Agriculture Reference
In-Depth Information
Obviously, nutrient-specific
cis
-elements are equally as important for reporter
expression as tissue-specific
cis
-elements. Nutrient-inducible plant promoters con-
tain multiple
cis
-acting elements, only some of which may specifically contribute to
nutrient inducibility. A number of potential nutrient responsive
cis
-elements have
recently been identified in the promoter of several nutrient responsive genes and
have been indicated as key regulatory factors of gene expression under different
nutritional conditions.
Sulfur-responsive elements (SUREs) have been identified in the promoter
regions of the
Arabidopsis NIT3
nitrilase and
β
-subunit
β
-conglycinin gene from
soybean (Awazuhara et al.
2002
; Kutz et al.
2002
), although no consensus
sequences have been shown yet. However, an interesting study on
Arabidopsis
sulfate transporter
SULTR1;1
promoter demonstrates that a 5 bp sequence is
essential to promote sulfur response of SULTR1;1 (Maruyama-Nakashita
et al.
2005
). Such a sequence also appears in the promoter regions of many
sulfur-responsive genes, suggesting its involvement in the transcriptional control
of a gene set required for adaptation to sulfur-limiting conditions. Deletion analysis
of the barley
IDS2
(iron deficiency-specific clone no. 2) gene promoter allowed the
identification of two
cis
-acting elements, iron-deficiency-responsive element 1 and
2 (IDE1 and IDE2), which synergistically induced iron-specific expression in
tobacco roots. Finally, comparative analyses of several nitrite reductase gene pro-
moters from various higher plants have recently allowed identification of a con-
served sequence motif as nitrate-responsive
cis
-element (Konishi and Yanagisawa
2010
).
What Do Bioindicators Sense? A Key Problem
Modification of promoter architecture necessary for manipulating gene reporter
activity requires accurate studies of the regulatory network involved in controlling
gene expression under different nutritional conditions. Unfortunately, for the most
part, these aspects are still largely unknown preventing the optimal design of a
synthetic nutrient-inducible promoter, particularly in cases where a mineral nutrient
undergoes complex assimilatory metabolisms (i.e. nitrate or sulfate) or interacts
with other nutrients. In all these cases the specific question to be answered is: what
do synthetic nutrient-specific promoters sense?
For example, the transcriptional regulatory mechanisms involved in sulfate
uptake and assimilation reasonably result from direct sensing of the plant nutri-
tional status rather than from the composition of the external soil solution
(Lappartient and Touraine
1997
; Lappartient et al.
1999
). This control involves
an inter-organ signaling mechanism in which key intermediates of the sulfate
assimilatory pathway may act as negative or positive signals in modulating the
expression of the sulfur-responsive genes. Adequate levels of sulfur compounds
would repress gene expression through a negative feedback loop preventing exces-
sive sulfate uptake and reduction; vice versa a contraction of the intermediates
