**4. Discussion**

Among soil contaminants, Pb is of particular concern due to its ability to biomagnify through the food chain, threatening food safety [47]. Lead is highly toxic for most plants, causing severe physiological damage [52]. However, some plant species have proven to be particularly resistant to lead toxicity and are widely used in the phytoremediation of Pb-contaminated soils [31,47]. The plant species tested in this experiment showed a growth capacity not affected by lead contamination. The reason for the selection of these two species is twofold: in addition to their satisfactory performances, as shown in other studies on Pb phytoremediation [53–57], they have recently been recognized as energy crop species, capable of providing a high level of biomass that can be reused in the bioenergy field [58–61].

This last aspect is currently very relevant as it allows further support for phytoremediation both in economic and sustainability terms.

The phytoextraction of Pb-contaminated soils with EDTA has been extensively studied. The results obtained in our study are in agreemen<sup>t</sup> with previous works that have shown how the addition of this complexing agen<sup>t</sup> was recognized as an effective technique to increase Pb absorption by plants [31,47,62,63].

Although several studies have reported that EDTA can promote plant growth in metal-polluted soils by stimulating auxin production and aiding nutrient translocation in plants [33,64,65], the rapid release of bioavailable metal in a short period could cause harmful transient plant phytotoxicity. In this regard, a good strategy for phytoremediation of Pb-contaminated sites could be the combined use of PGPRs and EDTA to simultaneously improve metal bioavailability in soil and plant growth under metal stress.

The stress caused in plants by the presence of heavy metals triggers complex physiological and molecular mechanisms. Among these, the production of radical exudates containing, e.g., low-molecular-weight organic acids (LMWOAs) that stimulate microbial growth of the rhizosphere and solubilize essential trace elements such as insoluble or poorly soluble (e.g., phosphorus, iron and zinc), can complex some metals such as arsenic, cadmium and lead [66].

Most metals are present in the soil in the form of insoluble and non-bioavailable salts. Chemical compounds such as EDTA, dipotassium phosphate or ammonium sulfate can separate metals from complexes bound to soil particles, favoring the absorption by the plants' roots.

PGPRs can increase the bioavailability of metals by producing microbial metabolites and siderophore molecules. Many studies show that adding chelating agents (such as EDTA) would further improve plant growth and metal uptake when combined with PGPR inoculation [67,68].

In this work, we have shown that the simple addition of the microbial inoculum (PGPR) led in *B. juncea* to a significant increase in the absorption of lead in the aerial part. A similar result was also shown in the work of He et al. [69], where the inoculation of two *Bacillus* strains, isolated from the soil, improved the rhizosphere soil environment promoting absorption of Pb by plants, enhancing the dry weight of shoots of plants growing in Pb-contaminated soil, and significantly increasing the total Pb content in aerial parts.

The possibility of avoiding the addition of chemical compounds to increase the bioavailability of the metals is undoubtedly a fascinating aspect that significantly increases the sustainability of phytoremediation.

New experiments with different plant species and endophytes isolated from metal contaminated soils are underway.
