Agriculture Reference
In-Depth Information
Organic acids like citric acid, aspartic acid and oxalic acid can convert inorganic
chromium to organically bound chromium, making it soluble for a longer period
of time and thereby available to plants (James and Bartlett 1983 ). Whether organic
acids can play significant role in chromium detoxification is still not completely
understood. It is interesting to note that recent studies have shown that soluble metal
ions can activate preexisting signaling pathways in the cell that can cause the cell
to respond to what it thinks are physiological signals (Ye and Shi 2001 ). In many
cases with a toxic metal ion, these signals are not physiological since they may be
multi-component. Hexavalent chromium has been shown to also affect cell signal-
ing by activating NFB, API, HIF-1, and VEGF (Ding and Shi 2002 ). However, it is
not clear whether these effects occur without its entry into cells.
5.2   Induction of Chelating and Sequestering Agents
Chelation and sequestration of metal ions is another defense line operative within
plants. This is done by a particular class of metal binding ligands denominated
metallothioneins (MTs) and phytochelatins (PCs) (Cobbett 2000 ; Cobbett and
Goldsbrough 2002 ; Schmidt 2003 ; Quartacci et al. 2005 ; Quartacci et al. 2006 ;
Diwan et al. 2010c ). The role of MTs and PCs in chromium detoxifictation in plants
has not been studied as thoroughly as for other heavy metals. However, it has been
reported that the production of ROS as a result of chromium exposure triggers sig-
nals to induce mRNA transcription (Shanker et al. 2004 ). Thus, MTs may have a
role in chromium detoxification in plants possibly through binding to chromium
ions and making them non-toxic. MTs are cysteine-rich polypeptides encoded by
a family of genes. MTs have a possible role in chromium detoxification in plants
and it has been reported for sorghum that MT-like proteins are expressed under
chromium stress (Shanker et al. 2004 ). These are products of mRNA translation and
are characterized as cysteine-rich metal-binding proteins (Kagi 1991 ). A study of
Cr(VI) effects on the MT3 gene expression using chromium tolerant and susceptible
varieties revealed a high intensity band matching the gene of interest in the tolerant
variety compared to the susceptible one (Shanker et al. 2004 ). This suggests that
under chromium stress, there could be high transcription rates of MTs, particularly
in the tolerant variety (Shanker et al. 2004 ). In a study on maize, an induction of
MTs synthesis was detected starting from 2 ppm potassium dichromate. However,
a consistent induction was observed starting from 100 ppm chromium. At the con-
centration of 300 ppm, the MTs content was seen to get doubled the value observed
at 100 ppm. The highest concentration of MTs was obtained at 600 ppm of potas-
sium dichromate. It is suggested that the production of ROS and H 2 O 2 as a result
of chromium exposure triggers signals to induce MT mRNA transcription (Shanker
et al. 2004 ). Thus, MTs may have a very important role in chromium detoxification
in plants, possibly by binding chromium ions and making them non-toxic. However,
the role of MTs in chromium detoxification in plants is not well understood nor thor-
oughly studied, so their role in this respect still remains a challenge for the future.
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