Agriculture Reference
In-Depth Information
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-gliadin and the high molecular weight subunits of glutenin is favoured at the
expense of sulfur-rich proteins which may cause unpredictable and unwanted
variations in wheat quality (Flæte et al.
2005
; Moss et al.
1981
). Another problem,
discovered relatively recently, is the formation of acrylamide during high-
temperature processing of potato and wheat products (Tareke et al.
2002
). Acryl-
amide is potentially carcinogenic to human. It also has negative neurological and
reproductive system effects (Friedman
2003
). The major determinant of acrylamide
forming potential is the concentration of free asparagine (Curtis et al.
2010
). The
accumulation of free asparagine in wheat grain in severe sulfur deprivation may
increase to 30-fold higher levels compared to sulfur-sufficient conditions, which
makes asparagine up to 50 % of the total free amino acid pool. For that reason, even
very small amounts of such grain entering the food chain could have a significant
effect on acrylamide formation, which makes the application of sulfur fertilisers
over entire fields very important (Halford et al.
2012
).
As sulfur deficiency in Europe appeared relatively recently, research on sulfur
use efficiency still lags behind that on the other major nutrients. Therefore, explor-
ing sulfur assimilation and the regulation of sulfur metabolism is of great interest
for agriculture and plant science, because it is important to understand the process
in order to optimise it for commercial use. However, there are still many gaps in our
knowledge in this area. Studies on model plants provide a great tool for further
exploration of the sulfur metabolic pathway mainly because of their rapid life cycle.
The knowledge obtained from research on model plants can subsequently be
transferred to crops and used for improving crop-breeding strategies. For these
reasons this review is focused on the sulfur metabolism of the model plant
Arabidopsis thaliana
.
Sulfate Transport
Sulfate is the major form of sulfur in plants. Sulfate uptake from the soil is the first
stage of plant sulfate metabolism. After entry into the plants, sulfate needs to be
delivered to the plastids for assimilation or to the vacuoles for storage. Cell-to-cell
transport as well as long-distance transport between organs required to fulfil the
source/sink demands during plant growth, involve specific sulfate transporter pro-
teins (Buchner et al.
2004b
). Genes encoding these proteins belong to the sulfate
transporter gene family and are divided into five groups. Members of different
groups vary in kinetics of transport and in patterns of expression indicating different
functions in the process of sulfate uptake and distribution. In general, high-affinity
transporters are responsible for the initial uptake of sulfate by the root epidermis
and cortex cells whereas low-affinity transporters are involved in the vascular
transport. Sulfate transporters may be expressed constitutively or depending on
sulfur availability. Decreased intracellular content of sulfate, cysteine and glutathi-
one (GSH) results in increased transporter activity (Smith et al.
1997
). The Group
3 sulfate transporters are not influenced by the sulfur status of the plant (Kataoka
