Page 114 - Plant-Based Remediation Processes

6 Metal Remediation via In Vitro Root Cultures 103

associated with most of the plants growing in the heavy metal polluted habitats
(Alves da Silva et al. 2005). The transport of the toxic metals absorbed by the
mycorrhizal surface to the aerial part of the remediating plants is an obvious
mechanism which can enhance the total uptake and transport of the toxic metals
in a defined period, due to an increased surface area of the rhizosphere by the
mycorrhizal associations (Khan et al. 2000; Schutzendu ¨bel and Polle 2002; Audet
and Charest 2009).
An in vitro screening reduces not only the growth period and the treatment time
length of the plants but also the space required for the experiments. Cell cultures are
also a useful system for metabolic engineering and for obtaining rapid evidence of
the ecotoxicological behavior of chemicals and heavy metals in plants with less
analytical expense (Golan-Goldhirsh et al. 2004). Moreover, the environmental
factor variability is also reduced, physiological activities can be increased by
modifying the culture conditions (for example, employing biotic and abiotic stress),
and it is easier to isolate and analyze metabolites (Shanks and Morgan 1999; Hu and
Du 2006).
De-differentiated cells, such as callus or cellular suspension, and differentiated
organs, such as roots and shoots, can be used for metal removal (Czuba 1987; Ros
et al. 1992; Ramgareeb et al. 1999; Rout et al. 1999; Nehnevajova et al. 2007;
Di Lonardo et al. 2011). When non-differentiated tissues are employed, genetic and
epigenetic changes can be observed due to Somaclonal Variation (Lee and Phillips
1988). However, this variation and in vitro selection seem to be an appropriate
technology for the development of new plant variants with enhanced metal accu-
mulation and extraction properties (Jan et al. 1997; Herzig et al. 2003; Nehnevajova
et al. 2007).
In vitro culture of roots and shoots allows indefinite propagation and experimen-
tation using tissues derived from the same plant, avoiding the risks of variability
between species (Pollard and Baker 1996; Huang and Cunningham 1996; Marmiroli
2007). This approach also allows the analysis of metal accumulation properties of
each organ (Kartosentono et al. 2001; Nedelkoska and Doran 2000a) and the
possibility to develop industrial bioreactor models (Kim et al. 2002; Giri and Narasu
2000). The in vitro root cultures are particularly important for studying the interac-
tion of contaminants because they are in direct contact with pollutants, besides
being metabolically very active. The roots not only participate in water and nutrient
uptake but also synthesize and release several compounds. Root exudations include
the release of ions, oxygen, and water but mainly consist of carbon-containing
compounds from low- and high-molecular weight. Low-molecular-weight
molecules include sugars and simple polysaccharides such as arabinose, fructose,
glucose, maltose, and rammnose; amino acids such as arginine, asparagine, aspartic,
cysteine, and glutamine; organic acids such as acetic, ascorbic, benzoic, folic, and
malic acids; and phenolic compounds. High molecular weight compounds include
flavonoids, enzymes, fatty acids, growth regulators, nucleotides, tannins,
carbohydrates, steroids, terpenoids, alkaloids, polyacetylenes, and vitamins (Uren
2000; Bertin et al. 2003). Organ root culture is used for the study of the transport
mechanisms of contaminants in roots with a focus on the interface among root tip,