Mechanisms of Nitrate Uptake in Plants
Nitrogen - The most important mineral nutrient needed by plants can be assimilated in various forms among which nitrate is a transient yet critical form for majority of plants. Nitrogen deficiency is a worldwide problem limiting agricultural production. N use efficiency is low for major crops. Understanding the mechanism of uptake of Nitrogen from soil could help in mitigating the poor utilization efficiency of nitrogenous fertilizers by plants and thus economize the cost of production and avoid environmental and health hazards. There is also high degree of heterogeneity with respect to availability of soil N and demand variation in plant requirement for Nitrogen, impose a need to regulate N fluxes across the Plasma membrane of roots in order to optimize plant N capture.
Nitrate uptake system in plants should be robust and versatile enough to transport sufficient amounts of nitrate to satisfy total demand for nitrogen in the face of external concentrations, which can vary by four orders of magnitude. So plants have evolved transport system, which is active regulated and multiphasic. Physiological studies using 15N 13N and electrophysical methods have indicated that plants have at least 3 distinct nitrate uptake systems viz.. One Low Affinity transport system (LATS) and two High Affinity Transport Systems.(HATS) (Garnett et al. 2003)
Two families of membrane proteins have been identified that can mediate active transport of nitrate presumably with symport of 2 protons across the plasmamembrane. PTRPeptide Transporter family-containing NRT1, which represents LATS and NNP Nitrate-Nitrite Porter Family containing NRT2. Which, represents HATS. Both NRT1 and NRT2 are assigned to Major Facilitating Super family; having 12 transmembrane domains. 58 putative nitrate transporters have been identified in plants (51PTR, 7 NRT2) of which 13 have been characterized. These are under strict regulation by reduced nitrogen metabolites like NH3, Glutamine, GABA, sucrose, auxins and light (Orsel et al. 2002, Little et al. 2005).
Nitrate analogue Chlorate (ClO3-) was used to characterize uptake mutants and to establish the molecular basis of nitrate uptake. AtNRT1 is a gene encoding a dual affinity transporter involved in both HATS and LATS Regulated by phosphorylation. It is preferentially expressed in actively growing regions and contributes to their growth. It also contributes in guard cell function. In Arabidopsis thaliana, 5 homologues of NRT1 and 7 homologues of NRT2 family have been characterized till date. Location of NRT2 genes on the 5 chromosomes of Arabidopsis also has been deciphered. Recently it is found that HATS in higher plants also need 2-component system for functioning. NRT2 also serve as a signal transducer for nitrate regulated root growth (Liu and Tsay 2003, Tong et al. 2005).
Thus far, the focus of attention in studies of inorganic N uptake at the physiological and molecular levels has been upon the regulation of root Plasma membrane transporters. In future Physiological and molecular studies will include fluxes to sub cellular compartments and between major organs of plant.
1. Mathilde Orsel , Sophie Filleur, Vincent Frazier and Francoise Daniel-Vedele (2002) Nitrate transport in plants: which gene and which control J.Exp.Bot 53:825-833.
2. Daniel Y. Little, Hongyu Rao, Sabrina Oliva, Francoise Daniel-Vedele, Anne Krapp, and Jocelyn.E.Malamy (2005) The putative high-affinity nitrate transporter NRT2.1 represses lateral root initiation in response to nutritional cues PNAS 102: 13693-13698
3.Trevor.P.Garnett, Sergy.N.Shabala, Philip.J.Smethurst, and Ian. A. Newman (2003) Kinetics of ammonium and nitrate uptake by eucalypt roots and associated proton fluxes measured using ion selective microelectrodes F.Plant Biology, 30: 1165-1176.
4.YipingTong, Jiang-J.Zhou, and A.J.Miller (2005) A two component high affinity nitrate uptake system in barley, The Plant Journel.41: 442-450.
5.Kun-HsingLiu, and Yi fang Tsay, (2003), Switching between the two action modes of the dual affinity nitrate transporter CHL1 by Phosphorylation, The Embo Journel, 22: 1005-1013.
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