【文章內(nèi)容簡介】 氯酸鹽，因而保持綠色。 The role of the CRNA NRT2 family was first uncovered in the chlorateresistant mutant crnA of Aspergillus nidulans (曲霉素 ). Nitrate uptake in Aspergillus is nitrateinducible and has an apparent Km of 200 181。M. CRNA is located in a gene cluster that includes the NR and NiR genes， which encodes a hydrophobic protein with 12 putative, membrane spanning regions typical of cotransporters. The mRNA transcripts of these genes are found primarily in roots, are induced by nitrate, and are downregulated by reduced forms of nitrogen such as NH4+ and glutamine. Levels of NRT2 mRNA correlate with nitrate uptake rates in both Nicotiana species and Arabidopsis. The NRT2 family of genes encode nitrateinducible, high affinity nitrate transporters. Three NRT2 genes were found that encode high affinity nitrate transporters with differing specificities. Nrt2。1 encodes a nitrate/nitritebispecific transporter. Nrt2。2 encodes a nitratespecific transporters. Nrt2。3 has been found that is not part of the cluster. All three genes are nitrateregulated, but Nrt2。1 and Nrt2。3 are more substantially induced by nitrite. When plants are shifted from media containing ammonium with no nitrate to media containing 25 mM nitrate, CHL1 mRNA levels increase dramatically 30 min after treatment, peak at 1–2 h and level off to a high steady state level after 8 h. This response is very similar to NR observed in roots of nitrateinduced plants. The first member of the NRT1 family was identified in a chlorateresistant mutant of Arabidopsis called chl1. CHL1 gene expression is nitrateinducible and occurs primarily in roots. WT 181。 chl8 181。 CHL1 transporter is a ponent of the low affinity nitrate uptake system. Two NRT1 genes have been isolated from tomato, one is constitutively expressed and the other nitrateinducible. The latter could correspond to CHL1 in Arabidopsis. CHL1 participates in both low and high affinity uptake. Table 1. NRT1 family of nitrate transporters Interactions between AtNRT2 and (also known as ) form a 150kDa plasma membrane plex, thought to constitute the highaffinity nitrate transporter of Arabidopsis roots. New Phytologist, 2022, 194: 724–731 Predicted functions for the rice NAR2/NRT2 nitrate transporters 1) NO3 induction 2) Freedback repression Glutamine, NH4+ , 3) Diurnal regulation Uptake rate peaks during the light period and reaches a minimum in the dark 4) Spatial and developmental regulation NRT2 genes are expressed more strongly in roots than in aerial. 5 Regulation In plants, the expression of NR, NiR, and HANT gene expression is coregulated with respect to the nitrogen source, the intracellular amounts of reducednitrogen pounds, light, hormones, and the carbon status 6 Nitrate assimilation A: NR單體有 3個(gè)結(jié)構(gòu)域 , 分別與鉬輔助因子，血紅素和 FAD結(jié)合。 B: NR的帶壯結(jié)構(gòu)圖，黑色為鉬輔助因子，紫色為血紅素，藍(lán)色為 FAD NR基因表達(dá)的調(diào)控 A: NR mRNA在大豆根和葉中的表達(dá)受現(xiàn)有的硝酸鹽影響（ 15 mM 硝酸鹽處理硝酸鹽匱乏條件下 7d的大麥苗） B: 有硝酸鹽供給條件下 NR mRNA表現(xiàn)為晝夜節(jié)律變化 and NIA1 Are UpRegulated in Various NRDeficient Mutants A large proportion of nitrate (NO3) acquired by plants from soil is actively transported by NRT families of NO3 transporters. There are three families of NO3 transporters —NRT1, NRT2 (or NAR2/NRT2): Most NRT1 members are lowaffinity besides (CHL1)。 NRT2 members are highaffinity, some require a partner protein (NAR2)。 CLC members amediates nitrate accumulation in the vacuole. In Arabidopsis, the NRT1 family has 8 members, NRT2 family has 7 members All grass species possessed additional orthologues and appear to lack There is significant separation between NRT2 genes from dicots and grass species. Minisummary Section 3 Molecular and physiological aspects of ammonium uptake in plants 1 Ammonium uptake Many plants do not grow well with NH4+ as its sole source of N, probably reflecting its adaptation for growth in aerobic soils, where NO3? is the main form of inanic N, and NH4+ concentrations rarely exceed 50 181。M. Significant leakage of NH4+ occurs from root cells, even when NO3? is the N source, so that an uptake system with a high affinity for NH4+ is important for conserving valuable N supplies. NH4+ transport is also important for a variety of processes in the shoot, which include unloading of NH4+ from the xylem, absorption of atmospheric NH3 and retrieval of photorespiratory NH3 r