Physiological Mechanisms of Nitrogen Absorption and Assimilation in Plants under Stressful Conditions

Abstract

Nitrogen is one of the most essential nutrients for plants. It is regarded as the single most important factor limiting growth of crops. The major inorganic forms of N that predominate in croplands are NO3- and NH4 + The availability of N in the soil is limited due to adverse environmental conditions such as salinity, water deficit, low light intensity, heat, chilling, excess levels of metals in the soil, and UV-B radiation. These stresses reduce N uptake, cause adverse effects on N assimilation in the tissues, and drastically affect crop yields. Plants possess multiple NO3- uptake and transport systems to optimize N use with changing soil and environmental conditions. Depending on the soil NO3- concentration, two NO3- uptake systems high-affinity transport system (HATS) and low-affinity transport system (LATS) operate in plants. Five gene families of nitrate transporters (NRTs) have been characterized that are involved in uptake, transport, and storage of NO3- within the tissues. From the soil, NO3- is taken up by plant roots via NRTs; thereafter, NO3 – gets assimilated into organic compounds by the action of NO3- assimilatory enzymes. The processes of NO3- uptake, its translocation within the tissues, and its reduction are coordinately regulated. Most of the stressful conditions cause a decrease in NO3- uptake and inhibition in the activities of N assimilatory enzymes nitrate reductase (NR) and glutamine synthetase (GS). The NR is inducible by NO3- and its activity is subject to regulation by a variety of environmental conditions that are influenced under stresses. Environmental stresses adversely affect the behavior of enzymes of NO3- and NH4 + assimilation. Genotypes of plants differing in stress tolerance show varying activity behaviors of NR and other N assimilatory enzymes. Though extensive studies have been performed to unveil the biochemical mechanisms underlying the uptake of NO3- by plants, the process of its assimilation, and the regulation of enzymes of NO3- assimilation, our knowledge is still insufficient to address the complexities associated with effects of the varieties of abiotic stresses on N uptake and assimilation processes. The precise biochemical mechanisms on how adverse conditions of the environment reduce NO3- uptake and inhibit NR activity need to be investigated in more detail. Little information is available regarding molecular events of NO3- uptake, the NO3 – sensor protein system, signal transduction of environmental NO3-, NO3- induction regulatory proteins, primary responsive genes that are transcribed and translated as a result of NO3- induction, etc. Besides this, the nature of different families of NRTs, NO3- translocators, events involving overall induction of NR by NO3-, and regulation of NR and other N assimilatory enzymes under various environmental stresses like salinity, drought, heat, chilling, light, and excessive levels of metals in the soil need to be examined in greater detail. In certain cases, such as salinity and water stresses, suppression of the GS/GOGAT pathway and a sustained level of induction of the glutamate dehydrogenase (GDH) pathway of ammonium assimilation are observed. Thus, the role of aminating GDH as antistress enzyme needs to be examined under a wide range of stresses. A detailed understanding of the physiological and molecular controls of N uptake and assimilation in crop plants under different stressful conditions will help in identifying suitable genotypes with better N use efficiency for cultivation in stress-prone areas.