was recorded between MT and WT under two ratios of NH4+ and NO3- (75/25 and 0/100) with total N concentration of 0.2mM. As a result, the initiation of adventitious and lateral roots in MT was inhibited under low NO3-concentration supply compared with that in WT.3. Hydroponic experiments transfering rice plants from lOmM NO3- to a series of 15N-NO3- solution were conducted to study the response of the 15N uptake and lateral root growth in MT and WT plants. The results showed that under low 15N-NO3- (0.05-0.5mM) condition,15N concentration of the shoot and root and the new number and length of lateral root in nar2.1-RNAi are lower than those in WT, while under 15N-NO3-(2-10mM) concentration the differences between nar2.1-RNAi and WT decreased with increasing NO3- concentration. When both numbers of new visible lateral roots and increment of lateral root length were plotted against cumulative 15NO3- uptake, it clearly appeared that in the osnar2.1 mutants had a strong defect in branching of the newly formed portion of the primary and the seminal roots. This cannot be explained by the lowed NO3- uptake in the MT, with the similar cumulative 15NO3- uptake in the range of 15N cumulative plant dry weight for both osnar2.1 lines had much lower visible laterals than WT plants. As a conclusion, the silence of OsNAR2.1 independently affects initiation of root NO3- uptake.4. Hydroponic experiments were conducted to study the expression level of four MADS genes which have in excess of 50% homology with ANRI (MADS-box transcription factor 25,Transcription factor MADS27, MADS-box transcription factor 57 and Transcription factor MADS57).The results showed that the expression level of MADS-box transcription factor 57 and transcription factor MADS57 genes in nar2.1-RNAi were significantly lower than that in WT. This may indicate OsNAR2.1 was placed upstream of ANRI in the signal transduction pathway.