Soil salinity refers to the concentration of inorganic solutes in the soil aqueous phase, including soluble charged species, non-ionic solutes, and ions that combine to form ion pairs. Since sodium cations are the most damaging to plant development, from an agricultural point of view, salt-affected soils are classified as saline, sodic, and saline-sodic soils [1]. In nature, salts accumulate in soils and water mainly due to the erosion and weathering of superficial rocks, seawater intrusion in coastal areas, and rising saline aquifers [1]. The natural increase of salt amounts in soils and water is referred to as primary salinisation, resulting in areas rarely used for agriculture because of the relative sensitivity of all major crops to salinity. Moreover, many traditional agricultural areas, mainly those cultivated under irrigation in arid and semiarid regions, are affected by the so-called secondary salinisation provoked by the accumulation in the soil of ions dissolved in irrigation water but also by improper agricultural practices, such as the abuse of amendments and agrochemicals (pesticides, herbicides, fertilisers) in soil and water, and uncontrolled farming wastes, amongst others [1].

Salinisation, either primary or natural or secondary or anthropogenic, entails considerable growth and viability limitations for most plant species, including all major crops. Moreover, the increase of salts in soils alters their structure, chemistry and biology, indirectly affecting plant performance [2]. The rise of global Earth temperature and the intensification of agricultural practices increase salinity in cultivated areas, thus presenting an expanding problem for producing food and other plant-derived goods. According to the Global Map of Salt-Affected Soils (GSASmap; Ref. [3]), over 3% and 6% of global land top soils and subsoils, respectively, are affected by salinity or sodicity, arid and semiarid climatic zones being the most affected [3]. Thus, the present and future of the conventional agriculture model must be revisited to guarantee the availability of natural resources and food for future generations. In this sense, the scientific community has made a considerable effort to obtain new crop varieties with better performance under adverse environmental conditions, including salinity [4]. However, there has been limited success in the obtention of new crop varieties resistant to salinity despite the deep understanding of plant genetic and physiological responses to salt, probably because salinity has not been a primary trait for breeders and affects localised agricultural areas [5].

Intraspecific variation has been successfully used to obtain several salt-tolerant rice cultivars [6,7], whereas results are more discrete in wheat [5]. Traditional breeding programmes based on the hybridisation of commercial tomatoes with wild relatives such as Solanum pennellii resulted in the generation of lines with improved performance under salinity conditions [8]. Similarly, introgression of Nax2 genes from an ancestral wheat relative (Triticum monococcum) into commercial durum wheat resulted in a lower sodium accumulation in shoots, thus in a greater performance under saline conditions with no apparent growth penalties [9]. The use of salinity-resistant rootstocks with more efficient sodium and/or chloride exclusion mechanisms has been tested for woody crops such as apples [10], citrus [11], or grapes [12], although further work is required to reach commercial application [5]. Similarly, salinity resistance in tomatoes has been achieved using Na⁺ exclusion tomato and eggplant rootstocks [13,14]. Finally, transgenic japonica rice plants overexpressing OsNHX1, the vacuolar Na⁺/H⁺ antiporter, responded better than control plants when growing under mild saline conditions [15]. All these are some examples of the relative success of the obtention of improved crops resistant to salinity. However, a proper and suitable breeding approach has not yet been reported to generate salt-resistant crops with competent agronomic features.

In recent years, the use of salinity-resistant plant growth-promoting rhizobacteria (PGPR) has emerged as an attractive possibility to mitigate the salinity problem in agriculture. Such microorganisms synthesise beneficial secondary metabolites and siderophores, as well as hormones and enzymes, and improve soil features, including ion homeostasis and nutrient and water availability, that are beneficial for plant performance. The interaction between plants and PGPR improves critical physiological processes and enhances photosynthesis efficiency, which promotes plant growth and development, especially under saline conditions [16]. Thus, the application of halotolerant PGPR inoculates in the field opens a new sustainable approach to increasing food production. As a complementary strategy to the genetic improvement of crop salt tolerance, through classical breeding and genetic engineering or NGTs (new genomic techniques) such as genome editing, as well as the application of PGPR inoculums, resilient wild plant species adapted to extreme environmental conditions in their natural habitats can be used for agricultural purposes; they can alleviate the pressure exerted by agricultural activity on the natural environment and contribute to the maintenance of cultivated lands. In the case of saline soils, the commercial cultivation of plant species adapted to saline conditions, the halophytes, represents the basis of the so-called (bio)saline agriculture [17]. Moreover, the study of halophyte biology will help us understand the strategies followed by these species to tolerate high salinity conditions [18], which could be translated, to some degree, to our conventional crops, which are glycophytes or salt-susceptible species.

