Sustaining nitrogen dynamics: A critical aspect for improving salt tolerance in plants

Abstract

In the current changing environment, salt stress has become a major concern for plant growth and food production worldwide. Understanding the mechanisms of how plants function in saline environments is critical for initiating efforts to mitigate the detrimental effects of salt stress. Agricultural productivity is linked to nutrient availability, and it is expected that the judicious metabolism of mineral nutrients has a positive impact on alleviating salt-induced losses in crop plants. Nitrogen (N) is a macronutrient that contributes significantly to sustainable agriculture by maintaining productivity and plant growth in both optimal and stressful environments. Significant progress has been made in comprehending the fundamental physiological and molecular mechanisms associated with N-mediated plant responses to salt stress. This review provided an (a) overview of N-sensing, transportation, and assimilation in plants; (b) assess the salt stress-mediated regulation of N dynamics and nitrogen use- efficiency; (c) critically appraise the role of N in plants exposed to salt stress. Furthermore, the existing but less explored crosstalk between N and phytohormones has been discussed that may be utilized to gain a better understanding of plant adaptive responses to salt stress. In addition, the shade of a small beam of light on the manipulation of N dynamics through genetic engineering with an aim of developing salt-tolerant plants is also highlighted.

Keywords: crop productivity; genetic engineering; nitrogen metabolism; phytohormones; salt stress resilience.

Figure 1

N supplementation exerts a defensive response on plant physiological and morphological machinery during salt stress and helps in alleviating the osmotic stress, oxidative damage and morphological alterations. N also help in maintaining the ionic homeostasis along with N metabolism in plants during salt stress, which in turn helps in improving the plant performance during salt stress. Cl^-, chloride; N, Nitrogen; Na⁺, sodium; K⁺, Potassium; ROS, reactive oxygen species.

Figure 2

Schematic diagram showing the routes of NO₃¯ and NH₄ ⁺ sensing, transportation and accumulation in plants. The green dots and arrows denote the transport of NO₃¯ and the red dots and arrows indicates the transport of NH₄ ⁺ ions in plants. In the uptake process, NO₃¯ and NH₄ ⁺ enter the plant roots via NRTs and AMTs transporters respectively. For example, NRT1.1/NPF6.3/CHL1, AMT1.1 and AMT1.3 involved in the sensing of NO₃¯ and NH₄ ⁺ respectively. These transporter are localized in the plasma membrane of root cells. After uptake, NO₃¯ transported via NRT1.5/NPF7.3, NPF2.3, NPF2.4, NPF2.5 and NPF6.5 and NH₄ ⁺ via AMT1.1, AMT1.2 and AMT1.3 to the shoot through xylem. NRT1.5 is involved in NO₃¯ loading in the xylem, while as NRT1.8, NRT1.9 are involved in NO₃¯ unloading from the xylem. However, NRT1.7, NRT1.9 transport NO₃¯ into the phloem. Furthermore, NRT1.4/NPF6.2 and AMT1.3 mediates NO₃¯ and NH₄ ⁺ transport to the leaf/petiole. NO₃¯ accumulation within the leaf vacuole is mediated by NPF7.3, NPF5.12, and NRT2.7. AMT1.1, AMT1.2 and AMT1.3 mediates NH₄ ⁺ transoction from root to shoot. NRT1.6/NPF2.12, NPF7.9 and AMT1.1a, AMT1.4 is involved in the transportation of NO₃¯ and NH₄ ⁺ respectively in the seed where they accumulated in the vacuoles. NRT2.7 is a tonoplast transporter of embryo which regulates NO₃¯accumulation within seed vacuole. AMTs, ammonium transporters; Gla, glutamine; Glu, glutamate; GOGAT, glutamate synthase; GS, glutamine synthetase; NH₄ ⁺, ammonium; NiR, nitrite reductase; NO₃¯, nitrate; NR, nitrate reductase; NRTs, nitrate transporters; 2-OG, 2-Oxoglutarate.

Figure 3

Phytohormones have been found to regulate nitrogen metabolism during salt stress, either in conjugation with nitrogen sources or by interacting with other phytohormones leading to improved physio-biochemical attributes, and promoting plant growth and development under salt stress.

Figure 4

Specific targets for manipulating nitrogen dynamics through genetic engineering. Nitrogen dynamics is mainly associated with higher expression of nitrate and ammonium transporter genes as well as transcription factors, and the overexpression of these genes in transgenic plants leads to increased nitrogen use efficiency, photosynthesis, crop productivity and nutritional quality, nitrate and ammonium uptake and salt-stress resistance.