salt stress dealing with osmotic and ionic stress, which overlap at some points. Earlier researchers assumed that the osmotic stress signaling initiates immediately after the salt stress. In contrast, the signaling cascade and response to the ionic imbalance initiate later due to the slow accumulation of sodium ions (Na+) in shoot tissues beyond a threshold level and corresponding inhibition of the photosynthesis (Zörb et al. 2019). Intriguingly, the new findings showed the root growth response specific to the Na+ accumulation and rapid signaling cascade mediated by reactive oxygen species (ROS) or calcium ion (Ca2+), specific to the salt ionic stress (Choi et al. 2014; Galvan‐Ampudia et al. 2013; van Zelm et al. 2020). Apart from the ROS and Ca2+ signals, the phytohormones viz. absiscic acid (ABA), jasmonic acid (JA), salicylic acid (SA), gibberelic acid (GA), and ethylene play crucial role in signal transduction and regulation of expression and function of several proteins during salt stress (reviewed in Zhao et al. 2020). These signals are perceived at the organelle level or at the level of the nucleus and responded by the plant cell in terms of stress‐responsive gene expression, different degrees of mRNA stability, and varied way of translational or post‐translational regulation to change protein abundance and the activity. These responses depend not only on the extent and duration of the stress but also on the plants’ genetic nature. The halophytes are evolutionary adapted to survive in the salt stress with unique genetic makeup, morphological, physiological, and anatomical adaptation (Munns and Tester 2008; van Zelm et al. 2020; Zhao et al. 2020). They are adapted to sequester the excess salt ions in the root or shoot vacuoles and secretion of excess salt through different kinds of salt glands and epidermal bladder cells [EBCs; (Zhao et al. 2020)]. However, salt‐stress tolerance is a complex trait regulated by several genes and pathways; engineering the crops using a single gene is inefficient. Moreover, the pyramiding of several genes is time‐consuming and seems less realistic to improve the salt‐tolerance capacity of conventional crops. Cultivation of halophytes for food, forage, renewable energy, and phytoremediation emerged as an alternative and economic strategy in the salt‐affected areas (Panta et al. 2014). Thus, in summary, to understand better the effect of salt stress on plants, a comprehensive approach is required to understand the cellular ion transport system in different tissues, major phytohormone, or osmotic stress‐specific signaling pathways not only in the model plant Arabidopsis thaliana but also in the halophytic plant species (van Zelm et al. 2020) in order to understand the advantageous differences in the halophytes.
2.2 Crop Loss Due to Salt Toxicity – An Estimation Worldwide
The soil salinization is one of the three soil degradation processes that pose a threat to human health and crop productivity by affecting more than one billion hectares land across the globe (Ondrasek et al. 2011). The negative effect of salt stress on crop productivity indirectly affects the economy dependent on the agricultural produce, resulting in the loss of billion dollars annually. The economic loss caused by the salt stress can include two components: first, the loss of crop productivity (presented in t/ha) and thus, the loss of income generated from the agricultural production and, second, the cost spent for the restoration of degraded land. Estimating the global loss due to soil salinization can be heterogeneous among the different countries or geographical regions because of factors such as labor costs, the market price of the agricultural produce, fertilizers, seeds, and other operational costs affecting the total input and income differentially. There is the possibility that in some regions of the world (developed countries), even the moderate salinization of the soil could result in higher economic loss due to higher operational and labor costs. In many developing countries, most of the poor farmers depend on agriculture for their livelihood and loss of crop productivity due to salt stress affects their livelihood. In Asia, the Maldives is a low‐lying country, always on risk of submergence due to increasing sealevel and salt deposition. The intrusion of seawater on its land area due to Tsunami and deposition of toxic salts caused degradation of more than 70% of the agricultural land (FAO 2005; Ondrasek et al. 2011). The salt deposition destroyed more than 3 70 000 fruit trees, with an estimated loss of AU $ 6.5 million, which affected around 15 000 farmers economically. In a previous report by Qadir et al. (2014), the total estimated economic loss globally was more than the 27 billion US dollars per year. The loss of productivity among the crop also varied depending upon their genetic makeup, for example, moderate salt stress of 8–10 dS/m results in the yield losses of 15%, 28%, and 55% in cotton, wheat, and corn cultivars, respectively (Satir and Berberoglu 2016; Zörb et al. 2019), showing that cotton performs better at moderate salt stress; however, at the higher salt stress, the cotton also became susceptible, and the yield loss at the 18 dS/m resulted in a 55% loss in cotton productivity (Satir and Berberoglu 2016). The yield loss estimation by salt stress in comparison with the healthy growth conditions in some of the major crops of Indian subcontinent revealed the loss of yield by 45, 39, 63, and 48%, in rice, wheat, cotton, and sugarcane, respectively (Qadir et al. 2014; Tripathi 2009), which again suggest the variation in yield losses could be the result of combined effects of many factors such as the cultivars used for the cultivation, environmental condition of that specific area, and extent and time of exposure to the saline soil conditions.
