Page 113 - Plant-Based Remediation Processes

102 M.d.S. Santos-Dı ´az

(Suresh and Ravishankar 2004; Ghosh and Singh 2005; Padmavathiamma and Li
2007; Capuana 2011). Generally fast growing plants with high biomass and different
kinds of root systems are used to clean up the pollutants. Metal removal can be done
by taking up the metals into the roots or by transporting the metals from the roots up
into the leaves (Pilon-Smits and Pilon 2002; Lasat 2002; Pilon-Smits 2005;Memon
and Schro ¨der 2009;Zhou et al. 2012).

6.2 General Characteristics of In Vitro Culture for Metal
Removal

Another approach with great potential to study the phytoremediation process and
the mechanisms involved in heavy metal tolerance is the use of in vitro cultures.
Using this system, the analysis of metal removal can be performed under conditions
that are more easily controlled than with soil-growing plants like medium compo-
sition, nutritional parameters, water potential, and plant growth regulator (PGR)
concentration. In addition, the substance (organic and inorganic pollutants) and
nutrient uptake is faster and more uniform because the barriers present in whole
plant such as leaf wax, bark, cuticle, epidermis, and endodermis are not present in
in vitro cultures (Buchanan et al. 2000; Hopkins and Hu ¨ner 2009).
In vitro culture involves growing plant cells and tissues on environmentally
controlled conditions (temperature, photoperiod, and darkness), on defined
medium, and in a microbe-free environment (George et al. 2008). Different
media have been used for in vitro root culture, but the most employed are those
derived from the original White medium (1934), Murashige and Skoog (MS)
(Murashige and Skoog 1962), and B5 medium (Gamborg et al. 1968). In general,
all media contain micro- and macronutrients, a carbon source (sucrose, glucose),
vitamins, and PGR. The auxins are the primary PGR involved in the formation of
adventitious roots and their promoting effect varied between species and cultivars
(Blakesley et al. 1991; Nandagopal and Ranjitha Kumari 2007; Khalafalla et al.
2009). Besides auxins, the ability of a plant to produce in vitro roots depends on
genotype, environmental conditions, and the level of nitrates in the culture medium
(Kusakari et al. 2000; Sudha and Seeni 2001; Valim Reis et al. 2011). Liquid
medium with or without agitation is more frequently used than solid media.
The in vitro systems allow the independent study of the complex interaction
among plant/soil/microbiota, to evaluate the participation of specific enzymes,
organic compounds, transporters, or peptides involved in the plant response to the
pollutants (Boominathan and Doran 2002; Flocco and Giulietti 2007; Doran 2009).
The axenic conditions in culture prevent microbial symbiosis disguising the metal
uptake characteristics of plants grown in soil. Lynch (1982) reported that the soil
associated with plant roots (rhizosphere soil) supports 10–100 times more
microorganisms per gram than unplanted soil, due to a supply of carbon-containing
compounds exuded by plant roots. Particularly, mycorrhizal fungal taxa, such as
species like Glomus, Gigaspora, and Entrophospora, have been reported to be