Halophytes are a limited group (less than 1% of the world flora) of plant species that present a natural resistance and/or tolerance to salinity. These can be either obligate halophytes, which require constant saline environments for optimum growth, or facultative halophytes, which can withstand a certain degree of salinity, although they perform better under non-saline or low salinity conditions. Nonetheless, the precise classification of particular species as halophytes is still under debate, and the exact number of halophytes is difficult to establish. The most generally accepted definition for halophytes considers these species as plants able to grow and complete their life cycle under 200 mM NaCl or higher salt concentrations [19]. Other authors, however, established a salt concentration threshold down to 85 mM NaCl to consider a species to be a halophyte [20]; according to this criterion, a total number of ca. 6000 species, including terrestrial and marine plants, could be considered as halophytes [20]. A broader definition considers halophytes as species that can naturally grow and complete their life cycle on saline soils where most plant species cannot [21]. A less restrictive list of worldwide halophytes that includes miohalophytes or species with the best performance under non-saline conditions that tolerate low salinity (i.e., 20–100 mM NaCl) resulted in 26,000 species [22]. Aronson defined 1560 salt-tolerant plant species, selected by their ability to grow under electrical conductivity levels of 7.8 dS m⁻¹, equivalent to 80 mM NaCl, during significant life cycle periods [23]. Moreover, halophytes are distributed within many plant families. From a list of only 1861 halophytes, a total of 139 families and 636 genera were identified [22]. Altogether, the exact definition and classification of halophytes is complex since salinity responses depend on many different factors, such as the soil and water salt concentration, the physiological stage of plants when salt is present, or the exposure interval. A more detailed discussion of halophyte definition and classification can be found in [24].

Finally, the web repository eHALOPH (https://ehaloph.uc.pt/ (accessed on 10 July 2024)) compiles more than 1200 species with a certain degree of natural salt tolerance, including strict halophytes that perform better under mild saline conditions (e.g., 100 mM NaCl) and can grow in the presence of 200 mM NaCl (20 dS m⁻¹) or higher salinity levels, and species that tolerate salt concentrations of ~80 mM NaCl (7.8 dS m⁻¹). Moreover, eHALOPH includes relevant published information about their morphological, physiological and biochemical adaptations to salinity, natural habitats, and reported economic uses [25], which is helpful for the halophyte biologist community.

Salt cellular response triggers the activation of an early signalling pathway, including the production of Reactive Oxygen Species (ROS) and Ca²⁺ waves, and a later mechanism to avoid the toxic effects of Na⁺ cations within cells. Sodium cellular perception provokes a rapid production of ROS that results in oxidative damage, characterised by alterations in redox homeostasis, lipid peroxidation, protein oxidation, and enzymatic inhibition [18]. Moreover, calcium accumulates in the cytosol, and calcium peaks are sensed by calcineurin B-like (CBLs) Ca²⁺ interacting proteins. Upon Ca²⁺ sensing, CBLs interact with CBL-interacting protein kinases that activate the phosphorylation of target proteins [26]. Later, sodium accumulation within cells causes an osmotic imbalance that is lethal for cells. This osmotic deregulation can be alleviated by the activity of proton pumps that promote sodium cell extrusion or sequestration within vacuoles [26].

How plants, and specifically halophytes, deal with sodium in soils depends on several built-in, non-exclusive mechanisms that rely on anatomical, physiological, biochemical, and genetic adaptations. Both halophytes and glycophytes share common response mechanisms, although halophytes’ cellular and enzymatic machinery is generally more effective for sodium detoxification [27]. One of the primary defence mechanisms under high salinity conditions is sodium extrusion through the Salt Overly Sensitive (SOS) pathway. Briefly, SOS3 is a calcium-binding protein that interacts with and activates the serine/threonine protein kinase SOS2 in response to high calcium peaks provoked by sodium accumulation within cells [28]. This interaction further leads to the phosphorylation and activation of SOS1, the plasma membrane Na⁺/H⁺ antiporter that actively transports sodium from the cytosol to the apoplast [29]. Although this response pathway is shared in both halophytes and glycophytes, it is more effective in the salt-resistant Eutrema salsugineum and Schrenkiella parvula than in their relative Arabidopsis thaliana [30]. Whether this is a general trend, although likely, is still unresolved.

Moreover, halophytes present an efficient antioxidant machinery that reduces the cellular damage caused by the rapid increase of ROS in the cytosol. Antioxidant response in plants can be enzymatic or non-enzymatic. Enzymes such as superoxide dismutase (SOD), catalase (CAT), dehydroascorbate reductase (DHAR), and ascorbate peroxidase (APX), amongst others, can metabolise toxic ROS into non-toxic products and water. On the other hand, non-enzymatic scavenging mechanisms exist in plants and are especially important to avoid the accumulation of ¹O₂ and OH∙ since these ROS cannot be enzymatically catabolised [31]. The basis of the non-enzymatic system relies upon the accumulation of antioxidant compounds such as ascorbate and glutathione, glycine-betaine, and proline, amongst others, which can scavenge ROS molecules [31]. Although ROS degradation and scavenging alleviate the toxic effects of ROS, these are not the primary mechanisms that allow strict halophytes to grow under high salinity conditions. Instead, such highly salt-tolerant species are able to maintain a low cellular sodium concentration, thus low ROS and oxidative stress, from the initial salinity perception [31].

In addition, halophytes have evolved specific anatomical adaptations, such as succulent leaves or stems, and thicker epidermal layers under salt stress conditions [32,33,34], sunken stomata, tissue lignification [35], or short root systems [18]. Some halophytes have developed specialised salt-secreting structures that either directly excrete salt from the inner to the outer part of the leaves (salt glands) or temporarily accumulate salt in cavities on the surface of the leaves (salt bladders) [35].

Different halophytes show one or more of these features that allow them to handle salinity. According to the main mechanisms used by a particular species, they can be divided into three categories: (i) excluders or species that deal with salinity through salt avoidance, typically due to root anatomical or morphological adaptations; (ii) accumulators or includers, which accumulate salt within cells and tissues but avoid its deleterious effects through cellular and biochemical responses, and (iii) conductors, which present specialised leaf glands able to excrete salt from the inner tissues to the leaf surface [36].