Page 179 - Plant-Based Remediation Processes

170 X.-Z. Yu and J.-D. Gu

9.5 Conclusions

The extensive use of Se-containing chemicals due to anthropogenic activities has
resulted in significant releases and its distribution in the environment. Speciation of
Se in different environmental compartments is substantially different. Among the
chemical form of Se, the most common species are selenate and selenite, which
display quite different chemical properties. The former is more water soluble,
while the latter has a strong affinity of sorption. Both Se species are bioavailable
for plants. Selenate is chiefly absorbed by roots through cell membranes by active
transport driven by ATP (ATPase), whereas selenite uptake may not be mediated
by membrane transporters and seems to be accumulated through passive diffusion.
Due to the chemical similarity between selenate and sulfate, both elements share
the common metabolic pathway in plants. Competitive inhibition in biochemical
processes between selenate and sulfate affects uptake, translocation, and assimila-
tion throughout plant development. Through reviewing the Se uptake, transport,
assimilation, and volatilization in plants, it is evident that the ability of plants to
accumulate Se in their tissues highly relies on their genetic traits and greater
differences in the capacities of tolerance, uptake, accumulation, assimilation, and
volatilization of Se among various species of plants. The ATP sulfurylase pathway
responsible for the botanical reduction of selenate has been widely observed in
plants. Since this biological process is rate limiting enzymatically, storage of
selenate in plant materials is more likely to be in selenate-supplied plants. Indeed,
ATP transgenic Indian mustards are able to increase the assimilation of selenate
into SeMet, whereas selenate accumulated in the wild-type plant supplied with
selenate. Due to the existence of a nonenzymatic pathway capable of reducing
selenite into selenide, selenite is more readily assimilated by plants than selenate.
The major gaseous Se of DMSe has been identified, which is the most significant
contributor to the environment Se mobility in atmosphere. Since the volatile DMSe
is much less toxic than other species of Se, phyto-volatilization is a suggestive
remediation strategy for phytoremediation of Se-contaminated soils. Although
hyperaccumulators exhibit much more promise in the removal and accumulation
of Se than non-accumulators, most hyperaccumulators belong to grasses, which
may serve as food sources for numerous higher animals. However, Se hyperaccu-
mulators are able to provide a source of genetic materials that can be used to
modify or alter the botanical capacity of Se uptake, transport, and assimilation
using molecular modification of genes encoding proteins. It is noted that the
introduction of any transgenic plant into ecosystems should never be taken lightly;
it needs to be accompanied by careful risk assessment since it is very difficult to
identify beforehand the ecological consequences of releasing transgenics into the
environment.

Acknowledgements We thank Dr. Sheng-Zhuo Huang from Institute of Tropical Bioscience and
Biotechnology, Chinese Academy of Tropical Agricultural Sciences, in preparation of graph.