Agriculture Reference
In-Depth Information
together, very low SAT activity in plastids, the presence of sulfide in the cytosol and
the cytosolic localisation of cysteine strongly suggest the cytosol as the main
cellular
compartment
for
cysteine biosynthesis
in
Arabidopsis
(Krueger
et al.
2009
).
Glutathione Biosynthesis and Functions
Formation of cysteine is the terminal step of sulfate assimilation pathway and the
starting point for production of methionine, GHS and many other sulfur-containing
compounds (Fig.
3.3
). GSH is the main thiol-containing molecule in plant cells and
is present in much higher concentrations than cysteine. It has a broad range of
functions, which include removal of reactive oxygen species (ROS), detoxification
of heavy metals and xenobiotics, sulfur donation, transport and storage (in catalytic
reactions), redox signalling and many others. It is synthesised from glutamate,
cysteine, and glycine by two enzymes:
-ECS)
and glutathione synthetase (GSHS). The reaction consumes two ATP molecules.
ʳ
ʳ
-glutamylcysteine synthetase (
ʳ
-ECS is redox sensitive; only its oxidised form has high activity, whereas in
reduced form the activity is much lower. The increase in
-ECS transcript level
in response to various environmental changes suggests its role as regulatory factor
(Xiang and Oliver
1998
).
ʳ
-ECS is also inhibited by higher concentrations of GSH.
Synthesis of GSH is regulated by cysteine availability (Noctor et al.
2002
). In
Arabidopsis thaliana
GSH is synthesised in cytosol and plastids, whereas
ʳ
-ECS is
localised to plastids only and GSHS activity is distributed between plastids and
cytosol. GSH from leaves is transported to roots, seeds and fruits via the phloem
(Leustek et al.
2000
), which supports an important role as a sulfur donor. In
addition, GSH degradation is an important process in plants, however, the mecha-
nism is not well understood. The main enzymes responsible for GSH degradation
include
ʳ
-glutamyltransferase (GGT), glutathione
S-transferase (GST), and glutaredoxin. It seems that GSH turnover in cells is
maintained mainly by GGT activities (Takahashi et al.
2011
). However, the intra-
cellular degradation of GSH is independent of GGT and it
glutathione
reductase,
ʳ
is initiated by
ʳ
-glutamylcyclotransferase (Ohkama-Ohtsu et al.
2008
).
Methionine Biosynthesis
Methionine is a sulfur-containing amino acid belonging to the aspartate family of
amino acids together with lysine, threonine, leucine and isoleucine. It is essential
amino acid for mammals and must be obtained entirely from the diet. Plants are able
to synthesise it
de novo
from cysteine or homocysteine. Methionine plays important
roles in plant metabolism. First of all it is a protein component. It is also involved in
initiation of translation.
S
-adenosylmethionine (SAM), which is produced from