2.3 Effect of Salt Stress on Target and Nontarget Plants and Microorganisms
The adoption of agricultural practices leads to the colonization of humans. For agriculture, domestication of crop plants began a million years ago, and after that, the crops were selected for numerous beneficial traits. The selection continued along with the development of civilization and till the recent. During the selection process, the plant species with the higher yield and less fruit dehiscent property became target species for the selection and agriculture. In comparison, several nontarget plant species with better stress tolerance but less yield were ignored or not prioritized. Their further improvement for the better yield through conventional breeding approaches narrowed down the plant genetic diversity. Approximately, 30 plant species fulfill 90% of the total food demand of the world human population, but they are highly sensitive to salt stress (Zörb et al. 2019). The horticultural crops, vegetables, or fruits are a nutritious source of minerals, carotenoids, vitamins, and fiber in the human diet and are more sensitive to salt stress than the cereals. The salt‐stress threshold level for the metabolic homeostasis in most horticultural crops is ≤2.5 dS/m (SNAPP et al. 1991). Among the cereal crops, the effect of salinity varies depending on the cultivars’ salt‐tolerance ability. Rice and barley are both the targeted cereal crops used in agriculture for food production but affected differently by salt stress. The rice is the most sensitive to salt stress, whereas the barley is the most tolerant of salt stress among all the cereal crops used for agriculture (Munns and Tester 2008). The salt tolerance ability among the different targeted dicot crop species varies greatly. For example, some legumes are very sensitive to the salt‐stress conditions among the legume species. In contrast, the Medicago is tolerant of the salt stress up to the level of salinity equivalent to the seawater (Munns and Tester 2008).
Recent advances in the field of salinity stress tolerance and research identified close relative of the crop species, which were never targeted for agriculture but are halophytes and survive the salt stress more efficiently. The halophyte Eutrema salsugineum belongs to the same family as the horticultural crop cabbage or the oilseed crop mustard, and the model plant A. thaliana. Under 100 mM of NaCl salt stress which is deleterious for cabbage (Pavlovic et al. 2019) and A. thaliana, the E. salsugineum not only survived but completed its life cycle with a negligible effect on their growth (Kant et al. 2006). The close relative of the wheat, the wheatgrass (Thinopyrum ponticum, syn. Agropyronelongatum) is one of the most salt‐tolerant monocotyledonous plants (Munns and Tester 2008), which can complete its life cycle at the soil salinity equivalent to the seawater salinity. Few halophytic nontarget plant species grow like weeds in the saltmarshes and have their ability to grow and complete the life cycle in the highly saline soil (Flowers and Colmer 2008; van Zelm et al. 2020).
The soil microbial community is an indispensable component of the soil nutrient cycling and supports crop plants growth and immunity. The microbial community helps plants absorb nutrients by participating in chemical modification of mineral, which becomes suitable for the plants’ easy absorption, increasing the soil porosity and organic matter,
