Page 120 - Plant-Based Remediation Processes

6 Metal Remediation via In Vitro Root Cultures 109

6.5.4 Zinc and Copper

The annual worldwide release of Zn to environment exceeded the levels of other
heavy metals reaching 1,350,000 Mg (Singh et al. 2003). Zinc is also a dominant
heavy metal that pollutes rivers in several countries (Mason 1991; Pistelok and
Galas 1999; Jain 2004). Solanum niger is a hyperaccumulator of zinc and therefore
is a good candidate to establish in vitro root cultures. Subroto et al. (2007) studied
the ability of the hairy root cultures of this species to absorb zinc. Two strains of
S. nigrum hairy roots were isolated, strain A4 and strain K1 (control). Both strains
-1
are able to grow in medium supplemented with 13.98 mg L Zn. The strain A4 was
capable of accumulating Zn from the culture medium better than the strain K1.
However, the two strains of hairy roots presented similar patterns of growth and
metal absorption and were able to remove as much as 98 % of the Zn from the
culture medium within 18 days. Both strains A4 and K1 actually reached maximum
Zn accumulation at day 9. Strain A4 accumulated slightly more Zn than the strain
K1. Only small amounts of Zn underwent an uptake–release pattern, suggesting that
metal strongly binds to cellular sites.
Regarding copper, its high toxicity to plants is due to inhibition of the activity of
many enzymes, photosynthesis, pigment synthesis, alteration of membrane integ-
rity, and blocking of photosynthetic electron transport, leading to the production of
free radicals (Fernandes and Henriques 1991).
Genetically transformed hairy root cultures were established from Hyptis
capitata, which is a widespread weedy species; Polycarpaea longiflora, belonging
to the Caryophyllaceae which includes copper indicator species; Euphorbia hirta,a
rhizomatous herb; and N. tabacum. These cultures were screened for their
-1
capabilities to uptake copper. After a short-term exposure to 1,000 μgg Cu, the
H. capitata, P. longiflora, and N. tabacum hairy root cultures accumulated similar
copper levels, but the Cu content in E. hirta hairy roots was 28 % lower. Equilibrium
Cu levels for the four species represent average concentration factors of 3.8–5.6
relative to the Cu initial concentration provided. Thus, the establishment of hairy
root cultures from a range of plant species demonstrates the utility of this system for
screening plants with capabilities to uptake metals (Nedelkoska and Doran 2000b).

6.6 Scaling Up of Hairy Roots and Bioreactors

Advances in design of proper bioreactors for hairy roots growth are being of great
interest, since scale-up will allow the integration of this technology to industrial
processes. Several challenges must be surpassed for commercial exploitation. For
example, mechanical agitation causes wounding of hairy roots and leads to callus
formation; meristem-dependent growth of root cultures in liquid medium results in
a root ball with young growing roots on the periphery and a core of older tissue
inside, and it is difficult to have a good distribution of roots which affects the supply
of nutrients and oxygen (Kim et al. 2002). However, techniques of inoculum
handling, root homogenization (Ramakrishnan et al. 1994), and an inoculation