Table 1.

Phytoremediation challenges and selected examples of transformation-based solutions

Phytoremediation challenges (acc. to Ali et al. 2013)	Examples of transformation-based solutions	Effects	References
Reduction of time required for clean-up	Accelerating heavy metal uptake by plants overexpressing genes encoding novel metal transporters Transporters of phytosiredophore–metal complexes: yellow stripe (YS1) and yellow stripe-like (YLS) Co-transformation with multiple gene constructs	In N. tabacum, BjYSL7 gene from B. juncea enhanced accumulation of Cd and Ni In A. thaliana, SnYSL3 from S. nigrum gene is up-regulated in the excess of Cd and enhances translocation of Cd to the shoots In A. thaliana, dual-gene transformation with phytochelatin synthase gene from garlic (AsPCS1) and YCF1 (yeast vacuolar transporter) gene increased Cd and As accumulation properties	Wang et al. (2013) Feng et al. (2017) Guo et al. (2012)
Increased bioavailability of the contaminants in the soil	Modulation of heavy metal uptake via transformation with components of bacterial efflux system Transformation with genes encoding components of Pseudomonas putida efflux system: CzcA, CzcB and CzcC Increasing heavy metal uptake from the soil due to secretion of phytosiderophores Potential candidate genes: nicotianiamine synthase (NAS) gene; nicotianamine amino transferase (NAAT) gene; deoxymugineic acid synthase (DMAS) gene	In N. tabacum, increased bioavailability of Cd and gene variant-dependent differences in Cd accumulation in plant organs In hypperaccumulator A. hallerii NAS regulates Zn bioavailability In O. sativa, HvNAS contributes to enhanced accumulation of Zn NAS and NAAT genes are expressed not only under Fe deficiency, but also in the presence of heavy metals (Cd, Pb and Ni)	Nesler et al. (2017) Tsednee et al. (2014) Masuda et al. (2008) Gupta and Singh (2017)
Accelerated growth rate and enhanced plant biomass accretion	Transformation-based organogenesis Transformation with rol genes from Agrobacterium rhizogenes Generation of hairy roots overexpressing modified copper resistance protein C (copC) gene from Pseudomonas fluorescens Stimulation of plant metabolism Transformation with heme oxigenase (HO) gene (regulation of phytochrome biosynthesis) Overexpression of isopentyl transferase (IPT) gene (increased synthesis of cytokinins)	In S. americanus, increased root biomass and plant tolerance to Pb and Cr In N. tabacum, increased root biomass, increased Cu accumulation and minimized toxicity symptoms In B. napus, higher plant biomass and increased tolerance of transformants to Hg stress, with decreased accumulation of Hg In N. tabacum, increased biomass accretion under Zn exposure	Alfaro-Saldaña et al. (2016) Perez-Palacios et al. (2017) Shen et al. (2011) Pavlikova et al. (2014)
Increased plant tolerance to different growth condition (i.e., elevated level of contaminants, climate, and light intensity)	Transformation towards enhanced general stress defense mechanisms Genes encoding components of antioxidant system Genes encoding transcription factors related to various abiotic stresses (salinity, drought, and cold) Genes encoding elements of DNA repair system Genes encoding cytochromes	In A. thaliana, overexpression of OsGSTL2 (glutathione S-transferase) gene confers tolerance to various heavy metals and other abiotic stresses Gene encoding DRAB (drought-responsive element binding) transcription factor increased Cu tolerance in transgenic N. tabacum; and Cd tolerance in transgenic S. tuberosum In M. truncatula, overexpression of MtTdp2α gene enhanced tolerance of plants exposed to Cu In A. thaliana, overexpression of CYP450-like gene (cytochrome P450) confers to tolerance to As, Cr, and Cd and other abiotic stresses	Kumar et al. (2013) Ban et al. (2011); Charfeddine et al. (2017) Faè et al. (2014) Rai et al. (2015)
Reduction of the risk of food chain contamination	The use of transgenic plants with increased ability to phytoremediation may itself accelerate soil clean-up and, an as a result, the risk of food chain contamination can be diminished