Chelate-assisted phytoextraction using Brassicaceae plants

The potentially toxic elements can occur naturally in soils at various amounts depending on the source rock geochemical compositions and soil formation processes like weathering sedimentation and volcanic eruptions. Meanwhile, anthropogenic activities such mining and smelting operations, industrial production, oil and gas production, agricultural activities and military practices can also result in elevated soil contaminant amounts. Indeed Figure 1 illustrates the potentially toxic element sources in soil ecosystems [4,5].


Introduction
The civilization of mankind and the agriculture sustainability rely on two precious bio-resources namely land and water [1,2]. So, the two last ones have been subjected to an hyper exploration and they were severely polluted by kinds of contaminants having its origin from various anthropogenic studies [2,3].
The potentially toxic elements can occur naturally in soils at various amounts depending on the source rock geochemical compositions and soil formation processes like weathering sedimentation and volcanic eruptions. Meanwhile, anthropogenic activities such mining and smelting operations, industrial production, oil and gas production, agricultural activities and military practices can also result in elevated soil contaminant amounts. Indeed Figure 1 illustrates the potentially toxic element sources in soil ecosystems [4,5].
It is crucial to note that the majority of contaminants get accumulated in plants and either directly or indirectly, fi nd their way into food web thus cause severe consequences [3].
In addition, the plants undergo several abiotic stresses such as salt, drought and heavy metal witch are known among the most limiting ones [6][7][8][9][10]. Indeed, it is crucial to signal that as much as one-half of the world irrigated areas are affected by the high salinity level [7,8,10,11], the drought and also the excessive soil metal amount [6,7,12]. The last cited one pose signifi cant hazard to human, plant, animal and health, hence to the ecosystem [7,13,14]. We indicate in this case that soil contamination by toxic metal has often resulted from human activities, such as those related to application of sewage sludge to agricultural soils, industrial emissions, mining, leakage and/ or disposal of industrial wastes and also the pesticide use [12,15]. It is basic also to signal that the phytoextraction is one of the metal phytoremediation technologies as illustrated in Table 1 [16].
Due to the potential toxicity and the high metals persistence, those facts constitute a serious environmental problem that requires an affordable solution [15,17]. Hence, phytoextraction seems to be the most promising technique and has received increasing attention from researchers since it was proposed by various research teams such as [18] as a technology for reclaiming metal polluted soils [3,7,12].
The metal phytoextraction from the soil relies on the use of plants to extract and translocate metals to their harvestable parts [3,16,19,20]. The phytoextraction aim consists on the reduction of the metal concentration in contaminated soils to regulatory

Abstract
In these last decades, excessive metal concentration pose serious contamination in soils. Therefore, it is urgent to develop and adopt a new strategy and technology to remove soil contaminants. Here, the phytoextraction was considered as a recently developed approach to clean up metal-polluted soils in that the plants are used to translocate the toxic metals from the contaminated soils. Therefore, this mini-review provides a perfection of recent knowledge related to the heavy metal phytoextraction from soils and their accumulation in plants. It has been reported that EDTA enhances the availability of heavy metal to plants and thus increases the accumulation in their shoots. In addition, fundamental and basic advancement has been made on the molecular aspects and the physiological regarding tolerance and metals phytoaccumulation in plants. A multidisciplinary strategy is warranted to make phytoextraction a plausible trade technology to remediate metal-polluted soils. Hence, the purpose is to integrate soil-related and biological progress to suggest research needs and future direction.

Metal extraction process
The heavy metal extraction process is due to two approaches that have currently been used to reach this purpose: the fi rst one results in the exploration of plants with exceptional, natural metal-accumulating capacity, so called hyperaccumulators [22], and the second one is the use of high-biomass crop plants with a chemically enhanced phytoextraction method [6,12,[21][22][23][24][25]. So, the basic properties of those two phytoextraction strategies of metals from soils are illustrated as followed in Table 2. In addition, the analysis of the Table 2 content and based on some of other's investigations give birth to a fundamental conclusion summarized on the fact that the natural metal-accumulating capacity seem to be the most one because of the negative repercussion of the chemically enhanced phytoextraction method [26][27][28].

Metal hyperaccumulation and tolerance in plants
The metal hyperaccumulation capability is a rare phenomenon in plants. Occurring ≈400 vascular plant species     [13].
The correlation and also the relationship between metal hyperaccumulation and tolerance is still a subject of discussion.
Some authors suggest the non-correlation between those traits, while others suggest that hyperaccumulators possess a high degree of metals tolerance. [6] compiled a number of studies in that the metal accumulation meaning tolerant and nontolerant plants had been compared [29]. Those funding's led to conclude that there is no pattern regarding accumulation and tolerance [6]. Both shoot and root concentrations are equally variable even when only one particular metal is [7,30,31].
However, at least in some cases, it is clear that the increased tolerance give birth to greater metal amount. Plants may use two strategies to deal with high metal amounts adjacent to their roots: (i) exclusion mechanisms by which the uptake and/or root to shoot transport of metals are restricted; and (2i) internal tolerance mechanisms which immobilize, compartmentalize or detoxify metals in the symplasm through production of metal binding compounds. Given that the phytoextraction purpose is to maximize metal accumulation effeciency in plant tissues, mechanisms of internal tolerance are likely to be crucial and [6,19,32].

Stress-associated protein provides tolerance to heavy metals
The boom in industrialization over the past few decades has led to the onset of long-term pollution by heavy metals as well as serious environmental and also ecological problems for humans, plants and animals. In addition, we signal that some heavy metals have a crucial role during various physiological processes in plants [33,34]

Chemical amendments upgrades metal phytoextraction
The solubility and availability of metal are both dependent on the soil characteristics and are strongly infl uenced by pH [4] and also the level of complexation with soluble ligands [37,38].
So, the metals exist in soil under various organization types: in solution as ionic or organically complexed species; on exchange sites of reactive soil components; complexed with organic matter; occluded in Fe, Al, and Mn oxides and hydroxides; entrapped in primary and secondary minerals [37]. Most metals in soils exist in unavailable forms, thus soil conditions have to be altered to elicit phytoextraction since the phenomenon, depends on a relatively abundant source of soluble metal to enable signifi cant metal uptake and translocation to shoots. In addition, some metals have their extraction rate limited by their inherently low solubility. In such a case, the organic compounds may be used as amendments to upgrade phytoextraction. Such substances can chelate metal ions, therefore modifying the metals availability in soils [4,37,38].
At the same case, we note that as indicated by [39],  Table 3 shows that EDTA and DTPA can solubilize up to 77 and 100% of Pb present in the soil samples, respectively. This was equivalent to bring 383 (EDTA) and 521 mg kg -1 (DTPA) of Pb into the soil solution.
Of course, such high values must be weighted against the use of these chelates for phytoextraction in the fi eld [6,39].

The chelating agent and its use in Brassicaceae plants for phyto-extraction
Concerning the chelating agent and its use in Brassicaceae plants for phyto-extraction, various studies and investigations were described the effect of the heavy metal and the EDTA  [40][41][42]. It has been reported that EDTA enhances the availability of heavy metal to plants and thus increases the accumulation in their shoots [43][44][45].