Page 115 - Plant-Based Remediation Processes

104 M.d.S. Santos-Dı ´az

root hairs, and the rhizosphere (Golan-Goldhirsh et al. 2004). The in vitro cultures
can be established from small sections of roots bearing a primary or lateral root
meristem. The culturing of adventitious root is an efficient means of biomass
production because of fast growth rates and stability (Shanks and Morgan 1999).

6.3 Metal Removal by In Vitro Root Cultures

An efficient approach to establishing in vitro root cultures capable of tolerating
metals is to obtain the tissue from plants growing in contaminated soils or water
bodies. Using this protocol, in vitro root cultures from Rubia tinctorum and from the
aquatic plants Scirpus americanus and Typha latifolia were established. These
cultures require the addition of auxins to the culture medium to stimulate growth.
Exposure of R. tinctorium cultures to Ag, As, Cd, Cu, Ca, Hg, In, Ni, Pb, Se, and Zn
induced the formation of phytochelatins (PCs) and many of them also induced the
corresponding desglycyl peptides (Maitani et al. 1996). The metal removal by
S. americanus and T. latifolia root cultures were characterized by an initially
rapid metal concentration decrease, followed by a slower decrease in the solution.
The first stage could be related to a sorption process at the root surface, and the
slower stage to an internalization of the metals. The cultures removed nearly 100 %
3+
of Pb 2+ and Cr , and 71–100 % of Mn 2+ from culture medium after 6–8 day
(Santos-Dı ´az et al. 2007). The quantification of metals in roots showed that Pb and
Cr uptake was due to an absorption process while Mn was principally adsorbed to
root surface. The concentration factors [CF - metal concentration in roots (mg g -1
-1
dry weight)/metal concentration in medium (mg L )] calculated for T. latifiolia
roots were 1,093, 1,473, and 122 for Pb, Cr, and Mn respectively, while for
S. americanus roots were 2,198, 2,433, and 419 for Pb, Cr, and Mn, respectively.
Thus, the root cultures of S. americanus were about two- to threefold more efficient
at removing the metals than the T. latifolia roots (Santos-Dı ´az and Barro ´n-Cruz
2011).

6.4 Hairy Root Cultures for Phytoremediation Research

A variant from the in vitro root culture that has enormous potential is the hairy root
culture, in which the root is infected with Agrobacterium rhizogenes, a Gram-
negative bacterium that belongs to the Rhizobiaceae family. The hairy root pheno-
type also can be induced by mutation (Suzuki et al. 2008), but only the roots
transformed with A. rhizogenes are considered in this chapter. This bacterium is
attracted to wounded sites of the plant and subsequently induces the formation of
adventitious roots in a wide range of plant species (Flocco and Giulietti 2007).
During infection, A. rhizogenes transfers a segment of DNA from about 10–30 kb
known as T-DNA, which is a portion of the large plasmid called the root-inducing
plasmid (pRi) from about 200 kb (Giri and Narasu 2000). When T-DNA is
integrated into the plant cell genome, it promotes the expression of enzymes that