Page 64 - Plant-Based Remediation Processes

52 M. Mleczek et al.

3.4.3 Comparative Transcriptomics in Hyperaccumulator
Gene Discovery

The comparative analyses of transcription profiles of closely related
hyperaccumulator and non-accumulator plants, wild-type (WT) plants with inser-
tion mutants of the same cultivar or plants with the same genetic background grown
on media containing different meta(loid) concentrations, led to the detection of
hundreds of differentially regulated “candidate genes”. In several cases, the
biological function of a candidate gene can be predicted and further tested; how-
ever, the function of a huge majority of these candidates cannot be easily predicted,
so they have not yet been tested. Cadmium treatment affects regulation of a broad
range of A. thaliana genes in several hours. Many genes involved in glucosinolate
biosynthesis and photosynthesis are repressed, while genes involved in sulphur
uptake and assimilation or cell wall and phenylpropanoid metabolism are induced,
and indeed in A. thaliana sulphur uptake increases rapidly upon Cd treatment
(Herbette et al. 2006). A. thaliana low affinity plasma membrane nitrate transporter
NRT1.8 is expressed in xylem parenchyma cells and is induced by Cd stress.
Disruption of the NRT1.8 gene shows that NTR1.8 takes up nitrate from xylem
vessels to parenchyma cells and that nitrate allocation to roots is important for Cd
tolerance (Li 2010). Unfortunately, several other genes important for Cd tolerance,
such as phytochelatin synthase AtPCS1 or transporters ABCC1 and ABCC2, are
expressed constitutively (Song et al. 2010b).
Roots of A. halleri grown in normal conditions revealed much higher constitu-
tive expression of NAS and Zn transporters ZIP9 and NRAMP3. Comparison of
shoot transcriptomes after exposure to low or high Zn revealed higher transcrip-
tional induction of over 50 genes in A. halleri than in A. thaliana in both treatments,
among them Zn transporters (HMA3, ZAP, ZIP6, CDF1), NAS and AGO5—a
microRNA binding protein (Becher et al. 2004). Transcriptomes of whole plants
grown under control conditions and upon short-term exposure to high Zn
concentrations, compared using ATH1 microarrays, revealed further candidate
transporter genes, induced more strongly in A. halleri than in A. thaliana, such as
HMA4, IRT3 and ZIP10 (Talke et al. 2006). Function of the huge majority of these
candidate genes in metal(loid) tolerance and hyperaccumulation remains to be
elucidated. RNAseq-based transcriptomics provides more detailed information
about gene expression and better transcriptome coverage than microarrays. Using
this technology Bernal et al. (2012) have recently discovered that copper uptake
depends on Cu(II) reduction to Cu(I) by FRO4/FRO5 (Bernal et al. 2012).

3.5 Conclusion

Prospects for the development of phytoremediation may stem from almost all
presented aspects; however, it is certain that the future of this method may be
associated with highly specialised plants (probably after complex genetic