Agriculture Reference
In-Depth Information
along the assimilatory pathway would unrepress gene transcription allowing sulfate
to enter the pathway. A second regulatory loop, involving OAS as a key interme-
diate, would act in promoting gene unrepression when nitrogen and carbon supply
exceeds sulfur availability within the cells (Hawkesford
2000
). In this context the
need to dissect the molecular mechanisms involved in the nutritional signal per-
ception and transduction is evident since, in several cases, the relationships existing
between gene expression and the levels of the signal-intermediates are not always
clear. Further research is needed to associate single gene expressions to a specific
nutritional signal or sulfur-nutritional status.
Genome-wide expression analyses have revealed that nitrate supply induces
changes in the expression of several genes, not only those involved in nitrate
reduction and assimilation. Such behaviour is likely both due to the direct effects
of nitrate itself and indirect effects caused by changes in nitrogen metabolite
content or nitrogen nutritional status. In fact, nitrate is thought to act as a signal
molecule influencing the expression of a number of genes, since their expression is
rapidly induced by nitrate even in mutants severely compromised for nitrate
reductase activity (Wang et al.
2004
). In addition, it has been shown that nitrate-
inducible expression NADH/nitrate reductase mRNA in maize roots, scutella and
leaves also occurs in the presence of inhibitors of protein synthesis, suggesting that
the signal transduction systemmediating this response is constitutively expressed in
plant cells, independently of the presence or the absence of nitrate in the growing
medium (Price et al.
2004
). Results of these studies clearly shows that dissection
analyses of the signal transduction pathways controlling gene expression under
different nitrogen supply should provide important information to define smart
plants able to sense the cellular level of nitrite or the general nitrogen nutritional
status of a crop system.
Conclusions
Growing varieties with enhanced efficiency and/or modifying the environ-
ment in which the crop is grown can increase the nutrient use efficiency of a
crop production system. The selection of varieties with improved nutrient use
efficiency is a more generic approach, which necessarily requires deep
knowledge of the genetic variation and inheritance of these traits. On the
other hand, the improvement of the agricultural practices aimed at sustaining
the nutrient needs of a crop may provide more immediate advantages in terms
of cost and environmental quality.
Although much progress has been made in improving fertilisation prac-
tices there remain considerable uncertainties about the persistence of nutri-
ents in the soil and their actual availability to the plants. The development of
quick and inexpensive methods to determine changes in nutrient bioavail-
ability in the soil or the nutritional status of the plants are desirable for better
fertiliser management for different crops in a variety of environmental con-
ditions. It could be particularly important not only for areas of intensive
agriculture but also for agriculture in developing countries where accessibil-
ity to fertilisers could be a problem.
