# Potential of Halophytes-Associated Microbes for the Phytoremediation of Metal-Polluted Saline Soils
## 1 Introduction
### Potential of Halophytes  
Salt stress induces plant responses similar to those caused by metals. Indeed, as the main stressful driver for species living in saline areas [61], salt stress has led to strong adaptations based on the exclusion, compartmentation, and excretion of ions and oxidative defense system responses that counteract the effects of reactive oxygen species (ROS) overproduced during stresses such as salt or metal stress [62]. Morphological adaptations to salinity can help plants to tolerate metals within tissues. Succulence, currently considered to be a halophytic or xerophytic trait, could also alleviate growth injuries due to metal stress without reducing metal bioaccumulation [63; 64]. Suculence can indeed improve the water-use efficiency, thus enhancing photosynthesis under salt stress [65]. Halophytes deal with salt stress owing to three main strategies: salt excluders, where plants limit toxic ions absorption by roots; salt includes or accumulators, which are able to tolerate high NaCl concentrations, especially in their aerial parts; and salt excretors, which possess salt glands that excrete the absorbed salt at the leaf surface [39; 66]. Metal phytoremediation requires the use of plants' resistant to contaminants able to reduce the bioavailability of metals or able to tolerate metals in shoots, the harvestable parts of plants [54]. Plants that preferentially translocate metals in shoots with a high specificity for the metal(s) that are easy to grow and offer the possibility of several harvesting seasons and are perfect candidates for phytoextraction, but they are not so easy to find. According to [PERSON] et al. [54], phytoextraction is more suitable than phytostabilization, as it allows us to definitely remove contaminants from the soil. However, phytoextraction applications must combine several characteristics, particularly a high metal translocation rate from roots to shoots, and ideally a hyperaccumulating ability, as recommended by the authors. Most of halophytes accumulate metals in the roots, and only rare halophyte species are considered to be metal hyperaccumulators [67; 68]. Anyhow, the choice of halophyte species that rather belong to salt includes, ideally capable of accumulating metals in shoots and in roots, is relevant in the development of phytoremediation applications [54; 69]. Despite variations in their mechanisms of adaptation to salinity, all halophytes can use exclusion and inclusion mechanisms [70]. Depending on their salt and metal toxicity thresholds, these plants limit or tolerate metals in their internal parts. However, some other parameters can influence metal toxicity and thus limit the tolerance mechanisms. Metals' bioavailability and uptake by the plants greatly depend on salinity [1; 64]. In the study of [PERSON] et al. [71], adding 200 mM NaCl to _Sesuvium portulacastrum_ plants alleviated the Cd stress by reducing the Cd concentrationsin the shoots. Moreover, the global accumulated amounts of Cd were unchanged, and the growth was enhanced compared to the plants without NaCl. According to [PERSON] et al. [2], the distribution patterns of metals in soil is influenced by plant communities because plant roots can influence the stabilization or release of metals. Some halophytes are described as relevant candidates of phytoremediation, such as _Seswium portulacastrum_[72; 73], _Atriplex halimus_[74], _Atriplex hortenis_[75; 76], or _Climacopteran crassa_[77]. Specific species can also accumulate large amounts of metals. For instance, _Arthrocennium_ can accumulate 724 mg/kg DW of Pb [78]. _Salsola kali_, a desert halophyte, can bear 2075 mg/kg Cd in the stems and 2016 mg/kg Cd in the leaves [79] and may be classified as \"obligate hyperaccumulators\" according to [PERSON] and [PERSON] [68]. The desert halophylic _Sueda fructiosa_ can accumulate 1379 mg/kg Cr and a maximum of 13,246 mg/kg Na in the shoots, while soil concentrations were 19.5 mg/kg [80].