Agriculture Reference
In-Depth Information
Strategies to Enhance the Specificity of Bioindicators
In the future, the use of smart plant technology in crops would provide rapid
bioassay methods to obtain valuable information about nutrient availability in the
soil solution and/or the nutritional status of the plants allowing efficient temporal
and special application of fertilisers and the development of decision-making
systems for precision farming. To date the exploitation of these technologies is
limited, not only by the lack of telemetry systems suitable for plant monitoring
across a large area, but also by the lack of precise information to design a
transformation-cassette that would enable the nutrient-specific control of reporter
activity. Thus, the choice of a core promoter to confer specific transgene expres-
sion, represent the major challenge we have to face in order to develop the next
generation of bioindicators.
A typical plant promoter consists of CAAT and TATA boxes for recognition of
DNA-dependent RNA polymerase, several-tens of bp upstream of the transcription
initiation site (Yoshida and Shinmyo
2000
). Specific DNA sequences, called
cis
-
elements, generally upstream of the core promoter, drive the cell- or organ-specific
expression of the downstream gene under certain environmental conditions. Spe-
cific factors, called
trans
-factors (or transcription factors), bind to the
cis
-elements
affecting RNA polymerase activity. Generally, multiple-
cis
-elements and
trans
-
factors work together to induce the full regulation of gene expression, since gene
expression is generally under the control of several factors (Yoshida and Shinmyo
2000
; Venter
2007
).
In recent years, a wide range of different promoters have been characterised and
extensively used for regulating the expression of transgenes in plant cells (Venter
2007
). In several cases, the
cis
-elements that are necessary for transcriptional
regulation and the
trans
-factors that interact with these elements have been iden-
tified. From these studies has emerged a complex picture in which DNA sequence
cis
-elements that are important for regulation are scattered over thousands of base
pairs, and these elements interact with
trans
-factors that can be either ubiquitous or
highly restricted in their distribution. In this way diverse expression patterns may be
achieved through combinations of a limited number of regulatory elements and
trans
-acting factors. The knowledge of these combinatorial mechanisms should
allow the generation different transcription patterns by
'
cut and pasting
'
the com-
ponents in different ways.
Analysis of the cauliflower mosaic virus (CaMV) 35S promoter has contributed
to the understanding of transcriptional regulatory mechanisms and has allowed the
design of inducible transgene expression cassettes. The
343 to
46 upstream
region relative to the site of initiation of transcription (+1) of the promoter is
responsible for the strength of transcription. Two regions,
343 to
208 and
46
region plays an accessory role by further increasing the transcriptional activity
(Odell et al.
1985
; Fang et al.
1989
). Artificial promoters are generally constructed
by a combinatorial design of different promoter elements, with the minimal core
208 to
90, are responsible for transcriptional activation, and the
90 to
